Electronics Circuits and Systems Fourth Edition Book

Electronics Circuits and Systems Fourth Edition Book
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Electronics Circuits and Systems Fourth Edition Book

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    ElectronicsCircuits and Systems

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    ElectronicsCircuits and SystemsFourth EditionOwen BishopAMSTERDAM GBOSTON GHEIDELBERG GLONDON GNEW YORK GOXFORD GPARISSAN DIEGO GSAN FRANCISCO GSINGAPORE GSYDNEY GTOKYONewnes is an imprint of Elsevier

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    Newnes is an imprint of ElsevierThe Boulevard, Langford Lane, Oxford OX5 1GB, UK30 Corporate Drive, Burlington, MA 01801First edition 1999Reprinted 2000Second edition 2003Third edition 2007Fourth edition 2011Copyrightr 2011, Owen Bishop. Published by Elsevier Ltd. All rights reservedThe right of ...

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    ContentsPrefaceviiPractical Circuits and SystemsixConventions Used in this BookixCompanion WebsiteixPart 1Circuits11. Diodes52. Transistor Switches11Switching a Lamp11Light-Sensitive Alarm13Heater Switch15Overheating Alert17Schmitt Trigger18Logical Control193. Potential Dividers254. Capacitors315...

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    22. Counters and Registers18523. Display Devices19724. Converter Circuits203Analogue-to-Digital Converters203Digital-to-Analogue Converters20725. Integrated Circuits211Part 2Systems21526. AudioandVideoSystems217Analogue Audio Systems22027. Noise22728. Telecommunications23329. Cable Transmission24...

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    PrefaceThis book is written for a wide range of pre-degreecourses in electronics. The contents have been care-fully matched to current UK syllabuses at Level 3 /A-level, but the topics covered, depth of coverage,and student activities have been designed so thatthe resulting book will be a student...

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    PRACTICAL CIRCUITS AND SYSTEMSCircuit IdeasAs well as being a textbook, this is a sourcebook ofcircuit ideas for laboratory work and as the basis ofpractical electronic projects.All circuits in this book have been tested on theworkbench or on computer, using a circuit simulator.Almost all circuit...

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    Part 1CircuitsAn electrical circuit is a pathway for the flow of elec-tric current.In a direct current(or DC) circuit, the currentflows in one direction. It flows from a point in thecircuit that is at a high potential to a point in the cir-cuit that is at a low potential. The circuit may besimple...

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    A conductoris a material in which electriccurrent can flow. All metals are conductors. Listthe conductors in this circuit. An insulator(or non-conductor) is a material in which current can notflow. List the insulators in this circuit (some answersin Supplement B, p. 361).The diode D1 is made from...

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    All of these convert one form of energy intoanother form, electrical energy.Each part of the circuit above has:Ga currentflowing through it, measured in amperes(or ampsfor short).Ga pdacross it, measured in volts.Ga resistanceto the flow of current, measured inohms.These three quantities are rela...

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    Topic 1DiodesA diode is made from semiconducting materials. Ithas two terminals, called the anode and the cathode.Current can flow easily from anode to cathode.CURRENT AND VOLTAGEWhen there is a voltage (a pd) across a resistor, thetwo quantities are related as in the equation pd/cur-rent5 resist...

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    through the diode until the forward bias of more thanabout 0.7 V. As the voltage is increased above 0.7 V,the current increases, slowly at first then morerapidly.REVERSE BIASIf a diode is connected the other way round, so thatits cathode is more positive than its anode, it isreverse biased.Only a...

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    EXTENSION BOX 2 Conduction by HolesAnother type of semiconductor is known as p-type semicon-ductor. In this, the added material is an element such asindium. The indium atoms have a ‘vacancy’ in their outerelectron orbit where an electron can be taken in. Such avacancy is known as a hole.What ...

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    EXTENSION BOX 3 Conduction in DiodesConsider a bar of silicon, doped so that half is p-type andhalf is n-type. The bar is not connected to a circuit, so noexternal electric field is applied to it. Electrons and holesare free to wander at random.Some of the electrons from the n-type material wande...

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    QUESTIONS ON CIRCUITS1 Draw a schematic diagram of a door alert circuitbuilt from a battery of 4 cells, a push-button, and abuzzer (see the chart of symbols on p. 357). On thediagram mark the point in the circuit that is at thehighest potential. Show the direction of flow ofcurrent.2 Draw a schem...

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    Companion SiteA box like this tells you that the Companion Site forElectronics À Circuits and Systems has more about thistopic.The URL of the site is:http://www.elsevierdirect.com/companions/9780080966342.The site has questions on electrical circuits and Ohm’sLaw. It includes Calculator window...

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    Topic 2Transistor SwitchesTransistors are used in one of two ways:Gas switchesGas amplifiers.Transistor amplifiers are described later in thebook. Transistor switches are described in this topic,using three different types of transistor.As we shall explain, the purpose of most transistorswitches ...

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    In this way, the MOSFET acts as a voltage-controlled switch. The voltage at which the transistorswitches on is called the threshold voltage.A MOSFET of the type just described is known asan n-channel enhancement MOSFET.There areother types (pp. 16 and 69), but this type is by far themost commonly...

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    Because the gate requires so little current, aMOSFET is the ideal type of transistor for use in thiscircuit. If we were to use a BJT, we might find thatthe sensor network was unable to provide enough cur-rent to turn it on.Self TestA BUZ73L MOSFET has an ‘on’ resistance of 0.4 Ω and israted ...

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    GInput is accepted by a light-dependent resistor (orLDR). This is made from a semiconductor materialsuch as cadmium sulphide. The resistance of this sub-stance varies according the amount of light falling onit. The resistance of an LDR ranges between 100Ω inbright light to about 1 MΩ in darknes...

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    Now we can calculate what voltage from the R1/R2potential divider is needed to switch on Q1 and sound thesiren. We need a minimum of 1.6 mA flowing throughR3. Given that R3 is 82Ω, the voltage across R3 must be:current3 resistance5 1:6 3 10233 825 1:31 VIf the voltage at the R1/R2 connection is ...

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    THERMISTORThermistors are available in several resistance ratings,the one chosen for this circuit having a resistance of1kΩ at 25C. VR1 is adjusted so that the voltage atpoint A is close to 0.6 V when the temperature is closeto that at which the heater is to be turned on. The tran-sistor is just...

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    The LED will burn out if the voltage across it ismuch more than 2 V. If the LED is being powered bya supply greater than 2 V, we need a resistor (R3) inseries with it. This drops the excess voltage.In the diagram above, the supply is 6 V, so theexcess voltage is 4 V. If we decide to have a curren...

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    This illustrates a point about electronic circuits, thatthere are often two or more ways of doing the samething. Sometimes both ways are equally effective andconvenient. In other instances we may have reason toprefer one way to the other.In practice, npn transistors are used far more oftenthan pn...

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    HYSTERESISIn this circuit the temperature at which the relay isturned on or off depends on whether the temperatureis rising or falling.The action of the circuit is such that the ‘turn-on’temperature is lower than the ‘turn-off’ temperature.We call this action hysteresis.The circuit works ...

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    attached to a sliding bolt, so that energising the coilcauses a door to be bolted. In this way the door maybe bolted by computer control.As with the relay circuit on p. 15, a solenoidswitching circuit requires a diode to protect theswitching transistor.EXTENSION BOX 4 Current GainThe gain of a BJ...

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    saturated. It is equal to the current passing through the load.The next column lists the maximum power rating, Ptot.The current gain specified in the table is the small signalcurrent gain, hfe, mentioned on p. 20. The value of hFE isalmost the same as hfe, so these values can be used in tran-sist...

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    BJTs or MOSFETs?When choosing between BJTs and MOSFETs, keep thesepoints in mind:BJTs need appreciable current to drive them;MOSFETs need none.For switching: MOSFETs have very low ‘on’ resistanceand very high ‘off’ resistance.For switching: MOSFETs are faster (but BJTs are usu-ally fast e...

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    20 mA through an LED. The power supply is6 V. Select a suitable MOSFET and design thecircuit.c The circuit is a morning ‘wake-up’ alarm. A pho-todiode is the sensor and the circuit sounds anelectronic buzzer when the light intensity risesabove a given level. The buzzer takes 15 mA at6 V. Sele...

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    Topic 3Potential DividersA potential divider is a resistor network that producesa fixed or variable potential (voltage). This potentialis lower than the potential of the supply. The poten-tial divider is probably the most often used circuitmodule. There are many instances of potential divi-ders b...

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    THE EFFECT OF THE LOADThe foregoing calculation assumes that no currentflows out of the output terminal. This isalmost truewhen we use a digital meter to measurevOUT.Wefind that the measured value ofvOUT is almost cor-rect, allowing for possible tolerance errors in theresistor values.However, cur...

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    The current flowing through the divider must be atleast ten times that amount, which is 16.7 mA. For acurrent of this size,R1 andR2 in series must total nomore than R5 vIN/0.01675 539Ω.Wehavetospec-ify two resistors that have the same ratio as in theexample, but total no more than 539Ω.Self Tes...

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    resistance of many megohms and takes virtually nocurrent from the network.VARIABLE POTENTIAL DIVIDERSThe diagrams show three ways of building a potentialdivider with variable output voltage:When calculatingvOUT, we think of the variableresistor as two resistors joined at the point of contactof th...

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    In the case of the circuit above this becomes:terminal pd 5 emf 3 Rext= Rint 1 RextðÞIf Rint issmallcomparedwithRext,asisthecasewith batteries and other power supplies, the terminalpotential is only slightly less than the emf. The volt-age drop can be ignored.If the source has high internal res...

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    Topic 4CapacitorsA capacitor consists of two metal plates parallel witheach other and very close together. There is an insu-lating layer between them, called the dielectric.Inmany types of capacitor this is a thin sheet of plastic,but some capacitors have a dielectric of air or otherinsulating ma...

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    Therefore, for several capacitancesC1,C2,.. . Cnin parallel, the combined capacitanceC is given by:C 5 C1 1 C2 1 ... 1 CnThe combined parallel capacitance is equal to thesum of the individual capacitances.ExampleWhat is the combined capacitance of the three capacitorsin the diagram above?Summing ...

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    Their tolerance is 5% or 10%. Tempcos are about100 ppm/C, and are fairly constant over the operatingrange of 55C to 100C. This makes them preferred topolyester capacitors for filtering and timing circuits.Polypropylene: These cover the range 100 pF to10μF but their main feature is their high wor...

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    Electrolytic capacitors are made to cover the range1μF to 10 000μF, and special types are available out-side this range.Electrolytics have a wide tolerance range, usually620% or more. They also show appreciable changeof capacitance with age and usage. If a capacitor isnot used for some months i...

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    MIC1 is a crystal microphone. When there is no sound,there is no pd across the crystal and both terminals ofMIC1 are at 0 V. Plate A of capacitor C1 is at 0 V. WhenMIC1 receives a sound, an alternating pd of a few milli-volts amplitude is generated. The voltage of A rises (goespositive) and falls...

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    amplifiers. The capacitor absorbs fluctuations in thesupply voltage and so prevents these from getting backto the sensitive part of the circuit, the pre-amplifier.A similar effect is found in logical circuits. Whendevices such as LEDs, lamps and motors, are switchedon or off, there is a sudden ch...

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    Finally, note that the graph is plotted for 5 timeconstants after the voltage step. It is seen that thecurve levels outalmost completely during this time.In theory, the capacitor never reaches full charge but,for all practical purposes, we can say that it takes 5time constants to fully charge the...

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    EXTENSION BOX 6 (continued)The capacitor is charged to 0.63 of the supply after 1time constant.3 After 5 time constants:t 5 5 RCvC 5 VSð12e25ÞBute255 0:0067vC 5 VSð120:0067Þvc 5 0:993 VSvc % VSThe capacitor is almost fully charged after 5 time constants.EXTENSION BOX 7 DischargingThe exponent...

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    15 In Question 13, how long does it take for the dis-charged capacitor to almost fully charge? How longdoes it take to charge to half the supply voltage?(Proofs not required.)16 Given that the capacitor in Question 13 is chargedfrom a constant 9 V DC source, what is the voltageacross the capacito...

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    8 When a capacitor is used to transfer a signal betweentwo parts of a circuit that are operating at differentvoltages, the capacitor is said to be:A coupling the circuits.B filtering the signal.C decoupling the circuits.D amplifying the signal.There are more calculations on capacitance and imped-...

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    Topic 5Using CapacitorsTopic 4 demonstrates that capacitors introduce timeinto the action of a circuit. The delay circuit belowuses a capacitor in this way.DELAYThe circuit below has a potential divider R2/R3which has an output voltage 4.8 V. The operationalamplifier (see Topic 11) compares this ...

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    The charging timet depends on the values of R1and C1 according to the equation:t 5 1:1R1C1R1C1 is the time constant, and the factor 1.1 isobtained by a calculation similar to those set out onpp. 36À37. Note thatt does not depend on supplyvoltage. This is one of the main assets of the 555,making ...

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    permanently wired to the positive supply (high) sothat resetting is not possible.The action of the monostable is as follows:1) In the quiescent state, the timing capacitor (C1) ischarged to one-third of the supply voltage. The out-put is low (0 V). Current flowing through R1 afterC1 is chargedisp...

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    source. The circuits we have looked at have manyuseful timing applications. Now we will look at acapacitor being charged from analternating voltagesource.The alternating voltage is a sinusoid —whatisoftencalleda sine wave. A sinusoid is a signal that,when plotted to show how the voltage or curr...

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    If we feed in the same mixed signal as before, theoutput signal looks like this:The 8 kHz signal is reduced in amplitude but the1 kHz signal passes through unchanged. The circuit isa lowpass filter.RESISTANCE AND REACTANCER is the symbol for resistance. This is a property ofany material that cond...

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    At low frequencies,ZC is high. Most ofvIN isdropped across thecapacitor. Only a small voltage isdropped across the resistor, so the amplitude ofvOUTis small.FREQUENCY RESPONSEThe way in which the amplitude of the output of a fil-ter varies with the frequency of the signal is calledthe frequency r...

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    Low-frequency signals pass through the filterwithout loss, up to about 3 kHz. Above this fre-quency, amplitude falls off to reach about248 dB at1 MHz.Reading from the graph, the cut-off frequency isabout 4 kHz. This is confirmed by using the formula:fC 5 1=ð2π 3 43 10Þ 1= kW3 nFðÞÂÃ5 4kHzD...

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    it does not become completely discharged until theend of that half-cycle. As a result of these delays, thepeaks and troughs ofVOUT coincide with theends ofthe half-cycles ofVIN. Putting it another way,VOUTlags about a quarter of a cycle behindVIN.CYCLES AND PHASEIt is conventional to measure diff...

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    Current is at its positive maximum (A) when itbegins to charge the capacitor (B). Current falls asthecapacitorgraduallyaccumulatescharge.Eventually, current is zero (C) when the capacitor isfully charged (D). Then the current reverses and thecapacitor begins to discharge. Current reaches its neg-...

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    Topic 6FieldsWhen one object interacts with another object withwhich it is not in physical contact, it is because of afield. Think of a field as something that produces aforce at a distance. If one or both objects are free tomove, they will do so as a result of the force of thefield.Gravitation i...

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    For example, if a current of electrons is movingalong in awireand thewireisinamagneticfield,each electron experiences a force. The resulting forceon all the electrons appears as a force acting on thewire. Practical applications of this effect include elec-tric motors and loudspeakers.When a force...

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    cell, for example, produces emf as a result of chemi-cal action, with no physical forces involved.Magnetic reluctance: The lines of forces of amagnet are closed loops. Each loop is a magnetic cir-cuit, equivalent to an electric circuit.On the WebThere are several unexplained terms and devices men...

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    out and further increase of H does not increase B.The material is saturated.If H is gradually reduced, B falls too. But when His reduced to zero, B is still greater than zero. Thematerial has become magnetised.Some of the magnetic field has remained (that is,some of the domains are permanently al...

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    Topic 7InductorsWhen a current is passed through a coil of wire, amagnetic field is generated. This is the principle onwhich solenoids, electric motors and loudspeakerswork.The converse happens too. When there is a changein the magnetic field through a coil, an emf is gener-ated in the coil. This...

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    self induction produces a very large current whichmay damage other components in the circuit (seep. 16).GThe direction of the current is such as to oppose thechanging current. If the current through the coil isincreasing, the induced current acts to prevent theincrease. Conversely, if the current...

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    The inductor may be a few turns of wire woundon a plastic former, as in Figure 7.4. A tuned induc-tor may have more than one winding, and is equiva-lent to a transformer that operates best at oneparticular frequency.One or more coils are wound on a plastic former.The iron-dust core is threaded, a...

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    Once the network is resonating (oscillating), itcontinues for a while even when the signal source isremoved. But energy is lost in heating the conductorsand also in the dielectric of the capacitor and thearmature of the inductor. Eventually the oscillationsdie out. The network can be kept oscilla...

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    Type RC filters RL filters Response Lowpass Highpass Lowpass Highpass Frequency Low High Low High Low High Low High Circuit Cut-off frequency 1/2πRC VOUTphase 0 lag lead 0 0 lag lead 0 IOUTphase lead 0 lead 0 0 lag 0 lag R/2πL FIGURE 7.9 The lowest two rows indicate phase with respect to ...

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    EXTENSION BOX 11 Transformer RulesFrequency: The frequency of the induced AC equals thatof the inducing AC.Amplitude: If VPis the amplitude of the voltage in theprimary coil, and VS is the amplitude of the voltage inthe secondary coil, then:VSVP5secondary turnsprimary turnsExampleA transformer ha...

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    QUESTIONS ON FIELDS1 What is the cause of a magnetic field?2 Explain the left-hand rule and describe a practicalapplication of it.3 A and B are two points situated in an electric field.The distance from A to B is 2.6 m. The pd betwen Aand B is 150 V. Calculate the electric field strength.4 What i...

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    Topic 8MOSFET AmplifiersThe common-source MOSFET amplifier with whichthis topic begins is a typical transistor amplifier.COMMON-SOURCE AMPLIFIERThe amplifier is built around an n-channel enhance-ment mode MOSFET. Given a fixed supply voltageVDD,the size of the currentiD flowing through thetransis...

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    ExampleFrom the graph, find gm when vGS is 3.5 V.By measurement on the graph (or by reading scalevalues on the simulator) as vGS rises by 0.5 V, from3.25 V to 3.75 V, iD rises by 70 mA.Applying the formula for gm:gm 5 70=0:5mA=VÂÃ5 140 mSThetypicaltransconductancequotedfortheZVN3306A in data sh...

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    direction without clipping or distortion. The value ofR3 is chosen to obtain this.ExampleGiven that the gate is held at 4 V, the vGS2iD curveshows that iD is then equal to 210 mA. We must arrangethat this current flowing through R3 causes a voltagedrop of VDD/2. The voltage drop required is 7.5 V...

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    amplifier. Putting it another way, the output is 180out of phase with the input.The expected voltage gain is20.54/0.1525.4.This compares well with the gain obtained from thetest measurements plotted on p. 65.FREQUENCY RESPONSEThe frequency response of an amplifier is obtainedby inputting sinusoid...

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    variable power pack forvGS. The graph on p. 63shows a suitable range of values to try.Plot a graph of your results and, from this, calculatetwo or three values ofgm at different values ofvGSCOMMON-DRAIN AMPLIFIERThe MOSFET amplifier in Figure 8.6 is known as acommon-drain amplifier because the dr...

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    reduced signal can eventually appear from theamplifier.In (b) a follower amplifier is connected as a bufferbetween the microphone and the audio amplifier.The follower amplifier has high input resistanceRIand low output resistanceRO. BecauseRI is muchgreater thanROUT,most ofvS appears acrossRI and...

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    QUESTIONS ON MOSFET AMPLIFIERS1 Explain what is meant by transconductance. How isthe output current of a MOSFET converted to anoutput voltage in a common-source amplifier?2 Why is it important to keep signal amplitudes smallin MOSFET amplifiers?3 Why is it usually preferable to use capacitor cou-...

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    2 A follower amplifier is often used as a bufferbetween:A a high resistance output and a low resistance inputB a low resistance output and a high resistance inputC a high resistance output and a high resistance inputD a low resistance output and a low resistance input.3 A follower amplifier has a...

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    Topic 9BJT AmplifiersThere are three basic ways in which a bipolar junc-tion transistor may be used as an amplifier: common-emitter amplifier, common-collector amplifier andcommon-base amplifier.These each have very different attributes andapplications. We look at the circuits of the first twoof ...

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    The gain for the BC548 is often listed as 400 (butsee below). Note that the graph is almost a perfectstraight line.Self TestWhat is the hfe of a BJT if iC increases by 2.06 mA wheniB is increased by 5 μA?Because the curve is almost a straight line we canmeasure gain in a slightly simpler way. Re...

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    isnot insulated from the body of the transistor.CurrentiB flows into the base, through the emitterlayer and out by the emitter terminal. Along this pathit encounters resistance, typically about 25Ω.Thisisthe emitter resistancere, which can be thought of as aresistor inside the transistor.In the ...

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    In summary, a change which increasesiC is coun-teredbyareductioniniC. The result is thatiC tendsto remain unaffected by variations inhfe. The gain ofthe amplifier is stable.Further improvement in stability is obtained bybiasing the base with two resistors, instead of onlyone:R1 and R2 act as a po...

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    cut-off point to be at 20 Hz, as calculated. In thehigher frequencies, the cut-off point is approximately11.7 MHz. This bandwidth of 11.7 MHz is satisfac-tory for many applications.An upper cut-off point of 11.7 MHz may be essen-tial for an amplifier in a radio receiver, but it isunnecessary for ...

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    VOLTAGE GAINWe calculate the voltage gain of the amplifier by con-sidering what happens when the input voltagevINchanges by a small amountvin. The change invIN iscarried across C1 to the base of Q1.The change in base voltage is given by:vb 5 vinAs the base voltage changes, the drop (vBE) acrossth...

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    Darlingtons may also be used as switching transis-tors, as in Topic 2. They require only a very smallcurrent to trigger them.The circuit below is a practical example of aDarlington pair being used as a switching transistor.Two metal plates about 1 cm3 2 cm are mountedside by side with a 0.5 mm ga...

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    the result shown in Figure 9.13. The amplitude of theoutput signal is only half that of the input signal andit is inverted:vOUT 5 vIN 3 common-mode gainThe common-mode gain is20.5.The differential amplifier has high differential-mode gain but very low common-mode gain. It issuited to measuring vo...

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    in the fifth column. Calculate the average value ofthis for the 10 pairs.(b) Next measure the common-mode voltage gain.Connect the two inputs together and connect a sin-gle voltage source to them. Draw a table with threecolumns and in the first two columns recordvINandvOUT as you obtain 10 sets o...

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    QUALITY FACTORThe output of the circuit varies mainly with the proper-ties of the inductor, including the resistance of its coil.The quality factor of the inductor is given byQ 5 2πfL/r,where r is the resistance of the coil. If theinductor has a highQ (say, 400), the amplifier outputis high but ...

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    QUESTIONS ON BJT AMPLIFIERS1 Explain how a small increase in the voltage input ofa common-emitter amplifier produces a much largerincrease in the output voltage.2 What is the difficulty with using a single resistor tobias a common-emitter amplifier?3 Explain what is meant by the small signal curr...

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    6 Describe the action and applications of a common-collector amplifier.7 Compare the common-emitter BJT amplifier withthe common-source MOSFET amplifier.8 Explain what is meant by negative feedback. Quotetwo examples of negative feedback in amplifiers.9 A transistor has anhfe of 110. By how much ...

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    Topic 10JFET AmplifiersJunction field effect transistors have been replaced byMOSFETs as the most commonly used field effecttransistors. If you are working for an examination,check the subject specification to find out whether ornot JFETs are included in it.Like MOSFETs, JFETS are unipolar transi...

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    ExampleIf vIN increases by an amount vin5100 mV, the corre-sponding increase in iD is:id 5 gm 3 vin 5 0:0004 3 0:1 5 40 μAIf the current through R2 increases by 40 μA, the volt-age across R2 increases by:vout 5 id 3 R2 5 0:0004 3 15 000 5 0:6VThe voltage at the drain falls by 0.6 V.Voltage gain...

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    QUESTIONS ON JFETs1 What are the main similarities and differences of thedesign and performance of MOSFET and JFET com-mon-source amplifiers?2 Why is it not possible to operate a JFET amplifierwhen the quiescent gate voltage is more than about0.5 V?3 Describe how to measure the transconductance o...

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    C lower thanÀ5VD less than 0.5 V.2 Compared with its MOSFET equivalent, the band-width of a JFET common-source amplifier is:A lessB much widerC about the sameD a little wider.3 The output resistance of a common-source amplifierdepends on:A the transconductance of the FETB the value of the output...

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    Topic 11Operational AmplifiersOperational amplifiers are precision, high-gain, differ-ential amplifiers. They were originally designed toperform mathematicaloperations in computers, butnowadays this function has been taken over by digitalmicroprocessors. Operational amplifiers (or ‘op amps’as...

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    IDEAL OP AMPAn ideal op amp has the following features:GInfinite voltage gain.GGain is independent of frequency.GInfinitely high input resistance.GZero output resistance.GZero input voltage offset.GOutput can swing positive or negative to the samevoltages as the supply rails.GOutput swings instan...

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    ExampleIf the open-loop gain is 20 at 100 kHz, it is 10 at 200 kHz,5 at 400 kHz, and so on to a gain of 1 at 2 MHz. In everycase:Gain-bandwidth product 5 gain 3 frequency 5 2MHzThe frequency at which gain becomes 1 is knownas the transition frequency, and is numerically equalto the gain-bandwidth...

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    ExampleThe TL081C has a typical input offset voltage of 5 mV.Inputs must differ by at least 5 mV to obtain a reliableoutput swing. Putting it the other way round, this op ampis unsuitable for use as a comparator if the voltages thatwe want to compare are likely to differ by less than5 mV. If diff...

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    voltage above or below zero. This allows us to statearule:An inverting amplifier comes to a stable state in which thetwo inputs are at equal voltage.GAIN SET BY RESISTORSThe diagram below shows typical resistor values. Thevoltages are shown for whenvINis a constant100 mV and the circuit has reach...

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    result, the input resistance of a high-gain invertingamplifier is nearly always a few tens of kilohms.For higher input resistance we use an additional opamp as a voltage follower, as explained later in thistopic.In the diagram on p. 91, we show zero voltage atboth ends of RB. Ideally no current f...

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    op amps (see data on p. 95). Good ones to try arethe popular 741 and the more recent OP-177GP, whichhas an offset so small that you may not be able tomeasure it.ACTIVITY — FREQUENCY RESPONSESet up an inverting amplifier circuit (p. 90) with aclosed loop gain of 100. Connect its input to a signa...

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    and RA is omitted. The fullvOUT is fed back to theinverting input. As before, the circuit settles so thatthe inputs are at equal voltage. The non-inverting inputis atvIN, so the inverting input must be atvIN too.But the inverting input is atvOUT, being connecteddirectly to the output. As a result...

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    versions save board space and simplify the pcb lay-out. Op amps marked † are suitable for running froma single supply voltage.SELECTING OP AMPSWhen selecting an op amp, input offset voltage, slewrate, and output swing are the usually the most impor-tant factors to consider. The open loop gain i...

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    C 180 kΩD 2MΩ6 The input resistor of a non-inverting op amp is 39 kΩand the feedback resistor is 1.2 MΩ. The output volt-age is20.5 V. The input voltage is:A 215.7 mVB 231.8 mVC 16.3 mVD 216.3 mV.7 An op amp has an open-loop gain of 10 at 250 kHz.Its gain-bandwidth product is:A 250 kHzB 25 kH...

  • Page 108

    Topic 12Applications of Op AmpsOperational amplifiers are such a useful and generallyinexpensive building block that they appear in a widerange of applications. These include adders, differ-ence amplifiers, integrators, ramp generators, trigger-ing circuits and active filters.ADDERThe amplifier s...

  • Page 109

    voltage drop ofv2 along the chain and, because R3and R4 are equal, the voltage at the (1) input isv2/2.The voltage at the (2) input isv2/2. For the samereasons as for the other amplifiers, the circuit comesto a stable state when the voltages at the (1)and (2)inputs are equal.Consider the resistor...

  • Page 110

    probes to the amplifier. They are common-mode sig-nals, and are largely rejected, depending on the com-mon mode rejection ratio (CMRR) of the amplifier.Most op amps have a high CMRR. Of the typeslisted on p. 97, most have a CMRR in the region of90À100 dB.Thismeansthatthedifferential-mode gain is...

  • Page 111

    fixed portion of R2. We now read off the resistanceof the variable portion of R2. The resistance of thedummy at the ambient temperature is taken from datatables. From these values we calculate the resistanceof the strain gauge. Such equipment could be used formeasuring forces, including weighing ...

  • Page 112

    divided into many very short time intervals of lengthδt. We can consider that the voltage is constant dur-ing each of these, but changes to a new value at thestart of the next interval.During each interval, charge accumulates accord-ing to the constant-voltage equation. The total charge,and also...

  • Page 113

    the characteristic ‘snap’ action of the Schmitt trigger.Note the inverting action.The other result is that, once the swing has begun,any slight decrease invIN canhavenoeffect.Oncebegun, the change in state can not easily be reversed.Once the output has fallen to 0 V, the potentialdivider R3-R...

  • Page 114

    The same formulae apply whenVM isnot half ofthe supply. For example, suppose that we reduce R1to 1.3 kΩ,which raisesVM to approximately 7.5 V.The thresholds become:VU 5 7:5 1 ð12 2 7:5Þ 3 200=600 5 9VVL5 7:5ð1 2 200=600Þ 5 5VNote that the thresholds are no longer symmetricalaboutVM, but the ...

  • Page 115

    As a check on the calculations, the graph inFigure 12.11 was plotted for a simulated trigger cir-cuit based on these results.The method above can be used for any op ampthat swings its output fully to the supply rails. Ityields approximate results for op amps which do notswing to the rails, provid...

  • Page 116

    the equations by puttingR 5 R4/(R31R4). The resultis shown below and we can see that:v1 5 vIN 2 vIN 3 RCollecting terms gives:v1 5 vINð1 2 RÞvIN rises untilv1 equalsv2, the voltage at the (2)input of the op amp. At that point:vIN 5v212 RThis is the upper threshold voltage, at whichvOUTswings fr...

  • Page 117

    ExampleDesign a non-inverting Schmitt trigger operating on a15 V supply, with LVT 5 6V and UVT 5 11 V.Taking the calculation from step 2, onward:2 R 5 (1126)/(1126115) 5 0.253 v1 5 11 (120.25) 5 8.254 Let R2 5 200 kΩ, then, working in kilohms:R1 5 200ð15 2 8:25Þ=8:25 5 164 kΩ5 Let R4 5 200 k...

  • Page 118

    RAMP GENERATORInstead of taking the output of this circuit fromthe op amp output (vOUT), we can take it from thepoint markedvIN.Thisisplottedingreyonthegraph opposite. The voltage ramps up and down,producing a sawtooth wave. However, the segmentsof the waveform are really segments of an expo-nent...

  • Page 119

    The time taken to discharge from UTV to LTV isthe difference between these two times, so:Discharging time5 ð1:100 2 0:405ÞRC 5 0:695RCCharging and discharging times are equal because, inthis trigger, UTV and LTV are symmetrical aboutVS/2.This does not necessarily apply to other trigger circuits...

  • Page 120

    17 Calculate the time constant of the circuit of Q. 16.How long is its period, in time constants? What isits period in seconds and its frequency in hertz?18 Draw a circuit diagram to show how to use this cir-cuit as a ramp generator.MULTIPLE CHOICE QUESTIONS1 An op amp used in an integrator circu...

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  • Page 122

    Topic 13Active FiltersActive filters are another important application ofoperational amplifiers.When a signal of mixed frequencies is passedthrough a filter circuit, some of the frequencies arereduced in amplitude. Some may be removed alto-gether. The simplest filter consists of a resistor and ac...

  • Page 123

    the filtered signal has an amplitude of only 0.7 V.This is not the only change. The output reaches itspeaks 125μs behind the input. This is a delay of 1/8of the period of the signal. In terms of angle, this is360/85 45. Because it is a delay, it has negativevalue. The output isout of phase with,...

  • Page 124

    Self TestA first-order lowpass filter has a 47 kΩ resistor and a220 pF capacitor. What is its cut-off frequency?A phase lag of245 at the cut-off point is anothercharacteristic of single-stage passive or active filters.The phase lag is less at lower frequencies. At 0 Hz(DC) the phase change is ze...

  • Page 125

    SECOND-ORDER ACTIVE FILTERAdding an extra stage to the first-order lowpass filterallows feedback to be introduced. The circuit belowhas two RC filters, one of which is connected to 0 V,as in the first-order lowpass filter. The other is part ofa feedback loop from the filter output.The feedback ac...

  • Page 126

    GR1 and C1 form a highpass filter; high frequencysignals passing through this are fed back negatively,partly cancelling out the high frequencies in the orig-inal signal.GR2 and C2 form a lowpass filter; low frequency sig-nals passing through this are fed back negatively,partly cancelling out the ...

  • Page 127

    At the resonant frequency, the capacitor andinductor have equal impedance. Their impedance inparallel is a maximum. This reduces the amplitude ofsignals of that frequency, producing the narrow notchin the frequency response.To calculate the notch frequency we use the equa-tion on p. 57. The depth...

  • Page 128

    2 An op amp used in a highpass filter should have:A a high gain-bandwidth productB lowslewrateC low input bias currentD high output resistance.3 At the cut-off point of a lowpass filter the phasechange is:A 190B 0C 245D 290.4 The cut-off frequency of a first-order active highpass filter is 5 kHz....

  • Page 129

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  • Page 130

    Topic 14OscillatorsAn oscillator produces an alternating output voltageof fixed or variable frequency.The waveform of many types of oscillator is a sinu-soid (p. 44) but other waveforms may be generatedsuch as triangular, sawtooth, and square waves. Circuitsthat produce square waves, often called...

  • Page 131

    COLPITTS OSCILLATORThis type of oscillator depends on a resonantnetwork consisting of two capacitors (total seriescapacitance5 C) and an inductor (L) in parallel withthem. As shown on p. 79, this L-C network resonatesat a frequency, f5 1/2πO(LC).The op amp is wired as an inverting amplifierwith ...

  • Page 132

    permanent signal is produced at the output terminal.The amplifier then has the required gain, close to 3.Things to doTry breadboarding these oscillators. View their output sig-nals with an oscilloscope, or measure them using a test-meter with a frequency measuring facility.On the WebThere are tho...

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  • Page 134

    Topic 15Power AmplifiersA power amplifier is used to produce a major effecton the surroundings. Examples are power audioamplifiers, producing sound at high volume, or motorcontrol circuits actuating the arm of an industrialrobot or aligning the dish of a radio telescope. Otherexamples include the...

  • Page 135

    CLASS A AMPLIFIERSThe main disadvantage of Class A amplifiers is thatcurrent is flowing through the output transistor andits resistor even when there is no signal. Power isbeing used but no sound or other form of outputactivityoccurs.Suchamplifiersareinefficientbecause they waste 50% of the energ...

  • Page 136

    fact that the voltage gain of each of the followeramplifiers is less than 1. In this case it is approxi-mately 0.8.ELIMINATING CROSSOVER DISTORTIONA common technique for eliminating crossover dis-tortion is to bias the transistors so that they are juston the point of conducting. With BJTs, this m...

  • Page 137

    though more in some than in others. Getting rid ofheat is an important aspect of practical circuit con-struction, especially in high-power circuits. If thetemperature rises only a few tens of degrees, it altersthe characteristics of a device and the performance ofthe circuit suffers. For example,...

  • Page 138

    It is of course essential that the circuit enclosureshould have adequate ventilation. This may be aidedby a fan if the circuit dissipates large amounts ofpower, for example, a microcomputer circuit.THERMAL RESISTANCEThermal resistance (Rθ) is the ability of something toresist the transfer of hea...

  • Page 139

    able to pass through the holes, as shown in the sec-tional diagram in Figure 15.8. A washer helps by dis-tributing the pressure more evenly.With certain types of transistor or other device thetag is in electrical contact with one of the terminalsof the device. An example is the 2N3055 transistor ...

  • Page 140

    The circuit has many of the features of an invert-ing op amp circuit. There are two inputs, (1)and(2), with feedback from the output to the (2) input.The input stage of the IC is a differential amplifier.Some of the external resistors and capacitors make upfilters. Others are there to hold the su...

  • Page 141

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  • Page 142

    Topic 16Thyristors and TriacsA thyristor is also known as silicon controlledswitch (SCS) and as a silicon controlled rectifier(SCR). The wordswitch tells us that it can be turnedon and off. The wordrectifier indicates that currentflows through it in only one direction.A thyristor consists of four...

  • Page 143

    Base-Emitter Voltage, VBEAn npn transistor begins to turn on when this exceedsabout 0.7 V.Drawing current from its base begins to turn thepnp transistor on. Current begins to flow through it.This current goes to the base of the npn transistor,adding to the current already flowing there and turn-i...

  • Page 144

    triggers the thyristor. Once triggered, the thyristorconducts for an unlimited period, even if lightincreases again. The circuit is sensitive to very shortinterruptions in the light, such as might be caused bythe shadow of a passing intruder falling on the LDR.This kind of switching action is mai...

  • Page 145

    pulses. It then takes longer for the pulse to reach avalue that will trigger the thyristor. Firing is delayeduntil later in the cycle. The plot below shows whathappens if R2 is reduced to 560Ω. The delay at thestart of the cycle is now 2.2 ms. This gives a firingangle of 3603 2.2/205 40. The con...

  • Page 146

    complete its path through the circuit. The thyristorturns the current on and off during both positive andnegative half-cycles, as shown in Figure 16.8.TRIACSOne of the chief limitations of thyristors is that theypass current in only one direction. At the most, theycan operate only at half-power.I...

  • Page 147

    connected to wires at mains voltage and there is dan-ger in this. In commercially produced, manually oper-ated circuits the control knobs are sufficiently wellinsulated to prevent the operator from receiving anelectric shock, but when a circuit is automaticallyoperated, perhaps by a computer, we ...

  • Page 148

    high as 750 V/ms without causing false triggering. Asnubber network is not required with this device.DIACA triac can be triggered directly by applying a posi-tive or negative pulse to its gate, as in circuits suchas that in Figure 16.13. Triggering is made more reli-able by including a diac in th...

  • Page 149

    As an example, we could supply power to a motorfor five half-cycles, then cut off the power for the samenumber of half-cycles. This supplies power for half thetime. Power is interrupted for five half-cycles (for 0.1 son a 50 Hz mains supply) but the inertia of the motorand the mechanism it is dri...

  • Page 150

    fired at the beginning of every half-cycle and fullpower is applied to the load.ACTIVITY — THYRISTOR ACTIONAssemble the circuit shown below. Apply the 12 VDC supply from a bench PSU.Record and explain what happens when:Gpower is first applied.GS1 is pressed and held.GS1 is released.GS2 is press...

  • Page 151

    9 Explain the difference betweenanoptoisolatorandan opto coupler.10 Explain the possible causes of false triggering in a triacpower circuit and what can be done to prevent it.11 Explain the differences between phase control andintegral cycle control, and their advantages anddisadvantages.12 Descr...

  • Page 152

    Topic 17Power SuppliesElectronic circuits normally operate at low voltagesand on direct current (DC). A typical circuit operateson 6 V or 9 V DC, but some can work on as little as1 V. Often this low voltage supply is provided by abattery. A battery is ideal for portable equipment suchas a mobile ...

  • Page 153

    RECTIFICATIONThe output from the transformer is a sinusoid. Thediode is able to conduct only during the positive half-cycle of the sinusoid.The waveform consists of a series of positive pulsesoccuring 50 times per second. This waveform is knownas pulsed DC. Because of the forward voltage dropacro...

  • Page 154

    FULL-WAVE RECTIFIERThe four diodes D1À D4 are connected to form arectifyingbridge. ThisreceivesACfromthetransformer.During the positive half-cycle, terminal A of T1 ispositive of terminal B. Current flows from A, throughD1, and through R1 to the load. It passes through theload and returns throug...

  • Page 155

    The depth of the ripple depends on the capacity ofC1 and the amount of current drawn by the load. Thegreater the current, the deeper the ripple.Ripple may be minimised by using a large-valuecapacitor (1000μFormore) forC1, sothat the pdacross it falls more slowly.CALCULATING RIPPLEIt can be seen ...

  • Page 156

    REGULATIONThe output voltage of this PSU is held steady undervarying load by using a regulator circuit. This isbasedona Zener diode. The Zener diode needsabout 5 mA or more passing through it to maintain itsZener action. This is the reason for the resistor R1.The value of this is chosen to allow ...

  • Page 157

    required. Further, when the maximum current is notbeing drawn by the load, a large current is passedthrough the Zener, which is wasteful.Self TestA Zener-regulated mains PSU using a full-wave rectifier isto provide a current of up to 400 mA at 7.5 V. The trans-fomer is rated to produce an output ...

  • Page 158

    without undue dissipation of power within the regula-tor IC. The maximum allowable supply voltage is 30to 40 V.The 7805 is rated to produce a current of 1 A.Similar regulators are available for currents of100 mA and 500 mA. A heat sink may be neededwith the higher current ratings. The IC has a me...

  • Page 159

    circuits and comparing their advantages and disad-vantages.Useasignalgenerator(NOT THEMAINS) to provide a low-voltage AC input. A signalof 10 V AC, running at 50 Hz, is suitable. Design thecircuit to produce an output of 5 V DC.Monitor the waveforms present at various stages,including amplitude a...

  • Page 160

    Topic 18Logical CircuitsLogical circuits perform logical operations.Thereare three basic logical operations:GNOT, sometimes called INVERT.GANDGORLogical operations are performed by circuit unitscalled gates. Because a typical logical circuit needsseveral, or maybe many, gates they are made as int...

  • Page 161

    InputsOutputBAZ000010100111A ‘0’ in the input columns indicates that the but-ton is not pressed, so the input is low. A ‘1’ indicatesthat the button is pressed, so the input is high.A ‘0’ in the output column indicates that the gateoutput is low, and the LED is off (the gate is wireda...

  • Page 162

    The small circle at the output indicates that theoutput is inverted. The truth table of the gate issimple:InputOutputAZ0110In symbols, the operation of the gate is:Z 5 AIn Boolean algebra, a bar over a variable is usedto show that it is inverted.NAND AND NOR GATESThe three basic gates that we hav...

  • Page 163

    InputsZ5 A" BBA000011101110Note the symbol" for the exclusive-OR opera-tion. Except for the last line, the table is identical tothe table for OR. The symbol for an EX-OR gate is:The EX-OR gate is sometimes known as the ‘sameor different’ gate. Its output is ‘0’ when its inputs a...

  • Page 164

    The 74HCXX series is another popular branch ofthe TTL family. It comprises CMOS versions of the74XX devices. One of its main advantages is that,like CMOS logic (see next section), it operates on awide range of voltages (2 V to 6 V) and does notrequire a regulated supply. It is fast-acting and has...

  • Page 165

    CMOS has insulated input gates (using ‘gate’ in theother sense) like any other FET device, so it is not sur-prising to find that input currents are very low inCMOS and 74HC. The output currents are lower thanthose of 74LS, the figures being quoted for a 5 V sup-ply and (in brackets for CMOS) ...

  • Page 166

    logic families with a propagation delay of 1 ns pergate and a maximum clock rate of 500 MHz.As well as avoiding transistor saturation, the cir-cuits have low input impedances to avoid the speed-reducing effects of capacitance. Because of thesemeasures, power consumption is high, as much as30 mW p...

  • Page 167

    On the Web (continued)to make them suitable for hand-held equipment. Othersoperate at high speed, and some are specialised for pro-cessing digital video data.Search the Web for ‘Logic families’, and note the fea-tures of a few of the newest additions to the range. Forwhat applications are the...

  • Page 168

    Topic 19Logical OperationsThere are four basic logical operators, TRUE, NOT(or INVERT), AND and OR. The last three of thesewere described in Topic 18. The first operator, TRUEis illustrated by its truth table:InputAOutputZ0011The output equals the input. This is the symbol fora TRUE gate:There ma...

  • Page 169

    need to try all possible combinations of inputs andwork out the output for each combination. If there aretwo inputs there are four combinations, as listed inthe preceding table. If there are 3 inputs there are 8combinations. If there are 4 inputs there are 16 com-binations. With even more inputs ...

  • Page 170

    ExampleProve the first identity of de Morgan’s Theorem.The truth table lists all four input combinations and thevarious logical operations. The contents of the fourth andseventh columns are identical, so proving the theorem.Self TestUse the truth table technique to prove the RaceHazard Theorem ...

  • Page 171

    KARNAUGH MAPSA Karnaugh map has the advantage that it tells us ifit is possible to simplify the logic circuit and, if so,how to do it.ExampleUse a Karnaugh map to simplify the logic circuit inFigure 19.2.Taking the output column of the truth table showndirectly below Figure 19.2, we enter these r...

  • Page 172

    illuminated when Z51. This indicates that there areseven combinations of inputs in which a vacancyoccurs.At this stage the way to build the logic circuitmay be obvious, and there is no need for a Karnaughmap. However, the logic is not obvious in this exam-ple. so we proceed to the map (see right)...

  • Page 173

    The trace for output Z shows an unexpected effect.There is a glitch in the output, shown as a brief low pulseoccurring as input changes from 011 to 100. The reasonfor this is discussed in Topic 22. It is not a fault in thelogic of the circuit.WORKING WITH NAND AND NORUsually we are given a circui...

  • Page 174

    gates. There are some circuits, such as decoders, thatneed a lot of AND gates, which makes it worth whileto use ICs of this type. Some kinds of circuit need alot of NOT gates. But, on the whole, NAND andNOR gates are preferred.ExampleThe first step in converting the circuit of Figure 19.3 toNAND ...

  • Page 175

    When trying to solve any particular logical prob-lem, it is useful to adopt more than one of the fourpossible approaches. The results from one approachact as a check on those produced in another way. So,to confirm the all-NAND version of the exclusive-ORgate we will try to arrive at the same resu...

  • Page 176

    This pattern of outputs (all ‘1’ except for one ‘0’)is typical of a NAND gate (see table, p. 157).Therefore the circuit is equivalent to a 3-input NANDgate with input A inverted.Draw some circuits based on 3 or 4 logic gates,each with 2 or 3 inputs (exclusive-OR and exclusive-NOR gates ca...

  • Page 177

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  • Page 178

    Topic 20Logical CombinationsUsing the design techniques described in Topic 19, itis possible to build a wide range of logical functions.In this topic we look at a number of useful functionsthat you might need when designing electronic pro-jects. All of these depend on logical combinations,oftenre...

  • Page 179

    There are three groups of 1’s, giving:Z 5 A B 1 A C 1 B CThree 2-input AND gates and one 3-input ORgate, a total of 4 gates, can obtain the same logic.This is only half the number of gates used by theoriginal logic and these have fewer inputs.The all-NAND version is easy to find because thee...

  • Page 180

    Looking at the column forCo,thisisamajoritylogic function.Cois high whenever two or moreinputs are high. We use majority logic but base it onfour NAND gates. If the two exclusive-OR gates arebuilt from NAND gates, the whole circuit is in NANDgates. A gate is saved because the first exclusive-ORga...

  • Page 181

    The circuit does not allow for a zero (0) input.This is because, in any given application, a zero inputmay or may not have a meaning different from ‘noinput’. In either case the output should be 00. If therange of possible inputs starts at 1, a zero input cannot occur and there is no confusio...

  • Page 182

    If only input 1 is high, the NOT gate has a highoutput. Both inputs to theZ0 gate are high.Z0 ishigh. Outputs are 01, indicating priority 1.With all three inputs low, both outputs are low.Whether this is to be taken as a zero input or no inputdepends on the application. As before, we can addextra...

  • Page 183

    InputsOutputsDataSelectZ3Z2Z1Z0BA0XX00001000001101001011001001111000ARBITRARY TRUTH TABLESVery often a logic circuit has to perform a compli-cated function that is not simply related to one ormoreofthestandardlogicalormathematicalfunctions.ExampleConsider a logic circuit controlling the hot water...

  • Page 184

    To make things even more complicated, we couldalso decode for hexa decimal digits A to F when theinput is 1010 to 1111.HexadecimalA counting system based on 16 digits, 0 to 9 and A to F,the letters representing decimal values 10 to 15. Thiscovers all the 16 binary values 0000 to 1111.Designing a ...

  • Page 185

    Another advantage of this technique is that theoutput for any particular combination of inputs caneasily be altered, simply by connecting the corre-sponding input to the other supply rail. If we wereusing an array of gates, a change of logic is likely torequire gates of a different type, and re-r...

  • Page 186

    needs to be changed or updated, it is relatively sim-pler and quicker to re-write software than it is toredesign and reconstruct hardware.ACTIVITY — MSI DEVICESInvestigate some of the MSI devices that perform thecombinational functions described in this topic. Workon a breadboard or use a simul...

  • Page 187

    6 A data distributor:A has one data input.B has two outputs.C is often called a multiplexer.D has two address inputs.7 In the circuit drawn here, if inputs 1 and 2 are madehigh and 3 is low, the outputs are:A Z05 0andZ15 1.B Both are 1.C Z05 1and Z15 0.D Both are 0.1221 AND 23Z0Z1FIGURE 20.14176P...

  • Page 188

    Topic 21Logical SequencesIf a logic circuit has feedback in it, then its responseto a given input may not be the same the second timean input is applied. The circuit is in one state to beginwith and then goes into a different state, in which itsresponse may be different. It goes through a sequenc...

  • Page 189

    has a 11 input, and its output is about to change to 0.The flip-flop can not stay in this state; it is unstable.(c) The output of Gate A has gone low, so that theinput to Gate B is now 00. This makes no differenceto its output, which stays high. The circuit is now inthe Reset state, with Q5 0andQ...

  • Page 190

    The way the delay works is shown in Figure 21.4and in the timing graphs in Figure 21.5. The sequenceof stages is as follows:(a) The normal input to the circuit (dark grey line) islow (0 V). This gives Gate A a high output (15 V).The capacitor has charged to 15 V through the resis-tor. So the inpu...

  • Page 191

    This is the truth table for the operation of the74LS221 monostable IC:InputsOutputsCLRABQQ0XX01X1X01XX00110m1k1m01The CLEAR input is active low. When it is madelow, the values of A and B have no effect; output Qis low and output Q is high. The monostable can alsobe cleared by making A high or B l...

  • Page 192

    (b) When the voltage across the capacitor reachesabout 7.5 V, the input to Gate 1 (which is at almostthe same voltage) is taken to be at logic high.Instantly, the levels change to those shown in (b).The sudden rise in the output of Gate 2 pulls bothplates of the capacitor up by 15 V. This means t...

  • Page 193

    graphs following. For a clocked mode operation suchas this, the Set input (S) and Reset input (R)areheldlow. Some kinds of D-type flip-flop do not have the Rinput.The graphs below are obtained from a simulator,modelling a 74LS74 D-type flip-flop. The clock isrunning at 10 MHz and the time scale i...

  • Page 194

    The toggle flip-flop demonstrates one of theadvantages of clocked logic. After the flip-flop hasbeen clocked, there is plenty of time before the nextclocking for the circuit to settle into its stable state,and for its output to be fed back to the data input.This makes the operation of the circuit...

  • Page 195

    Make a toggle flip-flop from a D-type flip-flop ora J-K flip-flop, and investigate its action.QUESTIONS ON LOGICAL SEQUENCES1 Explain the working of a flip-flop built from twoNAND gates.2 Explain how to make a pulse generator based on twoNAND gates.3 Describe how to make an astable using two CMOS...

  • Page 196

    Topic 22Counters and RegistersIn Topic 21 we showed how a data-type (or D-type)flip-flop could be connected to make it into a toggle-type (or T-type) flip-flop. This is simply done byconnecting its Q output to its D input. As a T-typeflip-flop, its output changes state whenever its clock(or C) in...

  • Page 197

    NoteThere is a table in Supplement A that give equivalents indifferent number systems.Summing up, a chain ofn flip-flops can be usedeither to divide a frequency by 2, 4, 8,.. .,2n,orasacounter from 0 to (2n2 1).Frequencydividersareusedforgeneratingfrequency-related audio tones of different pitch ...

  • Page 198

    DigitsDecimalCBA1117110610’ 1510040’ 1’ 13010200’ 1100001’ 1’ 17The arrows show every time a digit changes from0 to 1, the digit to its left is changed. This produces arepeating downward count from 7 to 0.MSI counters are made in up-counting and down-counting form. There are also up-d...

  • Page 199

    a convenient way to provide inputs 000 to 111 to thelogic network. However, when the counter inputchanged from 011 to 100 (at 800 ns), it went brieflythrough a stage 010. As can be seen from the plots atabout 500 ns, this is an input which gives a low out-put from the network. Consequently, there...

  • Page 200

    The outputs of Counter 1 are decoded by a 2-inputAND gate. If the count registered on Counter 1 isbetween 0000 and 1000 (0 and 8), the output of thegate is low. This means that Counter 2 can not regis-ter any counts — it is disabled.When Counter 1 reaches 1001 (9), both inputs ofthe AND gate ar...

  • Page 201

    The decoding circuit is:This example illustrates the technique for generat-ing a repeated sequence of actions:GDecide how many stages thesequencehas.GDesign a counter with that number of stages, usingdecoding to reset it earlier if there are fewer than 10or 16 stages.GWrite the truth table.GDesig...

  • Page 202

    Data atSerial InputData in RegistersData at SerialOutputABCD0wxyzz00wxyy000wxx0000ww00000Empty registerThere are also registers that operate in parallel-in-parallel-out(PIPO)andparallel-in-serial-out(PISO) modes. Registers also vary in length from 4bits and 8 bits (the most common) up to 64 bits....

  • Page 203

    The wiring for all six AND gates is fairly compli-cated but, fortunately, a 10-stage walking ring com-plete with decoding is available as an IC, the 4017.The graph below shows its output sequence:This type of counter is useful for triggering asequence of events. As an illustration of the way inwh...

  • Page 204

    Decoding the output from the ring is more compli-cated than for a 1-of-n counter, and is likely to needmore gates. Decoding must also detect and correct fordisallowed states.STATIC RAMFor storing a large amount of data we use an array ofmany flip-flops on a single chip. A large array of thiskind ...

  • Page 205

    photo, the data can be erased and a new set of photo-graphs recorded.A memory stick is a small device generally with abuilt-in plug that fits into a socket on a CD or DVDplayer or on a computer. It may contain as much as1 GB of flash memory. It is a convenient way oftransporting data between comp...

  • Page 206

    Run the 4-stage MSI ripple counter at, say10 kHz, and then use an audio amplifier to ‘listen’ tothe output from each stage.Design and build a circuit to check the output of a 4-stage ripple counter for glitches. An example is the out-put ‘6’ which occurs during the transition from ‘7’...

  • Page 207

    operate the traffic lights or the 7-segment displayautomatically.Try this activity with other small SRAMs with 4-or 8-bit words.QUESTIONS ON COUNTERS ANDREGISTERS1 Explain what a D-type or J-K flip-flop does andhow three such flip-flops may be connected to makea ripple counter.2 What is the main ...

  • Page 208

    Topic 23Display DevicesLIGHT-EMITTING DIODESLight-emitting diodes, or LEDs, are widely used asindicators and in displays. Although filament lampsand neon lamps are still used, they have been replacedby LEDs in very many applications. LEDs outlast fila-ment lamps, they are cheaper and they require...

  • Page 209

    SEVEN-SEGMENT DISPLAYSWhen an appliance, such as a clock or a frequencymeter, needs to display numerical values, it may usean LED display in the seven-segment format.Seven-segment displays are made in a wide rangeof types. Most are red but some are green or yellow.The height of the display ranges...

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    segments are lit by drawing current through their cath-odes to currentsinks. Each segment needs a seriesresistor.The common-cathode display (Figure 23.8) hasthe cathodes of all segments connected internally to acommon terminal. Currentsources are connected toeach anode, through a series resistor....

  • Page 211

    NOT gates. It is thefield (or potential difference) thatproduces the effect, but the amount ofcurrent passingthrough the material is very small. Because of this,the power required for driving LCDs is very muchless than that used for an LED display. A typical 4-digit LCD needs only 2 or 3µA compa...

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    All resistors = 330RBCD input+5V0Vcomabcdfeg1312111091514LT 0V38abcdefgRBI + VRBO‘1’‘2’‘4’‘8’5712674LS47164FIGURE 23.13 How to connect a common-anode display to a 74LS47 BCD decoder-driver. The ripple-blanking input and output may be leftunconnected in a single-digit display.All r...

  • Page 213

    The display goes blank for inputs 1010 to 1111. Ithas a STORE input to latch the display while inputchanges. This gives time for the user to read a rapidlychanging input.PROBLEMS ON DISPLAY DEVICES1 Make a list of the different types of displays andindicators used in appliances, including domesti...

  • Page 214

    Topic 24Converter CircuitsWe have already looked as some converters, such asthe 74LS47, which converts a BCD input into thecodes needed to drive a 7-segment display. There aremany other coder and decoder ICs and circuits.Another useful circuit is a voltage-to-frequency con-verter, and its convers...

  • Page 215

    The result is that, as vIN is increased from 0 V tovREF the outputs change like this:00000000 whenvIN5 0V0000000100000011000001110000111100011111001111110111111111111111 whenvIN5 vREFThe outputs are passed to a priority encoder. Thishas the same action as the circuit on Figure 20.8, buthas 8 inpu...

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    those with more bits (the largest have 12 bits) employa technique known as half-flash. This is a compro-mise that requires fewer comparators but works intwo stages and therefore takes longer.SUCCESSIVE APPROXIMATIONCONVERTERA successive approximation ADC operates on anentirely different principle...

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    Some ADCs of this type are not as fast as this,even those with fewer bits, and may take up to 100μsto convert.The searching routine of the successive approxi-mation method is unsuited to a rapidly changinginput. For example, if the input is 3.8 V as inExample 2, the first clock cycle decreases t...

  • Page 218

    converter at any instant is either logical high or logi-cal low. It produces a rapid succession of highs andlows, known as a bit stream.It is the varying proportion of highs to lows thatcorresponds with the varying value of vIN.Imagine the converter (below) without the feed-back loop. The output ...

  • Page 219

    ExampleIf RF 5 10 kΩ and vH 5 5V, then:i3 5 vH=R3 5 vH=2RF 5 5=20 000 5 250 μASimilarly, because of the doubling of resistances, thecurrents corresponding to other high-level data inputs are:i2 5 125 μAi1 5 62:5 μAi0 5 31:25 μAWhen a data input is low, the current through theresistor is zero...

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    is the same. In the diagram, the positions of theswitches correspond to a data input of 1010.Because of the resistor values, the current flowingthrough switch S2 is half that flowing through S3.Similarly, the current flowing through S1 is half thatthrough S2, and so on. We have binary weighted cu...

  • Page 221

    C 7D None of these.3 An op amp adder is used as a digital-to-analogueconverter. The input resistor for D0 is 15 kΩ.Theinput resistor for D2 is:A 30 kΩB 15 kΩC 3.75 kΩD 7.5 kΩ.4 The fastest analogue-to-digital converter is:A an R-2R ladderB an operational amplifierC a successive approximation...

  • Page 222

    Topic 25Integrated CircuitsOne of the most important developments in electroniccomponent manufacture is the integrated circuit orIC. Instead of having to build circuits from individualresistors, capacitors and transistors, standard circuitsare produced with all its components and their con-nectin...

  • Page 223

    TheinvertercomprisestwoMOSFETS,onen-channel and one p-channel. Depending on the logiclevel of the input A, one or the other of the transistorsis switched on. The output is connected to either thepositive supply or the 0 V line. The CMOS4016 IChas six such inverters, with individual inputs and out...

  • Page 224

    The basic low-pass filter comprises two capaci-tors, with CMOS switches that are turned on alter-nately by a built-in clock oscillator. The clock runs atseveral tens of kilohertz.C1 samples the input voltage when the first switchis closed and the second switch is open. This voltagemay be positive...

  • Page 225

    Another type of phase detector is used in somePLLs. This can operate with signals of other dutycyclesand itallowsthe PLLtolockontoawiderange of frequencies.PLLs have many applications in frequency genera-tion and detection. One of these is detecting signalsof particular tone (frequencies) in freq...

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    Part 2Systems

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    Topic 26Audio and Video SystemsSYSTEMSThe concept of systems was introduced in Topic 2.Those simple circuits are the simplest possible of sys-tems, usually consisting of no more than a singleelectronic component at each stage.Now, in Part 2 of the book, we look at more com-plicated systems. These...

  • Page 229

    RADIO TUNERA radio tuner receives radio-frequency signals thatcarry the audio signals in a way described inTopic 31. It may be monophonic or stereophonic. Astereo tuner has two outputs, for left and right chan-nels and produces analogue signals of a few hundredmillivolts in amplitude.CD/DVD PLAYE...

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    we shall not analyse them to such depth, the samecomplexity is found in most other electronic systems.HARD DISK DRIVEA hard disk drive has one or more disks (or ‘platters’)on the same spindle. The disks are made from non-magnetic material, usually aluminium or glass, andare 3.5 inches in diam...

  • Page 231

    connections are made. A crystal microphone does notrequire a power supply and produces a voltagesuitable for feeding direct to a preamplifier.Crystal microphones are cheap. However, theirresponse is not linear, so there is distortion in the out-put signal.The electret microphone is similar to a c...

  • Page 232

    In the figure above,v is the instantaneous value ofthe emf produced in the signal source.ZOUT is theoutput impedance of the source. This might be 600Ωfor an electret microphone or 1 MΩ for a crystalmicrophone. The output impedance of an op amp isonly about 75Ω.vIN is the voltage input to the p...

  • Page 233

    Although is possible to design an amplifier with areasonably level response over its full bandwidth,there remains the problem that different signalsources may have an uneven response. A microphonemay emphasise some frequencies and reduce others.CDs have different frequency characteristics.Somesys...

  • Page 234

    range from 30 Hz to 800 Hz. There may also be afourth speaker, a sub-woofer that handles 20 Hz to200 Hz.Each channel has only one power amplifier todrive the array of speakers. To avoid waste of power,signals in the appropriate frequency range must bedirected to each speaker. This is done by a cr...

  • Page 235

    4 Discuss the factors concerned in the transfer of avoltage signal from a radio tuner to a preamplifier.5 What is meant by a ‘crossover network’?6 Explain why each channel of a stereo system usuallyhas at least two speakers. Describe a typical cross-over network.7 What is a mixer? Describe a ...

  • Page 236

    On the WebYou have been asked to write a 1500-word article on thelatest developments in audio-video equipment for a special-interest magazine. The magazine caters for one of thesegroups: sportsmen and women, music lovers, female teen-agers, male teenagers, computer hobbyists, games players,retire...

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  • Page 238

    Topic 27NoiseSOURCES OF NOISENoise is any unwanted signal and every kind of cir-cuit is subject to it. When a radio receiver is turnedup to full volume, but is not tuned to a station, thereis a steady hiss from the speaker. This noise is noisein the electronic sense. It may have originated in the...

  • Page 239

    by filtering, or by good design and layout. They arenot discussed any further here.ELECTRICAL CONDUCTIONThe types of noise listed next all arise through thenature of electrical conduction. They occur in all cir-cuits and are difficult to reduce:GThermal noise: This is also known as Johnsonnoise. ...

  • Page 240

    SIGNAL-TO-NOISE RATIOThere are several ways of expressing the amount ofnoise present in a signal. It is not usually the absolutepower of the noise that is important, but rather thepower of the noise relative to the power of the signalitself. For this reason, one of the most often usedways of expr...

  • Page 241

    narrow range means that noise is very much reduced.In general, noise may be reduced by restricting thebandwidth of a circuit to just that range of frequen-cies it needs to do its job.NOISE IN TELECOMMUNICATIONSIn all three modes of telecommunications the signalgets progressively weaker as the dis...

  • Page 242

    6 Describe how to avoid noise in a radio-frequencyamplifier.7 Explain the difference between a repeater and are-generator on a long transmission line.8 What is companding and how does it help reducenoise?9 What is the relationship between the bandwidth of asystem and the noise generated in it?MUL...

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    Topic 28TelecommunicationsThe job of a communications system is to conveyinformation from one place to another place that issome distance away. In this book we are concernedwith communications systems that employ electronicdevices. These are known as telecommunications,and include communication b...

  • Page 245

    CARRIERS AND MODULATIONExcept for sending audio signals as a varying voltageon a local telephone line, analogue information ismore often sent by superimposing it on a signal thathas a considerably higher frequency. This process iscalled modulation and the signal of higher frequencyis known as the...

  • Page 246

    Both sidebands contain identical information sothere is no point in transmitting them both. In singlesideband (SSB) transmission, the carrier and one ofthe sidebands is suppressed before transmission. Thepower of the transmitter is devoted to transmittingthe single remaining sideband. This is muc...

  • Page 247

    The analogue signal is sampled at regular intervals asfor PAM.The PWM signal consists of a series of pulseswith their leading edges occurring at the regular sam-pling times. The width of each pulse is proportionalto the amplitude of the analogue signal at sampling.The scaling is such that an inst...

  • Page 248

    PCM is also used for signals that are digital in ori-gin, such as signals sent from one computer or otherdigital system to another such system. Whether thesignal originated in analogue or digital form, thevalues transmitted are either 0s or 1s.FREQUENCY SHIFT KEYINGThis is one of several related ...

  • Page 249

    FREQUENCY DIVISION MULTIPLEXINGPulses can be transmitted directly as pulses, but moreoften they are used to modulate a high-frequency car-rier signal, using PCM or other coding system.Modulation of any kind makes it possible to senda large number of different signals simultaneously onone channel ...

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    JITTERIdeally, a transmitted sequence of pulses shouldarrive at the receiver exactly as transmitted. Theyshould be of uniform height and width, and shouldbegin and end at the right times. For many reasons,this rarely happens. The extent to which the pulses ofa sequence differ from the ideal is ca...

  • Page 251

    GDigital signals are highly immune from noise. Thesignal shown in Figure 28.12 is noisy but there is noproblem in recognising it as a pulsed signal.GDistorted or noisy pulses can easily be squared upwith a Schmitt trigger. This is not true of analoguesignals.AnaloguesourceLowpassfilterSampleandho...

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    GOn long routes, regenerators can restore the shape ofthe pulses. This is not so easy with analogue signals,which need more repeater stations along the route.GDigital signals are suited to processing by computersand other digital logic circuits. This makes coding,decoding, error checking, and mul...

  • Page 253

    CTS: clear to send. On receiving the RTS signal, and ifit is ready to receive data, the modem makes thelevel on this line high. The computer then startssending the data.DTR: data terminal ready. This signals that the terminalis operating properly and that the modem may beconnected to the telephon...

  • Page 254

    The connections between subscriber and exchangesare unswitched. The subscriber picks up the telephoneand is immediately in contact with equipment in thelocal exchange, usually hearing the ‘dialling tone’. Allswitching is done in the exchange. When the sub-scriber dials the number of the calle...

  • Page 255

    disassembler or PAD. The packet begins with ablock of data or header which contains such informa-tion as:Synchronising bits.Address of origin.Destination address.Sequence number.Error checking code.The sequence number is used when the messageextends over two or more packages. It might happenthat ...

  • Page 256

    use its look-up table to find the correspondingnumerical address and thus begin the process of for-warding the packet to its destination.At a router, packets are continually being receivedand checked using complex error-checking proce-dures. The packet is then sent on to the next router onits way...

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    6 A carrier with frequency 250 kHz is amplitude modu-lated with a signal of which the maximum frequencyis 8 kHz. The bandwidth of the signal is:A 258 kHz.B 16 kHz.C 8kHz.D 242 kHz.7 A system operates with even parity. The seven bitsof a byte are 0110101. When the parity bit isincluded the byte is...

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    Topic 29Cable TransmissionTelecommunication by cable is an extension of thepassing of signals directly from the circuit to exter-nal conductors. In a wired intercom, for example,the sound signal is carried along a pair of wiresfrom the master station to the remote station.When the master station ...

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    Coaxial cable is widely used for carrying radio-frequency signals. The signal is carried in the innerconductor, which may be a single copper wire or mayconsist of a number of narrower copper strands.Stranded wire is used when flexibility is important.The inner conductor is insulated from the oute...

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    At high frequencies, the lumped equivalent circuitconsists of only capacitance and inductance, as illus-trated above. This arrangement of capacitance andinductance has the properties of a lowpass filter.Signals passing along the line lose relatively more oftheir high-frequency components. This ha...

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    Thisminimiseselectromagneticinterferencebecause such interference will affect both linesequally, making one more positive and the other lessnegative. The interference is common-mode and hasno effect at the receiver.If the source circuit has a high output impedance,a buffer is needed between it an...

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    PeripheralEquipment such as a monitor, keyboard, printer, modemor CD drive, which is connected to a computer.CROSSTALKWhen several lines are close together, the current pass-ing along one line may cause current to flow in one ormore adjacent lines. This may be the result of capaci-tance between t...

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    The MC1488P and MC1489P are suitable types.Examine pulse shape at the source and load.Other pairs of drivers and receivers that can beused include the 75172 line driver and 75173receiver, and the low-power CMOS pair, the 14C88line driver and 14C89A receiver. Refer to the datasheets for circuit di...

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    Topic 30Optical TransmissionOPTICAL FIBREWhen a ray of light meets the boundary between amedium (such as air) and another transparent medium(such as glass) which is optically denser, it may bepartly reflected and partly refracted. Ignoring the partthat is reflected, Figure 30.1 below shows what h...

  • Page 265

    The angles at which these effects occur depend onthe wavelength (colour) of the light. This causes abeam of ordinary white light to spread into its com-ponent colours. The diagrams assume that the light ismonochromatic so the ray remains narrow.In Figure 30.3 below, light is passing along insidea...

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    Avalanche photodiodes (APDs) are operated withreverse bias. When stimulated by light, a fewelectron-hole pairs are generated. These move in theelectric field, striking other atoms in the lattice andgenerating more electron-hole pairs. The effect issimilar to that in an avalanche Zener diode. Ther...

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    3 What is graduated index optical fibre and how does itwork?4 Describe how an LED is used as the light source foroptical fibre.5 Describe the action of an avalanche photodiode as anoptical fibre receiver.6 What are the advantages of optical fibre as a mediumof telecommunication?MULTIPLE CHOICE QU...

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    Topic 31Radio TransmissionTHE ELECTROMAGNETIC SPECTRUMWhen an electron is accelerated, it radiates energy inthe form of electromagnetic waves.Wecan thinkof the waves spreading through space like ripples ona pond, but in three dimensions. The wave fronts arespherical and continuously expanding.Ele...

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    BROADCAST RADIOMany kinds of radio transmitter have an antenna thatradiates the waves in all directions with more-or-lessequal strength. It is broadcast to everyone withinrange. This is a feature of radio communication thatother forms of telecommunication do not normallyhave. A radio signal sprea...

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    SKY WAVESAs frequency increases above 500 kHz the radiowaves are able to penetrate the lower ionised layers.As they pass up through the layers the refractiveindex is decreasing. The effect is similar to that oflight passing along a graded index optical fibre. Thewaves are refracted along a curved...

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    ANTENNASThere are many types of antenna, but the one mostoftenusedisthe dipole (below). It consists of a verti-cal metal rod or wire, slightly less than half a wave-length long, and divided into two sections.The two halves of the dipole are fed from the out-put of the transmitter, usually by a co...

  • Page 272

    EXTENSION BOX 20 Standing WavesWith high-frequency transmissions, the cable is severalwavelengths long. A succession of waves enters thecable from the source and these are carried to the farend. What happens there depends on what is connectedthere between the two conductors:Matched load circuit (...

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    They act as two signal sources, and are spacedcertain distances apart so that they reinforce the sig-nal being radiated from the dipole. The signal isstronger than that radiated from the simple dipole,and is concentrated in one direction.The array may have further directors added to it toincrease...

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    multiplied by a varying amount determined bythe instantaneous voltage at the other input (the sig-nal). The output of the IC is the amplitude-modulatedcarrier. In a simpler system of amplitude modulationthere is no modulator as such, as shown inFigure 31.10.VaractorA diode-like device the capacit...

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    RECEIVERSIn many ways a receiver is like a transmitter inreverse, but there are differences. The stages of atuned radio frequency receiver (TRF) are illustratedin Figure 31.12.GAntenna: This stage is described earlier.GRF amplifier: This not only amplifies the weak sig-nal received by the antenna...

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    GOutput: This is the recipient of the information thathasbeen received. Examplesarelistedunder‘Recipients of information’ in the same table inTopic 28.The block diagrams of the transmitter and receiverdo not include encoders or decoders (codecs), whichform part of certain digital radio commun...

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    GIF filter: This is a bandpass filter with a very nar-row pass-band centred on 455 kHz. The filter isdesigned very exactly with sharp cut-off on eitherside of 455 kHz, so as to cut out signals on nearbyfrequencies.GIF amplifier: Further amplification with an ampli-fier designed to work best at 45...

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    More distant cells can be allowed to operate onthe same frequencies. This is known as frequencyre-use. Frequency re-use is one technique for accom-modating large numbers of subscribers on the system.The frequency on which a given mobile transmitsand receives is controlled by signals from the com-...

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    C gamma raysD ultraviolet.2 Sky waves are refracted in:A the ionosphereB the atmosphereC the stratosphereD free space.3 The frequency of ground waves is:A up to 500 kHzB over 500 kHzC above 25 MHzD below 30 kHz.4 The demodulation stage of a radio receiver is oftencalled:A the IF amplifierB a lowp...

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    Topic 32Instrumentation SystemsThis topic refers back to several previous topics,which readers should revise in detail.The purpose of an instrumentation system is tomeasure a physical quantity, such as mass, tempera-ture, force, or length. In this book we are concernedonly with electronic instrum...

  • Page 281

    Radiation PyrometerAn instrument based on a thermistor, thermopile, orother thermally sensitive device, designed to be used ata distance from the site where temperature is beingmeasured.MEASURING VOLTAGEThe simplest instrumentation systems are those thatmeasure voltage differences. The measurand ...

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    SENSORSTo measure quantities other than voltage it is neces-sary to employ a sensor. By suitably interfacing thesensor to the processing stage, we obtain a measur-able voltage difference that is related in size to themeasurand. Ideally, a sensor has these characteristics:GSelectivity: It is affec...

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    are robust and are frequently used in industrial applica-tions for temperatures between2200C to 2500C.A thermopile consists of several thermocouples inseries, with their hot junctions grouped together, andtheir cold junctions separately grouped together.Since they are in series, they produce a la...

  • Page 284

    As the diaphragm vibrates, the distance between theplates of the capacitor varies and this varies thecapacitance. Another way of varying capacitance is tovary the nature of the dielectric. This is the principleon which some humidity sensors work.The dielectric is ofen a thin layer of aluminiumoxi...

  • Page 285

    As in previous examples, variations in the measurandhave been converted to variations in a voltage whichcan be measured, displayed, or recorded in a varietyof ways.SIGNAL PROCESSING AND OUTPUTSome of the ways in which signals from the sensorare processed have been described in the previousexample...

  • Page 286

    kitchen scales uses a form of strain gauge to measureweight and can display the result in metric units or inpounds and ounces at the touch of a switch.One of the chief disavantages of a digital meter isthat when the input is changing rapidly, the confus-ingly rapid succession of digits on the dig...

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    Topic 33Electronic Control SystemsThe simplest kind of electronic control system com-prises three stages: input, processing, output. Examplesare given on p. 13. A general diagram is drawn below.These are called open-loop systems.There are also closed-loop systems, in whichpartof the signal from t...

  • Page 289

    for growing bacterial cultures) the set point may befixed. In others it may be altered occasionally (forexample, a room heater).In a servo system the set point is often changedand the system is continually adjusting to new valuesof the set point. Examples are steering systems invehicles, in large...

  • Page 290

    the temperature is not absolutely constant, but risesand falls over a restricted range.ROTATIONAL SPEED CONTROLVarying the current through the coils of a DCpermanent-magnet electric motor controls its speed.There are several ways of doing this:GManual control: A variable resistor is wired inserie...

  • Page 291

    Step no.Coil 1Coil 2Coil 3Coil 40OnOffOnOff1OffOnOnOff2OffOnOffOn3OnOffOffOnTo get from step to step, first coils 1 and 2change state, then3and4changestate,then1and2, and so on. The result of this is to produce clock-wise turning of the rotor, 15 at a time. If thesequence is run in reverse, the r...

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    position. It is then moved toward the position fromthe opposite side, and may overshoot again. This maycontinue indefinitely, causing the object to oscillateabout the position instead of settling there. This iscalled hunting. To avoid hunting, most servo systemsprovide a dead band on both sides o...

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    Topic 34Process Control SystemsThis topic looks more closely at the structure andbehaviour of control systems, particularly those usedin industrial processing.We are concerned only with closed loop systemsas open-loop systems are unable to operate as regula-tors or as servos.The simplest closed-l...

  • Page 295

    opened wide to start filling rapidly. As the vat fillsand the level of solvent approaches the set point,ydecreases and the valve is gradually closed. It is shutoff (y5 0) when the vat is full.The computer simulation below shows the inputand output of a proportional control system, simu-latedonaco...

  • Page 296

    chemical processing, for example. In mechanical con-trol systems, such as the control of robot arms or theflight surfaces (elevators) of an aircraft, overshootingmay damage the mechanism.When a proportional system is being set up, the gainof the amplifier must be tuned to produce acceptablerespon...

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    For as long as the input to the integrator is posi-tive its output rises at a rate depending on the input.WhenE is zero, the input to the integrator is zeroand its output remains at the same level. Its outputfalls if a negative voltage is applied to it.The two plots below show how the integrator ...

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    programmed as a triangular wave. This could be thesystem controlling an oscillating motion like that ofthe heater-dryer fans in a car-wash. The outputchanges with the same frequency as the set point, butnever catches up with it.The next section shows howthe response can be speeded up.PID CONTROLP...

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    Topic 35Systems with FaultsCAUSESA system may develop faults because of:GDesign faultsGHandling or usage faultsGComponent faults.Most systems work properly for most of the time. Buteven a system that has been properly designed may showfaults when it is operated under unusual or extreme con-dition...

  • Page 301

    GThe heater does not switch off when the set tempera-ture is exceeded.GNothing happens when a control button is pressed.To be able to provide such answers, the engineermust be very clear about what the system should do.Read the User Manual and consult it often.With some systems it may also be inf...

  • Page 302

    ExampleIf the base-emitter voltage of a BJT is more than about0.7 V the transistor should be on.This means that its collector voltage should be rela-tively low, perhaps not much greater than its base volt-age. Conversely, if the base-emitter voltage is less than0.7 V, the transistor should be off...

  • Page 303

    A signal generator may be used for introducing anaudio-frequency signal into an audio system. In aproperly functioning system, a tone is then heardfrom the speaker. Beginning at the power outputstage, we work backward through the system, apply-ing the signal at each stage and noting whether itapp...

  • Page 304

    RaceWhen two or more signals are passing through a logicalsystem, one or more of them may be slightly delayed,perhaps because a certain device has a longer delay timethan the others. Logic levels do not change at preciselyexpected times and the result is often a glitch.There are also input circui...

  • Page 305

    (c) thermostat circuit using a thermistor, a BJT anda relay.(d) BJT differential amplifier.(e) tuned amplifier.(f) non-inverting op amp circuit.(g) op amp adder.(h) precision half-wave rectifier.(i) BJT push-pull power amplifier.(j) triac lamp dimmer.(k) half adder.(l) magnitude comparator.(m) pu...

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    Part 3Microelectronic Systems

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    Topic 36Input and OutputMicroelectronic systems are also called computersystems or intelligent systems. All use a computer-like circuit to perform their processing. None is trulyintelligent, though the complexity of the things theydo often seems like intelligence.In Part 3, the emphasis is on mic...

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    This circuit is used to indicate one of two states:the temperature is either below the set point (logic 0)or it is above it (logic 1). If the output of the circuitis likely to hover at the midway point, the output isfed to a Schmitt trigger circuit before being sent tothe controller. A Schmitt tr...

  • Page 310

    The plate is transparent but marked with an opa-que pattern. There are four LEDs on one side of theplate and four photodiodes on the other side. The pat-tern is not based on the binary number system but ona sequence of codes known as the Gray Code.The principle of the Gray code is that only onedi...

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    thus control the speed of the motor. The brightness ofa lamp can be controlled in a similar way.PLCs have output cards to interface the controllerto the higher voltages and currents met with in theexternal circuits. The signal from the controller goesto an opto-isolator. The output from the trans...

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    A logic high.B 0V.C an unknown voltage.D 5V.5 In the circuit shown on the right, the control inputcomes from a microprocessor. The speed of themotor is varied by varying the:A operating voltage,1V.B frequency of the pulses from the microprocressor.C voltage of the input.D mark-space ratio of the ...

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    Topic 37ProcessingPersonal computers, either the desktop or the laptopversions, are probably the kind of microelectronic sys-tem most familiar to most people. They are known asmicrocomputers. There are more complicated sys-tems, such as the workstations used by geologists, forexample, to analyse ...

  • Page 315

    BUSSESThe block diagram below shows the main features ofa microcomputer. It consists of a number of unitsconnected to each other by busses. A bus may beserial or parallel:GSerial bus: A single conductor along which the bitsare signalled one at a time by making the line volt-age high or low.GParal...

  • Page 316

    GClock: This is a square-wave oscillator that causesthe microprocessor to go through one cycle of opera-tion for each pulse. The clock is usually a crystaloscillator and operates at 2 GHz or more.Read-only memory (ROM): This is memory thathas permanent content, programmed into it duringthe manufa...

  • Page 317

    MICROPROCESSORSThere are many types of microprocessor, each with itsown particular features and architecture. Figure 37.2shows the main parts of an imaginary microproces-sor that has most of the features typical of all micro-processors. The heart of the microprocessor is thecontrol unit, which fi...

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    using a transistor switch, either a BJT or an FET, asexplained in Topic 2.The figure shows a reset input; a low level on thispin causes the controller to start running its programfrom the beginning. There is also a temporary con-nection to a computer, which is used to download theprogram into the...

  • Page 319

    A very wide range of controllers is available fromAtmel, Microchip and several other manufacturers.When selecting a controller for use in a given projectthere are a number of features to be considered:GSize of program memory: 1 kB or 2 kB is enoughfor controlling the simpler systems.GSize of RAM:...

  • Page 320

    There is a terminal which usually has a few keys foroperating the PLC and an LED or LCD display toshow the current status of the system. The terminalalso contains the program and data memory. The con-trol sequences and logic are set up on a PC or a spe-cial programming unit which is not part of t...

  • Page 321

    RAM for holding programs and data takes up themain part of the address space. Some addresses inRAM are reserved for temporary storage of data. Oneexample is the stack, which is discussed later.The memory map of a microcontroller has feweraddresses but is similar in principle. There is a pro-gram ...

  • Page 322

    What actually happens is that the lines of theinstruction bus are set to 0 V where there is a ‘0’, andto the supply voltage (say, 6 V) where there is a ‘1’.This particular combination of lows and highs causesthe CPU to perform a particular operation. Let usanalyse the opcode to see what t...

  • Page 323

    ACTIVITY — CONTROLLER SYSTEMSThe topics in this topic are best studied by using amicrocontroller development system. Use the hand-book to learn about the features of the controller thatyou are using. The handbook will probably containmany example programs for you to run. You may pre-fer to post...

  • Page 324

    Topic 38ProgrammingIn Topic 37 we saw that a controller is made to per-form its tasks by reading and obeying a sequence ofinstructions held in its program memory. The instruc-tions are stored as a binary code, a pattern of 0’s and1’s. There is a different code, or opcode, for eachoperation th...

  • Page 325

    accumulator. A left pointing arrow’ indicates copy-ing data from one register to another. For example:A’num1This means that the data held in the register whichhas previously been defined asnum1 is copied to theaccumulator. This leaves the contents ofnum1unchanged. The left pointing arrow is a...

  • Page 326

    labels) within the memory of the PC, and set variousregisters in the controller. All is ready now for the pro-gram to be run. The following sections outline some ofthe things the controller can be instructed to do.ARITHMETICAL AND LOGICAL ROUTINES1) Add two bytes stored in two memory locations an...

  • Page 327

    temperature is above that set point or not. If it isabove (Y), the program proceeds to an output box,with an instruction to turn on a fan. If it is not above(N), the controller switches on a heater.This flowchart is not a complete thermostat, as ittakes a once-for-all decision, but it can be easi...

  • Page 328

    in BASIC or ladder logic and some examples aregiven later for these languages.HANDLING DATA8) Storing data in data RAM. When the power isswitched off, the values in the controller’s registersare lost. This is because the data RAM is volatileÀit does not store data permanently.Most controllers ...

  • Page 329

    though there could be more) the main program callsup a subroutine.The subroutine is called after Action 1 and per-forms Action 5 if the key is pressed. The controlleris then returned to continue cycling through theother actions. It goes to the subroutine again afterAction 3, and there may perform...

  • Page 330

    used, there is a single pin that accepts an analoguevoltage. The voltage is converted and the resultappears on a special register, which can be read bythe CPU.The flowchart in Figure 38.13 receives multi-bitinput from a thermal sensor. This it compares witha set point value stored in a register. ...

  • Page 331

    level to settle at its new value. The flowchartbelow counts how many times a button is pressed.It leaves the loop after 10 keypresses.After detecting and registering a press there is ashort delay to give the input time to settle.The program loops back for the next press afterchecking that the but...

  • Page 332

    The base address of the table on the previouspage is 2000H. In other words, the table is storedin memory at addresses 2000H to 2009H.The codes use all except the first bit of eachbyte. The allocation of bits to the segments of thedisplay is:0a bc de f gIn the flowchart below, we are using 7 out o...

  • Page 333

    DAC. Alternatively, we use an on-board DAC andtake the analogue signal from its pin.The program runs in a continuous loop, incre-mentingcount each time. Socount graduallyincreases from 0 to 255 (8 bits). At the next incre-ment it changes to 0 again, the carry-bit beingignored. It increases from 0...

  • Page 334

    are two outputs to control the motor. One is a relayswitched by a transistor switch. This turns themotor off or on. The second output goes to a sec-ondrelay wiredtooperate as areversing switch. Inthis flowchart (see below) we do not show theseoutputs separately, but simply state ‘Close doors’...

  • Page 335

    To obtain this feedback, the system needs twomore sensors. A set of microswitches are placed sothat one switch closes when the doors are fullyopen and another switch closes when the doors arefully closed.We write these into the previous flowchart byincluding the program segment below in place oft...

  • Page 336

    user would be unaware that the program had beeninterrupted.GPeripherals: In a computer system there are severaldevices that frequently need to interrupt the micro-processor. These devices include the keyboard, thehard disk drive and the printer.It is possible that a microprocessor receives inter-...

  • Page 337

    Other block operations can be programmed likethis, such as adding, say, 20 to each stored value. Or ablock of data can be searched to find a register or reg-isters holding a given value and to perform a givenoperation on it. For example it could find all registersthat hold more than 100 and subtr...

  • Page 338

    GA traffic light controller that produces the standardsequence of red, amber and green lights. The lightsare at a crossroads (above), but your program needshow only the program for the light indicated by thearrow. Then modify the program so that traffic wait-ing at the indicated light is not kept...

  • Page 339

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  • Page 340

    Topic 39Programming LanguagesThe purpose of this topic is to outline the distinctivefeatures of four different ways of programming a con-troller. The examples are based on some of the pro-gramming flowcharts in Topic 38. This topic does notset out to teach the reader how to program. The onlyway t...

  • Page 341

    The solution to this problem is to write the pro-gram in assembler language. Assembler replacesopcodes with mnemonics.Mnemonics are a set of abbreviations usually ofthree letters, sometimes more, one for each opcode.They are intended to help the human programmer todistinguish the different operat...

  • Page 342

    If we want to add numbers that total more than 255,or that have many more significant figures, or that arenegative, or to add three or more numbers together, theroutines become even more complicated when writtenas single steps. In addition to this, there are dozens ofroutines that may be used sev...

  • Page 343

    SOME ASSEMBLER PROGRAMSAn assembler program consists of a list of instruc-tions, which are read and executed by the microcon-troller. Each step in the program is listed as amnemonic. A typical microcontroller has 35 differentinstructions in its instruction set. To help you under-stand assembler p...

  • Page 344

    memory. Instead of having to remember this numeri-cal address, we can refer to it by its nameÀINTCON. The name briefly reminds us what this reg-ister does.At the start of a program we allocate names to reg-isters which store important variables, such astotal,dividend,and randomno. We type in a s...

  • Page 345

    This program puts the value 240 (11110000binary) into register 1 (data direction), so that pinscorresponding to the four lower bits are configured asoutputs and the four upper bits as inputs. The LEDsof the traffic signal are driven by the lower three (D0to D2) of the four pins; Next, it puts the...

  • Page 346

    THE OCR IMAGINARYMICROCONTROLLERThe assembler of this microcontroller has many fea-tures in common with that of the AQA microcontrol-ler. The mnemonics NOP and JMP are found in bothinstruction sets, and the JZ has the same effect asJPZ. However, the third jump instructions differ inaction. The OC...

  • Page 347

    This program is intended to be run with seven ofthe Stamp’s input/output pins (P0 to P6) connected tothe anode pins of a 7-segment common cathode LEDdisplay. There is a 1 kΩ resistor in series with eachsegment to limit the current. The program begins byinitialising the input/output pins as inp...

  • Page 348

    particularly suitable for programming control sys-tems. There are several different forms of ladderlogic. Here we illustrate ladder logic using the sym-bols for the Mitsubishi PLCs.It is important to be clear from the start that pro-cessing a ladder logic program is very different fromprocessing ...

  • Page 349

    of the software turns on the lamp if the input fromthe light sensor is 0 AND the input from the motionsensor is 1. This instruction produces AND logic.Some PLCs are programmed as connected logicgates. In such a PLC we represent the logic of theprevious diagram with the standard logic circuit dia-...

  • Page 350

    A counter is another useful item in some ladderlogic programs. On the screen it is similar to thedrawing Figure 39.10.A pulse applied to input 1 resets the counter; itscontacts are open and the lamp is off. The counter reg-isters each pulse arriving at input 2. On the tenth pulse,the counter cont...

  • Page 351

    7 When the routine is running, the LED at bit 4 is:A always off.B glowing dimly.C always on.D flashing on and off.8 When the program is running, the LED at bit 5 is:A flashing alternately with the other LED.B always off.C glowing dimly.D flashing.9 The program stops running:A after 1500 ms.B unti...

  • Page 352

    Topic 40Robotic SystemsA robot is an autonomous machine that is capable oflearning new behaviour as the result of experience ortraining.AutonomousControls itself to a large extent.Robots are divided into several types:GMobile platform robots: Most robots are of thistype, which includes the androi...

  • Page 353

    balance. It is a fact of solid geometry that a plane sur-face can be defined by any three points in space, so athree-wheeled robot can be set down on an undulatingsurface with all three wheels contacting the surface atthe same time. Four-wheeled vehicles have the disad-vantage that, unless the wh...

  • Page 354

    obstacle-avoidance program runs around the roomindefinitely, changing direction whenever it encoun-ters an obstacle in its path.Touch switches are often used in robotics as limitswitches. It is not enough for the controller to switchon the motor to extend the arm. It needs to be toldwhen to switc...

  • Page 355

    microcontroller switches off the motor and the arm isstopped in the fully extended position.The robots illustrated in this topic were constructed andprogrammed using a LEGOs Mindstormss kit.MICROPHONEA microphone can provide input to a robot so that itcan be programmed to respond to sound. For ex...

  • Page 356

    numerals ‘one’ ‘two’ ‘three’ and so on, as well as anumber of useful sound effects, such as the sound ofapplause.DISPLAYSThe simplest form of display is an LED. These canbe driven directly from an output pin of the micro-controller, so there are no interfacing considerations.A robot c...

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  • Page 358

    Topic 41Neural NetworksNeural networks may be used for controlling or ana-lysing highly complex systems. A neural networkmight be used for controlling a power station, forexample, for landing a lunar module, or for driving acar. Other neural networks are concerned with recog-nising patterns, as d...

  • Page 359

    (all input signals are processed simultaneously). Eachtime it processes data it may modify the outcome as aresult of what it has been taught, or experienced.The program of a digital computer must necessar-ily be based on a known set of rules and equations.However, in a neurocomputer, it may be th...

  • Page 360

    closely to the expected outputs, the weightings arechanged further in the same direction. Graduallythe system discovers the most effective weightings.The mathematical techniques used for changing theweightings is beyond the scope of this account.In another learning procedure, reinforcementtrainin...

  • Page 361

    fairly low-resolution system, the image may be 10pixels by 10 pixels. This provides 100 inputs to theANN. There are 26 outputs, each corresponding to aletter of the alphabet.The ANN is ‘shown’ specimens of the letters invarious styles, including handwritten versions. As itis shown each specim...

  • Page 362

    C reinforcementD an expert system.5 In contrast to a neurocomputer is:A programmedB trainedC fasterD without a memory.On the WebNeural networks are continually finding new applicationsand there is much research in progress. Find out aboutthe latest developments by exploring the World WideWeb for ...

  • Page 363

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  • Page 364

    SupplementsA. Useful InformationELECTRICAL QUANTITIES AND UNITSCurrent: This is the basic electrical quantity. Its unit,the ampere, is defined by reference to the mechanicalforceacurrentproducesbetweentwoparallelconductors.Symbol for the quantity, current:I.Symbol for the unit, ampere: A.SymbolsS...

  • Page 365

    Conductance: The unit of conductance is the sie-mens. It is defined as the reciprocal of the resistance.If a conductor has a voltageV across it and a currentI flowing through it, its conductanceG is:G 5 I=VSors?This is another source of confusion. The symbol for sie-mens is a capital S, but the s...

  • Page 366

    There are three loops in Figure A.2, ABGD,BEFG, and ABEFGD. Going round each loop in aclockwise direction, the equations for each loop are:For ABGD:v2v12v2 5 0For BEFG:v22v32v4 5 0For ABEFGD:v2v12v32v4 5 0Sinusoids: A sinusoidal signal is one that, when plot-ted to show how the voltage or current...

  • Page 367

    The inverse relationship applies to rms currents:ipis5nsnpCOMPONENTSResistors: The preferred values of the E24 series, inohms, are:1.0 1.1 1.2 1.3 1.5 1.6 1.8 2.0 2.2 2.4 2.7 3.03.3 3.6 3.9 4.3 4.7 5.1 5.6 6.2 6.8 7.5 8.2 9.1After these 24 values, the sequence repeats in mul-tiples of ten:10 11 1...

  • Page 368

    LOGICAL IDENTITIESThese are useful for solving logical equations whendesigning combinational logic circuits. In these iden-tities, the symbol Gmeans AND, and can be omittedwhen writing out the identity and when solving equa-tions. The symbol1 means OR. A bar over a symbolmeans NOT.The order of AN...

  • Page 369

    Brackets can be inserted and removed just as inordinary arithmetic:A B C5 A BðÞ C5 A B CðÞA1 B1 C5 A1 BðÞ 1 C5 A1 B1 CðÞA B1 CðÞ 5 A B1 A CANDing A with itself and with true and false:A A5 AA 15 AA 05 0ORing A with itself and with true and false:A1 A5 AA1 15 1A1 05 ANegation:A1 A5 1A A...

  • Page 370

    ACKNOWLEDGEMENTSTheauthorthanksEasternGenerationLtd.,Ironbridge, Shropshire, for permission to take photo-graphs on their site (p. 275).Information about the use of a neural network todetect smells is based on ‘Electronic noses and theirapplications’ by Paul E. Keller et al., PacificNorthwest...

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  • Page 372

    SupplementsB. Answers to Self-Test QuestionsSome answers show the actual calculated value (to 3significant figures) and, in brackets, the most appro-priate standard value.Page 1: Conductors are copper wire, brass switchand breadboard contacts, metal film of resistor, elec-trodes and electrolyte o...

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  • Page 374

    Index4-input circuits and logical operations, 1597LS47 BCD decoder driver, 200À1, 20374LS221 monostable pulse generator, 179À80555 timer IC circuit, 42À34511 CMOS BCD decoder-latch-driver,200À24518 counter, 188À97805 integrated circuit, 146À7AAccumulators (microprocessors), 306Active filter...

  • Page 375

    Bugs in software, 293Bussessee processingByte definition, 303CC (programming language), 313C-Dsee common-drainC-Ssee common sourceCable transmission:cable types, 247À8characteristic impedance, 249crosstalk, 251description, 247line driving, 249À51transmission lines, 248À9Cache memory and microc...

  • Page 376

    Direct current (DC):description, 1switching, 132À3Directional transmission (radio), 261À2Displays:LEDs, 197À9liquid crystal, 199À200robotic systems, 345Dividing and counting, 185À6Domain addresses, 244Dot matrix display, (LEDs), 199Down counters, 186À7DRAMsee dynamic RAMDual slope ADC, 206D...

  • Page 377

    Instrumentation systems: (Continued)signal processing and output, 274telemetry, 269À70Insulators (non-conductors), 2INTCON and assembler programminglanguage, 332À3Integrated circuits (ICs): 7805, 146À7CMOS4016, 212description, 211exclusive-NOR, 163exclusive-OR gates, 163Intel Teraflop, 211low ...

  • Page 378

    Majority logic, 167À8Mathematical models for neural networks,348À9Measuring voltage and instrumentationsystems, 270Mechanical effects and noise, 227À8Medium scale integration (MSI), 167Megabyte (MB) definition, 303Memory:control for counters/registers, 194maps, 309microprocessors, 332see also ...

  • Page 379

    Oscillators:Colpitts, 120description, 119phase shift, 119Wien Bridge, 120À1Outputs:multi-bit, 300one-bit, 299À300 see also inputs andoutputsOverheating alerts, 17Pp-type:semiconductors, 7silicon, 7Packer assembler/dissembler (PAD), 243À4Packet switching in telecommunications,243À4PADsee packe...

  • Page 380

    Ramp/square wave generator operationalamplifiers, 106À8Random-access memory (RAM):data, 317microcomputers, 305, 308the stack, 326Reactance and inductors, 56Read-only memory (ROM):assembler language, 330logical combinations, 174microcomputers, 305microcontrollers, 307Receivers for radio transmiss...

  • Page 381

    Telecommunications: (Continued)pulse code modulation, 236À7, 239À40pulse modulation, 235À6pulse position modulation, 236pulse rate, 239pulse width modulation, 235À6repeaters/regenerators, 228RS-232 standard, 241À2stages, 233À4TCP/IP, 244À6time division multiplexing, 238transmission rates, ...