What is Single Stage Transistor Amplifiers – Electronics Tutorial

What is Single Stage Transistor Amplifiers – Electronics Tutorial
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What is Single Stage Transistor Amplifiers – Electronics Tutorial

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    240 Principles of Electronics 10.1 Single Stage TransistorAmplifier 10.2 How Transistor Amplifies? 10.3 Graphical Demonstration ofTransistor Amplifier 10.4 Practical Circuit of TransistorAmplifier 10.5 Phase Reversal 10.6 Input/Output Phase Relationships 10.7 D.C. and A.C. Equivalent Circui...

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    Single Stage Transistor Amplifiers 241Our purpose here will be to discuss single stage transistor amplifier. By a stage we mean asingle transistor with its bias and auxiliary equipment. It may be emphasised here that a practicalamplifier is always a multistage amplifier i.e. it has a number...

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    242 Principles of ElectronicsFor example, consider a sinusoidal signal which increases or decreases the base current by 5 µAin the two half-cycles of the signal. Referring to Fig. 10.2, it is clear that in the absence of signal, thebase current is 10μA and the collector current is 1 mA. ...

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    Single Stage Transistor Amplifiers 243fication to the next stage. If it is not used, the bias conditions of the next stage will be drasticallychanged due to the shunting effect of RC. This is because RC will come in parallel with the upperresistance R1 of the biasing network of the next sta...

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    244 Principles of ElectronicsFig. 10.4ECX=10ERIn the given problem, fmin = 2kHz ; RE = 560Ω.∴10 ECX= 560orECX= 560/10 = 56Ωor12minEfCπ=56∴CE =3112562(2 10 ) 56minf=ππ××× = 1.42 × 10– 6 F = 1.42 μμμμμFNote. While discussing CE amplifier, the reader should be very particu...

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    Single Stage Transistor Amplifiers 245Fig. 10.5When the signal voltage increases in the positive half-cycle, the base current also increases. Theresult is that collector current and hence voltage drop iC RC increases. As VCC is constant, therefore,output voltage vCE decreases. In other wor...

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    246 Principles of Electronicsput and output voltages are in phase opposition or equivalently, the transistor is said to produce a180º phase reversal of output voltage w.r.t. signal voltage.Note. No phase reversal of voltage occurs in common base and common collector amplifier. The a.c.outp...

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    Single Stage Transistor Amplifiers 247(c) Output current is 180° out of phase with the output voltage (vC). Therefore, input voltageand output voltage are 180° out of phase.10.6 Input/Output Phase RelationshipsThe following points regarding the input / output phase relationships between cur...

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    248 Principles of ElectronicsApplying these two steps to the circuit shown in Fig. 10.8, we getthe d.c. equivalent circuit shown in Fig. 10.9. We can easily calculatethe d.c. currents and voltages from this circuit.(ii) A.C. equivalent circuit. In the a.c. equivalent circuit of atransist...

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    Single Stage Transistor Amplifiers 249d.c. load = RC + REReferring to the a.c. equivalentcircuit shown in Fig. 10.10, it is clearthat as far as a.c. signal is concerned,resistance RC appears in parallelwith RL. In other words, transistoramplifier sees an a.c. load equal toRC || RL i.e.a.c. l...

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    250 Principles of ElectronicsConsider the transistor amplifier shown in Fig. 10.12. In the absence of signal, d.c. conditionsprevail in the circuit as shown in Fig. 10.13 (i). Referring to this circuit and applying Kirchhoff’svoltage law,VCE = VCC − IC RC − IE REorVCE = VCC − IC ...

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    Single Stage Transistor Amplifiers 251The value of IC will be maximum when VCE = 0.∴Max. IC =CCCEVRR+This locates the second point A (OA = VCC/RC + RE) of the d.c. load line. By joining points A andB, d.c. load line AB is constructed [See Fig. 10.13 (ii)].Alternatively. The two end points...

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    252 Principles of Electronics Maximum collector current = IC + CEACVRwhereRAC = RC || RL = CLCLRRRR+This locates the point D of a.c. load line on the collector-current axis. By joining points C and D,the a.c.load line CD is constructed [See Fig. 10.14 (ii)].Example 10.5. For the trans...

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    Single Stage Transistor Amplifiers 253(ii) Operating point Q. The voltage across R2 (= 5 kΩ ) is *5 V i.e. V2 = 5 V. NowV2 = VBE + IE RE∴IE =2(5 0.7)2kBEEVVVR−−=Ω = 2.15 mA∴IC = IE = 2.15 mA Now VCE = VCC...

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    254 Principles of ElectronicsThis locates the point C (OC = 2.33 mA) on the iC axis. By joining points C and D, a.c. load lineCD is constructed (See Fig. 10.17).Fig. 10.17Comments. The reader may see that the operating point lies on both a.c. and d.c. load lines. It isnot surprising beca...

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    Single Stage Transistor Amplifiers 255Solution. Fig. 10.18 shows the whole process. When no signal is applied, vCE = 8 V and iC =1mA. This is represented by the operating point Q on the a.c. load line. During the positive half-cycle of a.c. signal, iC swings from 1 mA to 1.5 mA and vCE sw...

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    256 Principles of ElectronicsExample 10.8. In the circuit shown in Fig. 10.20, find the voltage gain. Given thatβ = 60 and input resistance Rin = 1 kΩ.Fig. 10.20Solution. So far as voltage gain of the circuit is concerned, we need only RAC, β and Rin.Effective load, RAC = RC || RL...

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    Single Stage Transistor Amplifiers 257(ii)Input impedance, Rin =0.0210BEBVVIAΔ=Δμ = 2 kΩΩΩΩΩ(iii)a.c. load, RAC =510510CLCLRRRR××=++ = 3.3 kΩΩΩΩΩ(iv)Voltage gain, Av =3.31002ACinRRβ×=× = 165(v)Power gain, Ap = current gain × voltage gain = 100 × 165 = 16500Examp...

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    258 Principles of ElectronicsFig. 10.22The circuit is not operating properly. It is because the voltage at point B should be 2 V instead of4 V.10.10 A.C. Emitter ResistanceThe ac or dynamic resistance of emitter-base junction diode of a tran-sistor is called ac emitter resistance. It is d...

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    Single Stage Transistor Amplifiers 259Fig. 10.24Solution. Voltage across R2, V2=21210102 V40 10CCVRRR×=×=++Voltage across RE, VE= V2 − VBE = 2 − 0.7 = 1.3 VEmitter current, IE=1.30.65 mA2kEEVVR==Ω∴AC emitter resistance, re′=25 mV25 mV0.65 mAEI== 38.46ΩΩΩΩΩ10.12 ...

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    260 Principles of ElectronicsNote that emitter bypass capacitor CE is connected in parallel with emitter resistance RE. Thecapacitor CE acts as a *short to the a.c. signal so that it bypasses the a.c. signal to the ground. There-fore, the emitter is effectively at a.c. ground. It is importa...

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    Single Stage Transistor Amplifiers 261Derivation. Fig. 10.25 shows the common emitter amplifier. The ac equivalent circuit of theamplifier is shown in Fig. 10.26. (i). Replacing the transistor by its *equivalent circuit, we get thecircuit shown in Fig. 10.26 (ii). Note that current source ...

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    262 Principles of Electronics○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○*Note that CC behaves as a short for a.c. and is replaced by a wire in the two a.c. circuits.Fig. 10.29CE amplif...

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    Single Stage Transistor Amplifiers 263In order to find D.C. IE, we shall proceed as under :D.C. voltage across R2, V2 =21220202.35 V150 20CCVRRR×=×=++D.C. voltage across RE, VE= V2 − VBE = 2.35 − 0.7 = 1.65 V∴D.C. emitter current, IE=1.650.75 mA2.2 kEEVVR==Ω∴ AC emit...

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    264 Principles of ElectronicsFig. 10.31(ii)Voltage gain, Av=20 kΩ250 ΩCeRr=′= 80(iii) The d.c. voltage across input capacitor is equal to the d.c. voltage at the base of the transistorwhich is V2 = 1V. Therefore, d.c. voltage across Cin is 1V.Similarly, d.c. voltage across CE = d.c v...

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    Single Stage Transistor Amplifiers 265D.C. collector current, IC= IE = 2.3 mAD.C. base current, IB= IC/β = 2.3 mA/100 = 0.023 mAD.C. collector voltage, VC= VCC – IC RC= 15V – 2.3 mA × 2 kΩ = 10.4VTherefore, all d.c. bias levels stand calculated.(ii)D.C. voltage across Cin= V2 = 3VD.C. ...

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    266 Principles of ElectronicsFig. 10.34* Zin(base) = .ine ebbVi rii′= Since ebii is approximately equal to β, Zin (base) = β re′.○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○(i...

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    Single Stage Transistor Amplifiers 267Voltage across RE, VE= V2 − VBE = 7.5 − 0.7 j 7.5 VEmitter current, IE=7.5 V1mA7.5 kEEVR==ΩAC emitter resistance, r′e= 25 mV/IE = 25 mV/1 mA = 25 ΩZin (base)=β r′e = 200 × 25 = 5 × 103 Ω = 5 kΩ∴Zin= R1 || R2 || Zin (base)= ...

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    268 Principles of ElectronicsRE1 is not. This method swamps or minimises the effect of er′ on the voltage gain without reducing thevoltage gain too much. Now the total a.c. emitter resistance is (er′ + RE1) instead of er′ as in a standardCE amplifier. Therefore, the voltage gain of a ...

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    Single Stage Transistor Amplifiers 269*Original Av = CeRr′ ; Final Av = 2CeRr′ . Obviously, a change of 50% from the original value.** Remember that in a standard CE amplifier, the emitter resistance RE is completely bypassed by the capaci-tor CE.○○○○○○○○○○○○○○...

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    270 Principles of ElectronicsFig. 10.39Fig. 10.40Solution. Both the circuits have the same value of a.c. emitter resistance er′ . Therefore, follow-ing the standard procedure for finding er′ gives us a value of *25Ω for both circuits.(i) Zin (base)For the standard CE amplifier shown i...

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    Single Stage Transistor Amplifiers 271Av =14kΩ25 Ω +210 ΩCeERrR=′ +4kΩ235 Ω= = 17The following points may be noted ;(i) The two circuits are identical for d.c. analysis purposes. Both have a total of 1.1. kΩ d.c.resistance in their emitter circuits.(ii) For a standard CE amplifie...

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    272 Principles of Electronics(iv) It means that power amplification is being done, coupling is by transformer and mode ofoperation is class A.10.19 Amplifier Equivalent CircuitAn amplifier can be replaced by an equivalent circuit for the purpose of analysis. Fig. 10.41 (i) showsthe amplifie...

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    Single Stage Transistor Amplifiers 273Fig. 10.42Current gain, Ai=021inoutLARIIRR=+Power gain, Ap=22222111()()LLininIRI RIIRIIR==22221111VIVIVIVI⎛⎞ ⎛ ⎞=×⎜⎟ ⎜ ⎟⎝⎠ ⎝ ⎠= Av × AiNote. The use of such an equivalent circuit is restricted to the signal quantities only. Fur...

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    274 Principles of ElectronicsNowV1 = I1 Rin = (1.25 × 10−6) × 2000 = 2.5 × 10−3 V = 2.5 mVExample 10.27. An amplifier has an open circuit voltage gain of 1000, an output resistance of15Ω and an input resistance of 7kΩ. It is supplied from a signal source of e.m.f. 10mV and...

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    Single Stage Transistor Amplifiers 275Fig. 10.45I1 = V1/Rin = 12.5 mV/3 kΩ = 4.17 µAPower gain = Av × Ai = 80 × 120 = 960010.21 Gain and Transistor ConfigurationsWe know that the process of raising the strength of an a.c. signal is called amplification and the circuitused to pref...

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    276 Principles of Electronicsinput voltage is 10 V, then its input imped-ance is .........(i) 20 kΩ(ii) 0.2 kΩ(iii) 10 kΩ(iv)5 kΩ11. When a transistor amplifier is operating, thecurrent in any branch is ........(i) sum of a.c. and d.c.(ii) a.c. only(iii) d.c. only(iv) difference of a.c...

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    Single Stage Transistor Amplifiers 277(iii) signal will not reach the base(iv) none of the above29. The radio wave picked up by the receivingantenna is amplified about ........ times tohave reasonable sound output.(i) 1000(ii) a million(iii) 100(iv) 1000030. A CE amplifier is also called ......

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    278 Principles of Electronics3. Draw the circuit of a practical single stage transistor amplifier. Explain the function of each compo-nent.4. Show the various currents and voltages in a single stage transistor amplifier.5. Show that the output voltage of a single stage common emitter transi...

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