What is Transistors – Electronics Tutorial

What is Transistors – Electronics Tutorial
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What is Transistors – Electronics Tutorial

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    Transistors8.1 Transistor8.3 Some Facts about the Transistor8.5 Transistor Symbols8.7 Transistor Connections8.9 Characteristics of Common BaseConnection8.11 Measurement of LeakageCurrent8.13 Common Collector Connection8.15 Commonly Used TransistorConnection8.17 Transistor Load Line Analysis8.19 P...

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    142 Principles of ElectronicsInvented in 1948 by J. Bardeen and W.H. Brattain of Bell Telephone Laboratories, U.S.A.; tran-sistor has now become the heart of most electronic applications. Though transistor is only slightlymore than 58 years old, yet it is fast replacing vacuum tubes in alm...

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    Transistors 1438.2 Naming the Transistor TerminalsA transistor (pnp or npn) has three sections of doped semiconductors. The section on one side is theemitter and the section on the opposite side is the collector. The middle section is called the base andforms two junctions between the emitt...

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    144 Principles of ElectronicsFig. 8.3(iii) The transistor has two pn junctions i.e. it is like two diodes. The junction between emitterand base may be called emitter-base diode or simply the emitter diode. The junction between thebase and collector may be called collector-base diode or si...

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    Transistors 145Fig. 8.4(ii) Working of pnp transistor. Fig. 8.5 shows the basic connection of a pnp transistor. Theforward bias causes the holes in the p-type emitter to flow towards the base. This constitutes theemitter current IE. As these holes cross into n-type base, they tend to co...

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    146 Principles of Electronicsthe amplifying capability of the transistor. We shall discuss the amplifying property of transistor laterin this chapter.Note. There are two basic transistor types : the bipolar junction transistor (BJT) and field-effect transistor (FET). As we shall see, these...

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    Transistors 147shown. This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps theinput circuit forward biased regardless of the polarity of the signal.As the input circuit has low resistance, therefore, a small change in signal voltage causes anappreciable ...

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    148 Principles of ElectronicsInput current, IE = 500 mVSignal20inR=Ω = 25 mA. Since αac is nearly 1, output current, IC = IE =25 mA.Output voltage, Vout = IC RC = 25 mA × 1 kΩ = 25 V∴Voltage amplification, Av =25signal500outVVmV= = 50Comments. The reader may note that bas...

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    Transistors 149*α =CEIIΔΔ at constant VCBIt is clear that current amplification factor is less than **unity.This value can be increased (but not more than unity) by decreasingthe base current. This is achieved by making the base thin anddoping it lightly. Practical values of α in comme...

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    150 Principles of ElectronicsFig. 8.11Example 8.2. In a common base connection, IE = 1mA, IC = 0.95mA. Calculate the value of IB.Solution. Using the relation,IE= IB + ICor1 = IB + 0.95∴IB=1 − 0.95 = 0.05 mAExample 8.3. In a common base connection, current amplification factor is 0...

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    Transistors 151Fig. 8.13∴IE=11.05 mA0.95CI==αUsing the relation, IE= IB + IC∴IB= IE − IC = 1.05 − 1= 0.05 mAExample 8.7. For the common base circuit shown inFig. 8.13, determine IC and VCB. Assume the transistor tobe of silicon.Solution. Since the transistor is of silicon, VBE ...

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    152 Principles of Electronicschange in emitter current (ΔIE) at constant collector-base voltage (VCB) i.e.Input resistance, ri = BEEVIΔΔ at constant VCBIn fact, input resistance is the opposition offered to the signal current. As a very small VEB issufficient to produce a large flow of...

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    Transistors 153Fig. 8.161.Base current amplification factor ( βββββ). In common emitter connection, input current is IBand output current is IC.The ratio of change in collector current (ΔIC) to the change in base current (ΔIB) is known asbase current amplification factor i.e.β* =CB...

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    154 Principles of Electronics2.Expression for collector current. In common emitter circuit, IB is the input current and ICis the output current.We know IE= IB + IC...(i)andIC=α IE + ICBO...(ii)From exp. (ii), we get,IC=α IE + ICBO = α (IB + IC) + ICBOorIC (1 − α)= α IB + ICBOorI...

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    Transistors 155(IB = 0), the transistor is in cut off. Ideally, IC = 0 but actually there is a small current from collector toemitter due to minority carriers. It is called ICEO (collector-to-emitter current with base open). Thiscurrent is usually in the nA range for silicon. A faulty transi...

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    156 Principles of ElectronicsExample 8.11. For a transistor, β = 45 and voltagedrop across 1kΩ which is connected in the collector circuitis 1 volt. Find the base current for common emitter connec-tion.Solution. Fig. 8.21 shows the required common emit-ter connection. The voltage drop...

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    Transistors 157Solution. When the emitter circuit is open [See Fig. 8.23 (i)], the collector-base junction isreverse biased. A small leakage current ICBO flows due to minority carriers.∴ICBO = 0.2 µA...givenWhen base is open [See Fig. 8.23 (ii)], a small leakage current ICEO flows due t...

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    158 Principles of ElectronicsFig. 8.24Fig. 8.25Example 8.17 Determine VCB in the transistor * circuit shown in Fig. 8.26 (i). The transistor isof silicon and has β = 150.Fig. 8.26Solution. Fig. 8.26 (i) shows the transistor circuit while Fig. 8.26 (ii) shows the various currentsand volta...

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    Transistors 159IC=α IE = (0.9977) (30 mA) = 29.93 mAAlsoIC=β IB = (440) (68 μA) = 29.93 mAExample 8.19. A transistor has the following ratings : IC (max) = 500 mA and βmax = 300.Determine the maximum allowable value of IB for the device.Solution.IB (max) =()500 mA300CmaxmaxI=β = 1.67 m...

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    160 Principles of ElectronicsFig. 8.28Solution. Fig. 8.28 shows the short circuit failures in a transistor. We shall discuss the circuitbehaviour in each case.(i) Collector-emitter short. Fig. 8.28 (i) shows a short between collector and emitter. Theemitter diode is still forward biased, ...

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    Transistors 1611.Input characteristic. It is the curve between base current IB and base-emitter voltage VBEat constant collector-emitter voltage VCE.The input characteristics of a CE connection can be determined by the circuit shown in Fig. 8.29.Keeping VCE constant (say at 10 V), note the ...

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    162 Principles of Electronicscurrent IC changes with VCE is called the knee voltage (Vknee). The transistors are always operated inthe region above knee voltage.(ii) Above knee voltage, IC is almost constant. However, a small increase in IC with increasingVCE is caused by the collector d...

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    Transistors 163NowIE= IB + ICorΔIE=ΔIB + ΔICorΔIB=ΔIE – ΔICSubstituting the value of ΔIB in exp. (i), we get,γ =EECIIIΔΔ− ΔDividing the numerator and denominator of R.H.S. by ΔIE, we get,γ =11EECEEEIIIIIIΔΔ=ΔΔ−α−ΔΔαCEIIΔ⎛⎞=⎜⎟Δ⎝⎠Q∴γ =11−α(ii) ...

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    164 Principles of Electronics(i) CB Circuit. The input resistance (ri) of CB circuit is low because IE is high. The outputresistance (ro) is high because of reverse voltage at the collector. It has no current gain (α < 1) butvoltage gain can be high. The CB circuit is seldom used. The ...

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    Transistors 165signal, the forward bias across emitter-base junction is decreased. Therefore, collector current de-creases. This results in the decreased output voltage (in the opposite direction). Hence, an amplifiedoutput is obtained across the load.Fig. 8.33Fig. 8.34Analysis of collecto...

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    166 Principles of Electronicsd.c. load line. Consider a common emitter npn transistor circuit shown in Fig. 8.35 (i) where nosignal is applied. Therefore, d.c. conditions prevail in the circuit. The output characteristics of thiscircuit are shown in Fig. 8.35 (ii).The value of collector-...

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    Transistors 167and is equal to VCC. For any other value of collector current say OC, the collector-emitter voltage VCE= OD. It follows, therefore, that load line gives a far more convenient and direct solution to theproblem.Note. If we plot the load line on the output characteristic of the...

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    168 Principles of ElectronicsWhen VCE= 0, then,IC= VCC/RC = 12.5 V/2.5 kΩ = 5 mAThis locates the point A of the load line on the collector current axis. By joining these two points,we get the d.c. load line AB as shown in Fig. 8.38 (ii).Example 8.23. In the circuit diagram shown ...

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    Transistors 169Fig 8.40Fig. 8.41Solution. The presence of resistor RB in the base circuit should not disturb you because we canapply Kirchhoff’s voltage law to find the value of IB and hence IC (= βIB). Referring to Fig. 8.40 andapplying Kirchhoff’s voltage law to base-emitter loop, we...

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    170 Principles of ElectronicsSolution. The transistor circuit shown in Fig. 8.42 may look complex but we can easily applyKirchhoff’s voltage law to find the various voltages and currents in the * circuit.Applying Kirchhoff’s voltage law to the base-emitter loop, we have,– IB RB – V...

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    Transistors 171Solution. Refer to Fig. 8.44.(i) The emitter voltage w.r.t. ground isVE = – VEE + IE RE = – 10V + 1.8 mA × 4.7 kΩ = – 1.54V(ii) The base voltage w.r.t. ground isVB = VE + VBE = 10V + 0.7V = 10.7V(iii) The collector voltage w.r.t. ground isVC = VCC – IC RC = 10V – ...

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    172 Principles of ElectronicsFrom exps. (i) and (ii), it is clear that magnitude of output is the same whether we take outputacross collector load or terminals 1 and 2. The minus sign in exp. (ii) simply indicates the phasereversal. The second method of taking output is used in multistage...

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    Transistors 173 = *RC || Ri = CiCiRRRR+As input resistance Ri is quite small (25 Ω to 500 Ω), therefore, effective load is reduced.(iv) Current gain. It is the ratio of change in collector current (ΔIC) to the change in basecurrent (ΔIB) i.e.Current gain, β = CBIIΔΔThe ...

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    174 Principles of ElectronicsΔVBE= 200 mVChange in base current, ΔIB= 100 µA∴ Input resistance, Ri= 200 mV100 µ ABEBVIΔ=Δ = 2 kΩΩΩΩΩExample 8.29. If the collector current changes from 2 mA to 3mA in a transistor when collec-tor-emitter voltage is increased from ...

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    Transistors 175IC (sat) j ();CCCECE satkneeCVVVVR==If base current is greater than IB(sat), then collector current cannot increase because collector-basejunction is no longer reverse-biased.(iii) Active region. The region between cut off and saturation is known as active region. In theac...

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    176 Principles of ElectronicsAs we increase RB, base current and hence collector current decreases. This decreases the volt-age drop across RC. This increases the collector-emitter voltage. Eventually, when IB = 0, the emitter-base junction is no longer forward biased and transistor acti...

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    Transistors 177We have VCE (off ) when IC = 0. Therefore, putting IC = 0 in eq. (i), we have,VCE (off) = VCC + VEE = 12 + 12 = 24VWe have IC (sat) when VCE = 0.∴IC ( sat ) =(12 12)(750 1500)CCEECEVVVRR++=++Ω = 10.67 mAExample 8.33. Determine whether or not the transistor in Fig. 8.54 is ...

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    178 Principles of ElectronicsSolution.IC =βIB = (100)(100 μA) = 10 mAVCE = VCC – IC RC= 10V – (10 mA)(970Ω) = 0.3VLet us relate the values found to the transistor shown in Fig. 8.56.As you can see, the value of VBE is 0.95V and the value of VCE = 0.3V.This leaves VCB of 0.65V (Note t...

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    Transistors 179Collector voltage, VC = VCC – IC RC= 10V – 2 mA×2 kΩ = 10V – 4V = 6VSince VC (= 6V) is greater than VE (= 2V), the transistor isactive. Therefore, our assumption that transistor is active is cor-rect.(ii) When RC = 4 kΩΩΩΩΩ. Suppose the transistor is active.∴IC...

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    180 Principles of ElectronicsAssuming the transistor is active, we have,IC = IE = 2.3 mA ; IB = IC/β = 2.3/100 = 0.023 mACollector voltage, VC = VCC – IC RC= 15V – 2.3 mA × 10 kΩ = 15V – 23V = – 8VSince VC < VE, the transistor is saturated and our assumption is not correct.8.2...

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    Transistors 181Fig. 8.60Example 8.38. The maximum power dissipation of a transistor is 100mW. If VCE = 20V, what isthe maximum collector current that can be allowed without destruction of the transistor?Solution.PD (max) = VCE × IC (max)or100 mW = 20 V × IC (max)∴IC (max) = 100 mW20 V= 5...

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    182 Principles of ElectronicsReferring to Fig. 8.62 and applying Kirchhoff’s voltage law to the base side, we have,VBB – IB RB – VBE = 0∴IB =10.70.31010BBBEBVVVVVRkk−−==ΩΩ = 0.03 mANowIC =βIB = 100 × 0.03 = 3 mA∴VCE = VCC – IC RC = 5V – 3 mA × 1 kΩ = 5V – 3V = 2V...

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    Transistors 183applied to the base and the a.c output is taken from the collector, then common terminal is the emitter.Hence the transistor is connected in common emitter configuration. If the a.c. input is applied to thebase and a.c output is taken from the emitter, then common terminal is ...

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    184 Principles of Electronics(ii) When the leads of a transistor are in the same plane but evenly spaced [See Fig. 8.64 (ii)],the central lead is the base, the lead identified by dot is the collector and the remaining lead is theemitter.(iii) When the leads of a transistor are spaced around...

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    Transistors 185would increase the voltage without increasing the current. This is illustrated in Fig. 8.66. The CBamplifier will provide voltage gain without any current gain.Fig. 8.66Fig. 8.67(ii) For impedance matching in high frequency applications. Most high-frequency voltagesources h...

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    186 Principles of Electronics8.29 Transistors Versus Vacuum TubesAdvantages of transistorsA transistor is a solid-state device that performs the same functions as the grid-controlled vacuumtube. However, due to the following advantages, the transistors have upstaged the vacuum tubes inmost ...

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    Transistors 187MULTIPLE-CHOICE QUESTIONS1.A transistor has ........(i) one pn junction(ii) two pn junctions(iii) three pn junctions(iv) four pn junctions2. The number of depletion layers in a transis-tor is ........(i) four(ii) three(iii) one(iv) two3. The base of a transistor is ....... dop...

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    188 Principles of Electronics22. The value of β for a transistor is generally........(i)1(ii) less than 1(iii) between 20 and 500(iv) above 50023. The most commonly used transistor arrange-ment is ........ arrangement.(i) common emitter(ii) common base(iii) common collector(iv) none of the...

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    Transistors 189....... that in CB arrangement.(i) more than(ii) less than(iii) the same as(iv) none of the above43. A heat sink is generally used with a transis-tor to ........(i) increase the forward current(ii) decrease the forward current(iii) compensate for excessive doping(iv) prevent e...

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    190 Principles of ElectronicsProblems1. In a transistor if IC = 4.9mA and IE = 5mA, what is the value of α ?[0.98]2. In a transistor circuit, IE = 1mA and IC = 0.9mA. What is the value of IB ?[0.1 mA]3. Find the value of β if α = 0.99.[100]4. In a transistor, β = 45, the voltage across...

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    Transistors 191Fig. 8.7212. Determine the intercept points of the d.c. load line on the vertical and horizontal axes of thecollector curves in Fig. 8.72.[2 mA ; 20 V]13. For the circuit shown in Fig. 8.73, find (i) the state of the transistor and (ii) transistor power.[(i) active (ii) 4.52 m...