The Transistor Amplifier

The Transistor Amplifier
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The Transistor Amplifier

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    The Transistor AmplifierHomeSave P1 as:.doc (700kB .pdf (600kB)Save P2 as:.doc (1.6MB) .pdf (1.2MB)See: 1- 100 Transistor Circuits101 - 200 Transistor CircuitsP1 P2 P3 testThe Transistor Amplifier is available as a .pdf but this file is not updated as fast as the web page. New items are...

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    A "Stage"Back EMFBase BiasBiasing A Transistor2 Biasing Diodes in push Pull AmplifierBiasing the baseBlocking OscillatorBridge - theBoost ConverterBootstrap CircuitBuck Converter - the Changing A TransistorClass-A -B and -CClipping and DistortionColpitts OscillatorCommon Base AmplifierC...

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    HysteresisIlluminating a globe (lamp)Impedance MatchingIncreasing mobile handset volumeInput and Output ImpedanceIntegration and DifferentiationInterfacingInverter - transistor as an Latch CircuitLeakage - the small leakage current due to combining two or more transistorsLevel ConversionLighting ...

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    Voice Operated Switch - see VOXVoltage Amplifier CircuitVoltage Buffer CircuitVoltage Doubler - theVoltage to Current ConverterVoltage RegulatorVoltages - measuring VoltagesVOX - Voice Operated SwitchZener TesterZener The transistor as a zener Regulator 1 watt LED - driving a high-power LEDTHE D...

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    Fig 71aeTHE CONSTANT-CURRENT CIRCUITFig 71a Constant-Current CircuitsThe three circuits above provide a constant current through the LED (or LEDs) when the supply rises to 15v and higher. The second and third circuits can be turned on and off via the input line.

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    Fig 71b Constant-Current CircuitThe first circuit in Fig 71b is a constant-current arrangement, providing a fixed current to the LEDs, no matter the supply voltage.This is done by turning on the top transistor via the 2k2 resistor. It keeps turning on until the voltage-drop across resistor R i...

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    supplyTHE CURRENT MIRROR CIRCUITFig 71bac Current Mirror CircuitThis is not a constant current circuit. It is a CURRENT SOURCE circuit. A constant current circuit means the current will not change if the supply voltage is increased or decreased. This circuit simply supplies a DC signal (in the fo...

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    value of current as selected by the 100R pot is reached the output voltage will have dropped by 0.6v. This is the voltage developed across the current-sensing resistor and this voltage is detected by the BC547 to to start to reduce the output voltage. As soon as the maximum current is reached, th...

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    Fig 71be The Unregulated Voltage is regulated by the diode (zener)In Fig 71be, the supply voltage is called the UNREGULATED VOLTAGE and it is connected to resistor R and a diode. The voltage at the top of the diode is called the REGULATED VOLTAGE. The diode produces a fixed 0.65v and the zener pr...

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    Fig 71bf Buffered Shunt Regulatorcalled a PASS TRANSISTOR RegulatorThe transistor operates as an amplifier and if the DC gain of the transistor is 100, the output current of a Buffered Shunt Regulator can be 100 times more than a Shunt regulator. See more circuits on the Zener Regulator and the T...

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    value. Both the supply and the load can change at the same time and the circuit will compensate. All we have to do is re-draw the circuit as a standard 2-Transistor Regulator as shown in Fig 71bcand you have covered the principle of its operation.Fig 71bc 2-Transistor Voltage RegulatorTHE TRAN...

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    analogy described previously. But the question is: 1. How much energy will a capacitor pass under ideal conditions?2. How do you work out if a capacitor needs to be: 100n, 1u, 10u or 100u?Without going into any mathematics, we will explain how to select a capacitor. Many text books talk about the...

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    separates the DC voltages - the capacitor allows the naturally-occurring voltages to be maintained. Fig 71eThe capacitive reactance of the 100u ranges from 10R to lessthan 1R (depending on the frequency being processed).In Fig 71d you can see the "resistance" of a capacitor is very smal...

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    Fig 71fThe waveform on the output of the electret microphone is 20mV p-p (peak-to-peak). This amplitude passes through the 100n capacitor, which we have drawn as a 10k resistor, (to represent the capacitive reactance of the capacitor at 200Hz). The input impedance of the common-emitter amplifier ...

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    Fig 71gFig 71g shows each transistor stage has an input and output impedance. This really means an input and output resistance, but because we cannot measure the value with a multimeter, we have to find the value of resistance by measuring other things such as "waveform amplitudes" and ...

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    value "63%" has been chosen because the voltage on the capacitor is increasing very little (each second) when it is nearly fully charged and waiting for it to reach 65% may take many seconds. Trying to detect an extra 10% or 25% is very hard to do and since it takes a long time for the ...

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    Fig 71kFig 71k shows two Time Delay Circuits as well as a latching circuit (the 4k7 resistor), a buffer transistor (BD136) and a high frequency filter (the 15n capacitor).When the circuit is turned ON, the relay is not energised. The signal on the base of the first transistor has any high frequen...

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    reverse voltage. A typical circuit that takes advantage of this high voltage is the: Wailing SirenHIGH FREQUENCY "NOISE"Before we move on to the next phase of this discussion, there is one interesting point that needs covering. When a circuit has a number of amplifying stages, there is ...

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    To understand how a filter works, you need to know "HOW A CAPACITOR WORKS."Fig 72a.Fig 72a shows a capacitor with a low-frequency signal entering the left terminal. The output amplitude from the capacitor in diag a will be small because the capacitor is able to charge and discharge as t...

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    Fig 72c.Fig 72c Fig a shows a capacitor and resistor connected in series on the "signal line." With a low-frequency signal, the capacitor reduces the amplitude because most of the signal is absorbed by the capacitor charging and discharging. As the frequency increases (fig b), the outpu...

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    input:Fig 72e.Fig 72e is a low-pass filter that provides unity voltage gain to all frequencies below 10KHz, but it rejects all frequencies above 10KHz at 12dB per octave. It is used to remove high frequency noise from audio recordings.Fig 72f.Fig 72f is a high-pass filter that provides unity volt...

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    Fig 72. Fig 72. A TORCH is an ON-OFF circuit. A Digital circuit is an ON-OFF circuit. Fig 73.Fig 73. This is the simplest DIGITAL CIRCUIT. The globe illuminates when the switch is closed. Two reasons why a Digital Circuit was invented:1. It produces either "0" or "1" (LOW or H...

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    ON state VERY QUICKLY. A transistor in this type of circuit is called a SWITCHING TRANSISTOR and it may be an ordinary audio transistor but it is called a switching transistor when used in a switching circuit. The two Darlington transistors in Fig 74 are SWITCHING TRANSISTORS and the circuit is a...

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    Fig 75. Latch CircuitFig 75. Circuit B is a LATCH. The two transistors instantly change from the OFF state to the ON state. This is also classified as a DIGITAL CIRCUIT. The circuit can also be called an SCR made with transistors. Circuit A shows an SCR in action. The top switch turns the SCR ON ...

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    This means the 10u discharges and when the first transistor turns off, the positive lead rises and takes the negative lead with it. This reduces the voltage on the emitter of the BC547 and the transistor turns OFF.This is how the LED turns off. Further blowing into the microphone will make the em...

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    Fig 77.Fig 77. This circuit has only two states. ON and OFF. The ON button turns off the first transistor so the second transistor turns the globe ON. This is called a TOGGLE ACTION and the circuit is a BINARY CIRCUIT or BISTABLE CIRCUIT called a BISTABLE SWITCH or a bistable of the MULTIVIBRATOR...

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    Fig 77b.Fig 77b. This is part of a counting circuit and since it takes many transistors to create a circuit to count to "2" it is not practical to make it using discrete components. That's why INTEGRATED CIRCUITS were invented where dozens, then hundreds then thousands then millions of ...

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    Fig 79. Schmitt Trigger CircuitFig 79. This circuit takes a slowly rising or falling voltage and turns it into a fast-acting ON-OFF signal to operate a LED or relay. This is done via the positive feedback line shown in red. It is called positive feedback because it ADDS to the change to speed it ...

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    This circuit is also called a PULSE EXTENDER. GATES We have described the transistor as an amplifier and the fact that POSITIVE FEEDBACK can turn a transistor ON more and more, so it changes from: "not-turned-ON" to "fully-turned-ON" in a very short period of time. When a tra...

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    Fig 79b.Fig 79b. When the speaker (LOAD) is placed above the transistor, the circuit is said to be SINKING the current. A BC547 does not have the collector-current to adequately supply an 8R speaker. You really need a BC338. There is no advantage in one placement over the other. If the load is co...

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    If the input requires more current, the voltage (or voltage-swing) from the previous circuit or device will be reduced. If the input requires less current, the voltage-swing will be affected a very small amount. But in ALL cases the voltage-swing will be reduced - because you ARE supplying SOME e...

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    0v.Fig 79e.Fig 79e. This circuit has been taken from Fig 71acc. It is a bootstrap circuit and produces a very clever "switch."The circuit sits with the first transistor turned ON and the second turned OFF as can be seen in the first line at the top of the output waveform -up to the red ...

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    Delivering a high amplitude analogue signal may be recognised by a digital circuit when it reaches a peak or goes to 0v, but this is not guaranteed or reliable. In addition we may want the signal to be a CONSTANT HIGH when the audio is present. This is what an ANALOG TO DIGITAL circuit will do. I...

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    Fig 80c.The amplifier maybe one or two stages, depending on the amplitude of the original signal.Each stage of an amplifier will increase the size of the signal about 70 times. If you are very lucky, you may get an amplification of 100x (100 times). Thus a 5mV signal with one stage of amplificati...

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    Fig 80e.The second transistor in Fig 80e is called a DIGITAL STAGE. This simply means a biasing resistor is not connected to the base of the second transistor so it turns on fully when a signal greater than 650mV is detected and is turned off at other times. This stage is ideal for a micro or oth...

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    Fig 80g.Fig 80g shows an electret microphone connected directly to the base of a two-transistor amplifier. This arrangement will work and provides the best transfer of signal from the microphone to the base. But biasing the first transistor is a very difficult thing to do. The electret microphone...

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    LEVEL CONVERSION withDiodesLevel conversion can be done with 3 diodes to change 3.3v to about 5v, but you have to know the current-capability of the 3.3v source and the current requirement of the 5v section. This arrangement is only suitable for very small current requirements. The output is allo...

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    Fig 80. A Feedback OscillatorFig 80. The 10n capacitor provides the positive feedback to keep the circuit oscillating. Fig 81. A feedback oscillatorFig 81. The 10n capacitor provides the positive feedback to keep the circuit oscillating. Fig 82. The positive feedback line creates the CALL tone

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    Fig 83.Fig 83. When the third transistor is turning OFF, the collector voltage is rising and this is passed to the base of the first transistor, to turn it ON. This is how the circuit keeps "cycling" or oscillating. Fig 83a. Globe flashes at 1HzFig 83a. The high-gain amplifier we studie...

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    voltage and it is being charged via the bas-emitter junction of the first transistor. As it becomes fully charged, the current into the base of the first transistor reduces slightly and the transistor turns off slightly. This effect is passed to the second transistor and it turns off slightly too...

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    Fig 84.When two transistors are cross-coupled as shown in Fig 84, you can safely assume the circuit will oscillate. The frequency of oscillation will depend on the value of the components but the oscillator is known as a FREE-RUNNING OSCILLATOR or ASTABLE(ay-stable) MULTIVIBRATOR and the output i...

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    Fig 86a.Fig 87.Fig 87. The ASTABLE ("ay" - meaning not-stable) MULTIVIBRATOR circuit is rich in harmonics and is ideal for testing amplifier circuits. To find a fault in an amplifier, connect the earth clip to the 0v rail and move through each stage, starting at the speaker. An increase...

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    Fig 90. The Sinewave OscillatorFig 90. This circuit produces a sinewave very nearly equal to rail voltage. The important feature is the need for the emitter resistor and 10u bypass electrolytic. It is a most-important feature of the circuit. It provides reliable start-up and guaranteed operation....

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    THE BLOCKING OSCILLATOR Fig 92.Fig 92a.Fig 92b.Fig 92. The BLOCKING OSCILLATOR circuit uses a transformer to produce POSITIVE FEEDBACK to the base. The circuit starts by Rbias charging Cbb to deliver voltage to the base of the transistor via Rb (and also a small current). The transistor turns on ...

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    Fig 94.Fig 94. This LED Torch circuit uses the "flyback" voltage of a BLOCKING OSCILLATOR to illuminate a 3.6v super-bright LED from a 1.5v supply. Note: the 10n capacitor prevents the energy from the feedback winding being lost. All the energy from the feedback goes into the base of th...

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    Fig 96. 2-transistors in PUSH-PULL - as a Blocking Oscillator circuitFig 96a.Fig 96. A simple extension of the Blocking Oscillator in Fig 92c above, is shown in this diagram. It consists of two BLOCKING OSCILLATORtransistors that are turning each other off. The circuit starts by one transistor be...

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    off. When the 330p is charged the BC547 is not turned on as much and the 2k2 can start to turn on the BC338. It pushes the charge on the 330p into the base of the BC547 to keep it off. The 330p gets discharged by the 330R and the voltage across the *R resistor turns on the BC547 to turn off the B...

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    THE BUCK CONVERTER Fig 97bAny circuit that converts a high voltage toa lower voltage is classified as a BUCK CONVERTER. Fig 97b will drive a 1watt white LED from a 12v supply and is capable of delivering 300mA. The driver transistor is BD 327 and the inductor is 70 turns of 0.25mm wire wound on t...

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    the signal taken from the active end. The Hartley Oscillator has a tapped coil and these are difficult to obtain. Fig 99. Fig 99aFig 100. Colpitts OscillatorFig 101. Colpitts OscillatorFig 102. Hartley OscillatorFig 103. Hartley Oscillator

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    Fig 103a. Door Knob AlarmDOOR-KNOB ALARMThis circuit can be used to detect when someone touches the handle of a door. A loop of bare wire is connected to the point "touch plate" and the project is hung on the door-knob. Anyone touching the metal door-knob will kill the pulses going to t...

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    Fig 103b.In circuit 103b, this high voltage is produced at some point in the cycle and it pulls the emitter UP a small amount via the 47p and this turns the transistor OFF. We are not going into what part of the cycle produces the high voltage via the inductor but it DOES. That's how the circuit ...

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    Every electronic component has a value of resistance. You can measure this value with a multimeter. But sometimes the value changes according to the light it receives, the frequency it is operating-at, or the voltage it is connected-to, or the sound it receives, or its temperature or many other i...

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    We have already studied "Impedance Matching" in the circuits above, but did not identify the concept. We will now go over some of the circuits and show where impedance matching took place:Fig 6In Fig 6, the transistor matches the HIGH IMPEDANCE of your finger to the LOW IMPEDANCE ne...

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    Fig 71eSometimes Impedance Matching is needed to separate or remove the DC component of a signal. In Fig 71e, the electrolytic matches the LOW IMPEDANCE output of the amplifier to the LOW IMPEDANCE of the speaker. The two impedances are almost identical and you could connect the speaker directly ...

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    the output to change state. This may produce an undesirable effect of the circuit turning "on and off" at the wrong time due to supply voltage fluctuations. By increasing the gap between these two voltages, the circuit will remain in one state or the other - until the input voltage (the...

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    NEGATIVE FEEDBACK The circuit shows a capacitor between the base and collector. It provides NEGATIVE FEEDBACK. If we remove the capacitor, when the base "moves down," the collector "moves up." In other words the signal is inverted. When the capacitor is fitted, we have to star...

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    signal stage. BASE CURRENTThe following 4 diagrams show how base current is delivered to a small-signal stage in a "self-biasing" arrangement.In this arrangement, the base-bias resistor is selected so the voltage on the collector is half-rail voltage. In this case it is 3v. Circuits A a...

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    Circuits A and F produce about the same gain and the only difference is two extra resistors in circuit F. The only problem with circuits E and F is the rail voltage must be above 4v. If the rail voltage is less than 3v, the transistor will not be turned ON as the base will see less than 0.6v. The...

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    Different transistors have varying base voltages. You will need to set the base voltage very accurately for the circuit to work.What do we mean by: DIFFERENT BASE VOLTAGES?Most silicon transistors start to turn ON when the base voltage reaches 0.65v. But some transistors start at 0.55v. And some ...

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    further increase in collector-emitter current is possible. Increasing the current into the base will simply overheat the transistor and damage it. LEAKAGEThis topic covers small, unwanted, currents produced when two or more transistors are connected together. Faulty Headlight Extender CircuitHead...

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    Devices such as globes, motors, relays etc are called LOADS. They can be placed above or below the driver transistor. When the driver transistor is above the load, as shown in diagrams A and B the circuit is called HIGH-SIDE SWITCHING. When the driver transistor is below the load, as shown in dia...

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    Diagram D shows the problem trying to switch a load on a 12v supply, from a 5v microcontroller. When the microcontroller is HIGH, the voltage is not high enough to turn off the transistor. The voltage on the base must be nearly 12v for the transistor to turn off. This circuit will NOT WORK. In di...

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    A resistor is aVOLTAGE TO CURRENT CONVERTER A red LED must be delivered a voltage of exactly 1.7v for it to work. In other words it must be connected to a 1.7v supply. But a 1.7v supply is very hard to obtain, so we use a 3v supply and a dropper resistor. The resistor converts the 3v to 1.7v. Thi...

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    Fig 103f.A common-emitter stage is aVOLTAGE TO CURRENT CONVERTER A slight increase in the voltage on the base of a common emitter transistor will increase the current through the load by a large amount. As you can see, there are lots of circuits that perform VOLTAGE TO CURRENT CONVERSION but we u...

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    Its function is to produce a very small voltage across it and this voltage is detected by a circuit (basically a voltmeter (or milli voltmeter). When a current flows though a resistor, a voltage is produced across the resistor. You can also say a VOLTAGE DROP is produced across the resistor. If t...

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    oscillator. The following circuit will not work: Fig 104a.The three stages of amplification will produce so much gain that the circuit will self-oscillate. The output will be a "buzzing-sound" and the fault will be impossible to find because it comes from withinthe design of the circuit...

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    Fig 104c. The positive feedback loop producing "Motor-boating"This effect can be reduce and eliminated by a term called DECOUPLING. Decoupling is achieved by adding capacitors [electrolytics] (and resistors) across the power rails so that each stage is effectively powered by a separate ...

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    Fig 104e. Fig 104e shows an electrolytic connected across the supply that feeds the electret microphone and 1k2 resistor to separate the supply we have just created, from the main supply rail. We have effectively created a separate power supply. It is fed by a 1k2 and kept "tight" by th...

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    BREAKDOWN and ZENER MODEThere are two conditions or states where a transistor can be instantly damaged. This is due to voltage applied in the wrong direction or the application of voltage that is higher than the rating of the transistor. Voltage will kill a transistor faster than excess current. ...

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    In zener mode, the base-emitter junction is connected to a voltage higher than 9v via a resistor. The junction will breakdown and a voltage of about 7v will appear across the base-emitter leads and the excess voltage will be dropped across the resistor. The zener-effect or zener mode can be used ...

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    ZENER TESTERThe maximum voltage a transistor can withstand is called the ZENER VOLTAGE of the transistor. It is Vce - the voltage between (across) collector and emitter. It is also the maximum supply voltage or circuit voltage or the voltage generated by an inductor in the collector-circuit and c...

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    TESTING A TRANSISTORWhen testing a transistor, fit it into the pins marked C B E. If you have a LED connected to the LED terminals, it will glow. If you remove the LED and measure the voltage across the 1u electrolytic, it will provide the maximum working voltage for the transistor. TESTING A ZEN...

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    THE TRANSISTOR & ZENER REGULATORA transistor can be used to amplify the characteristics of a zener. You can also say the transistor is a BUFFER or EMITTER-FOLLOWER. It is another example of the transistor as an AMPLIFIER - a DC AMPLIFIER - indicating it amplifies the "steady-state" ...

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    When the load takes 50mA, the zener takes 50mA. Fig (b)When the load takes 90mA, the zener takes 10mA. Fig (c)When the load takes 100mA the zener takes 0mA. Fig (d)Current-sharing between the zener and outputUp to this point the circuit works perfectly. Even though the zener takes 0mA, the circ...

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    emitter-follower transistor must be a POWER TRANSISTOR. Here are some examples from 100mA to 2Amp:The transistor has a gain of 50In the circuits above, the output current can range from 100mA to 2Amp. The zener will pass 48mA when the load is 100mA and drop to 10mA when the load is 2Amp. If the o...

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    The value of resistance for the Dropper Resistor is obtained by the formula:Suppose the supply is 15v and the zener is 12v . The value of the Dropper Resistor is:The output voltage is 0.7v less than the voltage of the zener. The following diagram shows an example of the voltages on a typical regu...

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    The regulator circuit re-drawnIMPROVING THE SMOOTHNESS OF THE OUTPUTThe quality of the output (meaning the smoothness of the output) of a regulator - also called the smoothness of a POWER SUPPLY - can be improved by adding a transistor that detects any increase or decrease in the the output volta...

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    The resistance of Rb = 0.65/.009 = 72 ohms. The resistance of Ra = 10.65/0.01 = 1.065kThe resistance of the dropper resistor = 3/0.01 = 300 ohmsThe circuit turns on via the 300 ohm dropper resistor pulling the base UP. As the output voltage rises, a point is reached where the voltage into the fee...

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    The circuit is suitable for up to 100mA. A power transistor can be used, but the 1k will have to be reduced to 220R for 500mA output. The output of the circuit is about 2v less than the output of the plug pack.By reducing the 1k or adding a zener across the electro, the voltage will remain much m...

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    current, the current though the collector-emitter terminals can be adjusted. In most cases the transistor is in series with a LOAD RESISTOR and the two items can be adjusted to remove unwanted energy. In addition, the percentage dissipated by the transistor compared to the load resistor depends o...

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    The diagrams we have provided show percentages but not the base current required to create the load sharing. Even simulation software will produce false data as the actual characteristics of the transistor you are using will be unknown. Rather than spending time on trying to work out the probable...

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    5v to 15v Non-InverterThe following circuit does not work because the second transistor is never turned off. Both transistors MUST be connected to the high voltage rail. This circuit does not workThe following circuit converts a signal that starts as 5v HIGH and goes LOW. During this signal trans...

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    Push-Pull InverterAll the circuits above convert an analogue signal or A DIGITAL SIGNAL into a digital signal. This is due to the gain of the transistor. In other words the output does not respond in a linear manner, (to the input voltage). The output changes when the input moves from a voltage o...

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    discussed above but it does produce a reverse voltage that can be as high as 99.99% of the applied voltage. But this is getting away from the point we want to cover. The secondary winding produces an exact copy of the voltage flowing into the primary and if you measure it on a piece of test equip...

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    Designing a transformer is very difficult and complex. The easy approach is to buy a product that contains a circuit similar to your requirement and use the transformer. It is very difficult to take a transformer apart as the laminations or the ferrite core is dipped or glued or sealed so the win...

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    If the pot in the last diagram is turned fully clockwise, the wiper will each rail voltage. If the wiper is connected to a low resistance, a high current will flow and damage the pot. A "safety resistor" will reduce the high current. There are three reasons why a pot is included in a c...

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    The resistance of a pot is selected from one of the following values: 100R, 500R, 1k, 5k, 10k, 50k 100k, 1M and 2M. In most cases you will copy a circuit and use the same value for the pot. Working out the value is quite a complex task. Here are three different circuits. The voltage on the top a...

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    well as creating the required voltage on the top of the pot. TRIM POTA trim pot is simply a pot without a shaft. It usually has a screw-driver slot and is adjusted once in the life of a circuit. It is usually small in size and can be any resistance value to suit the circuit.It can be connected a...

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    CLAP SWITCH USING PIEZO DIAPHRAGM PICK-UPThe circuit above takes about 20uA when "sitting around." That's because the piezo diaphragm does not require any current. The same circuit can use an electret microphone for the input but the idle current rises to 200uA. CLAP SWITCH USING ELECTR...

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    CLAP SWITCH TOGGLES THE 2 LEDSThe second 10u is charged via the 5k6 and 33k and when a sound is detected, the negative excursion of the waveform takes the positive end of the 10u towards the 0v rail. The negative end of the 10u will actually go below 0v and this will pull the two 1N4148 diodes so...

  • Page 91

    The circuit above is the best design as it uses the least number of components and drives a relay. The next circuit comes from Engineers Garage website. It uses fewer components but takes more current (about 6mA) in the quiescent mode and does not have any delay to hold the relay ON:A 6v VOX CIRC...

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    3v to 6v VOX CIRCUIT(Good design - circuit takes 0.25mA. Circuit keeps 100u discharged)The addition of the diode in the 3v circuit is needed to discharge the 22u so that it produces its "full effect" to saturate the output transistor when required. It is not needed in the 12v circuit as...

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    12v VOX CIRCUIT(Not a good design - circuit takes 12mA ) Here is the circuit re-designed to take less quiescent current (0.5mA) and provide a longer delay with 100u electrolytic (20 seconds). 12v VOX CIRCUIT(Good design - circuit takes 0.5mA.) The next circuit can be Voice Operated or activated b...

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    you to use the tuner built into your VCR to turn on and off older TVs that are not equipped with a remote. It can also be used to activate surround-sound equipment, turn off room lights, turn on video game consoles, etc. When power is applied, the first transistor is not turned on and the second ...

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    VOX TOGGLE CIRCUITShort tone = ON Long tone = OFFThe circuit allows a whistle to turn an appliance ON and OFF by sending a short whistle to turn a circuit ON and a long whistle to turn a circuit OFF. This is handy when you cannot see the result of your operation. A simple toggle operation is no...

  • Page 96

    Sound makes LED stay ON The electret mic and first transistor are active when the circuit is "waiting for a sound" and the 3rd and fourth transistors are biased OFF due to the 1M and 100k voltage-dividing resistors putting a voltage of between 0.27v and 0.54v on the base of the second t...

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    An OVERDRIVE Circuit

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    The following website has SOUND CLIPS for lots of different effects:http://www.home-wrecker.com/salvo.html#bazzfussIntegration and DifferentiationWe are going to show how two components, (a resistor and capacitor - in series) produces different results according to the frequency of a voltage (sig...

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    But if we connect the same two components to a rising and falling voltage, a completely different result is produced. If the input is a sine-wave, the following results are produced when a low-frequency, medium-frequency or high-frequency is supplied to the input:The circuit is a LOW PASS FILTERY...

  • Page 107

    the effect of the waveform when it is falling, by adding a diode to the input. You will notice a voltage builds-up on the capacitor. This is because the input voltage is charging the capacitor a little-bit more on each cycle. The circuit becomes an INTEGRATOR. It does not matter if the waveform i...

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    The 47k is a "Pull-Up" resistor. The designer of the circuit wants the base of the transistor to be at a known voltage when the circuit is sitting around, waiting for a signal. The 47k forms a voltage-divider with the 100k and BC547 and it makes sure the BC557 is turned off when the cir...

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    CLASS-A CLASS-B CLASS-CEach stage in a circuit can be given a name according to its efficiency. Normally the output stage is the only stage that is classified as "A" "B" or "C" because this is where most of the efficiency or losses occur. However the same criteria ...

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    applications with very good results.THE DRIVER STAGE Any transistor that drives a LOAD is said to be a DRIVER in a DRIVER STAGE or OUTPUT STAGE. A LOAD is generally a relay, motor, globe, LED or other device that requires a CURRENT. The following diagrams show a DRIVER (or OUTPUT) TRANSISTOR driv...

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    5v relay 100 ohms 50mA5v relay 240 ohms 20mA9v relay 100 ohms 90mA9v relay 240 ohms 40mA12v relay 100 ohms 120mA12v relay 240 ohms 50mAThe current for some loads is unknown. For example, a 6v or 12v motor will take more current when starting or when heavily loaded. The starting cur...

  • Page 112

    However if you follow our suggestions, you will be able to achieve the maximum gain:Select a transistor capable of delivering 2, 3 or 5 times more current than is needed for the project you are designing. This will allow it to operate in its HIGH GAIN region because it will not be over-loaded. Fe...

  • Page 113

    Base current supplied via a LOAD resistorIn the circuit above, current is always flowing through the 100R LOAD Resistor. The BC547 is merely diverting the current from the base of the output transistor to the 0v rail. We will not be describing this circuit arrangement. It is much more efficient t...

  • Page 114

    This means 1mA will control (deliver) 1amp (1,000mA) into a LOAD.Here is the PNP-output circuit:In Summary: When designing a circuit, we allow a current-gain of 100 when a transistor is lightly loaded and a gain of 10 when fully loaded. The driver transistor will have a gain of between 100 and ma...

  • Page 115

    The current for a relay can also be provided by a small-signal transistor and the gain can be 100. The base current must come from a previous stageFor low-current requirements, use a BC547 for the driver/output transistor. A BC547 will handle up to 100mA. Similar transistors include: 2N2222A, BC1...

  • Page 116

    on the collector. But the problem with all these circuits above is the high current taken by the stage. As you can see, the load is only 8R and if the transistor is partly turned ON, the idle current will be very high. This can can be reduced by using the following circuits. The output volume wil...

  • Page 117

    between the base voltage and the emitter will be reduced.This will turn the transistor OFF slightly and maintain the original conditions. The second feature of the circuit is the electrolytic in series with the speaker. This allows only the AC portion of the waveform to enter the speaker and the ...

  • Page 118

    A simpler circuit is shown here. The base-bias resistor is chosen so that half-voltage appears on the collector and the transistor is self-biased so transistors with a slightly different gain will operate correctly. This operation is commonly called "Mid-point operation" or "Q-poin...

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    Transistor NOT biasedDiode discharges capacitorThis transistor is not biased AT ALL. However it will work if the incoming signal is large. The signal needs to provide all the energy to turn on the transistor and it only amplifies the positive portions of the waveform. The amplifier is classified ...

  • Page 120

    To make the operating-point of "Class-A" amplifier stable, the base-bias resistor is connected as shown. If the gain of the transistor reduces due to temperature rise, the collector voltage will increase. This will increase the current through the base bias resistor and turn the transis...

  • Page 121

    fully turned ON and the same collector voltage is produced for a wide range of gains. Transistors with internal Resistors Transistor with ResistorsSome transistors have internal resistors. This includes small-signal transistors, power transistors, Darlington and surface mount devices. You must k...

  • Page 122

    Using a Transistor as a CURRENT LIMITER10 Second DelayThe circuit produces a delay of a few seconds due to the TIME DELAYcircuit (made up of the 470k and 100u) taking time to charge the 100u. The transistor is an EMITTER FOLLOWER and the emitter rises at the same rate as the base but is about 0.7...

  • Page 123

    In other words, the load can be connected between the positive and relay to create HIGH-SIDE. (the other terminal of the relay is connected to 0v.) Or it can be connected between the relay and 0v to create LOW-SIDE. (the other terminal of the relay is connected to positive rail.) This will determ...

  • Page 124

    Circuit B is a VOLTAGE DOUBLER. The two circuits appear to be very similar. The component values are the same. We have reversed the 10u and placed the lower diode above the other diode. The operation of the circuit is completely different. When the BC547 turns ON, the 10u charges via the top diod...

  • Page 125

    SUMMARYHere is a summary of the features of the three basic ways a transistor can be connected:CharacteristicCommon EmitterCommon CollectorCommon BaseInput Impedance MediumHighLowOutput ImpedanceHighLowVery HighPhase Angle180o0o0oVoltage GainMedium slightly less than 1HighCurrent GainMediumHighLo...

  • Page 126

    You can see the input impedance of a COMMON BASE stage is equal to the resistance of the emitter resistor it is designed to interface (connect) a low impedance device to the circuit. This is normally very hard to do as the speaker or inductor may be only 8 to 50 ohms. Trying to connect an 8 ohm d...

  • Page 127

    You would think the input is like driving into a 2k2 resistor. But this is not the case. Placing a 2M2 between the base and 5v rail will turn the transistor ON fully. The 2uA via the 2M2 is sufficient to turn the transistor ON. This is obviously nothing like driving into a 2k2 and you should avoi...

  • Page 128

    The situation gets worse when a Darlington pair is connected as a common-collector output. The voltage across the two transistors will be a minimum of 1.35v and will normally be more than 1.6v. Figs E and F show some solutions to driving a load with the most-efficient design.Fig E is called a NPN...

  • Page 129

    The transistor as a VARIABLE RESISTOR SHORT CIRCUIT CURRENT / shoot-through currentHere is one of the major hidden problems when designing a circuit. It's called a SHORT CIRCUIT current or "Shoot-through" current and occurs when two transistors are directly connected together as shown i...

  • Page 130

    This current will also flow through the lower transistor and is WASTED CURRENT. It creates an almost SHORT-CIRCUIT across the supply rails.The solution is to add a low value resistor such as 100R to limit the current. NO CURRENT . . . No ON-OFF switch neededIn all the circuits we have discuss...

  • Page 131

    Thus no current will flow through the circuit when "sitting around." LOGIC PROBE with HIGH, LOW and PULSE The same "cut-off" condition applies to the Logic Probe circuit above. But this time an orange LED is added to the circuit. The Pulse LED is connected to the HIGH sect...

  • Page 132

    CIRCUIT PROBLEMS:CIRCUIT 1The input to a microcontroller needs a HIGH when a microphone picks up audio. This is the requirement from a customer. The circuit in Fig 104 was designed to meet the customers requirements. The 10mV audio waveform from a microphone is converted to a 4v-5v CONSTANT HIGH....

  • Page 133

    Fig 105.One of the most difficult amplifiers to design and service is a DC (Directly-Coupled) amplifier. The voltage on the output is fed back to the input to create the idle (quiescent) state and the biasing of the input is created from the output. So, where do you start?All the facts we have le...

  • Page 134

    At the moment the join of the two one-ohm resistors has a very low voltage on it and the BC547 is also an emitter-follower and the emitter is about 10v minus 0.7v. This puts a current through the 22k resistor of less than 1mA however this current also flows through the emitter-base junction of th...

  • Page 135

    Fig 107.From the theory discussed above, can you see the problem with driving the BC237 in Fig106.It is being pulled HIGH via the 1k resistor. If the transistor has a gain of 100, Q4 will effectively be equal to a 10 ohm resistor. For 100mA current delivered to the output, 1v will be dropped acro...

  • Page 136

    Fig 107b.The high-value collector load also gives the transistor in the first circuit a high possibility to pick up noise on the base and produce pulses on the collector.Fig107b shows a transistor that is turned on via a diode on the base. This is a BAD design. The transistor is said to be in a H...

  • Page 137

    He has a choice of using an emitter-follower or common-emitter amplifier, as shown in the two circuits. The first circuit will only increase the current. The second circuit will increase the current AND the voltage of the waveform and is the best circuit to use.CIRCUIT 8This circuit is a "se...

  • Page 138

    Fig 108.Fig 109.Fig 109 shows the circuit for the trainer and how it can be wired to produce all the stages we have covered in this discussion. By feeding each stage with a sinewave at the input, you can view the output on a CRO and see how it works. This is only part of the picture to understand...

  • Page 139

    CRO. This trainer does not give you a full understanding of the operation of the three stages. (33u and 15v is rarely used in modern designs), I would give it a MISS. Fig 110.Fig 110 shows another trainer. It covers the common-emitter stage. When a common-emitter stage drives a transformer or spe...

  • Page 140

    Fig 111.The 4 circuits in Fig 111 drive an 8 ohm speaker and are called OUTPUT STAGES or DRIVER STAGES. They are all different in performance and have different input voltage requirements. Circuit A is really only a one transistor emitter-follower amplifier as the other transistor discharges the ...

  • Page 141

    Fig 111a.Here is a text book containing a series of questions and answers.:Self Teaching GuideI have included it to show you that even electronics authors who have been in electronics for decades, make mistakes. See the question on base and collector current for a 24v globe. The author has made n...

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  • Page 143

    For the selection of the transistor-type you can click on the button: "Select transistor type from ", and you will see a list of all supported transistor types. TransistorAmp supports some thousand transistor types - even some Germanium transistors. Select your desired transistor type a...

  • Page 144

    Decibel (dB) CalculatorDecibels are defined as ten times the log of a power ratio. This calculator converts between decibels, voltage gain (or current), and power gain. Just fill in one field and the calculator will convert the other two fields.dB= 20log(V1/V2)= 10log(P1/P2)Decibels (dB)Voltage G...