What is Transistor Biasing – Electronics Tutorial

What is Transistor Biasing – Electronics Tutorial
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What is Transistor Biasing – Electronics Tutorial

  • Page 1

    192 Principles of Electronics 9.1 Faithful Amplification 9.2 Transistor Biasing 9.3 Inherent Variations of TransistorParameters 9.4 Stabilisation 9.5 Essentials of a TransistorBiasing Circuit9.6Stability Factor9.7Methods of Transistor Biasing9.8Base Resistor Method9.9Emitter Bias Circuit9.1...

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    Transistor Biasing 1939.1 Faithful AmplificationThe process of raising the strength of a weak signal without any change in its general shape is knownas faithful amplification.The theory of transistor reveals that it will function properly if its input circuit (i.e. base-emitterjunction) remai...

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    194 Principles of ElectronicsZero signal collector current ≥ Max. collector current due to signal aloneIllustration. Suppose a signal applied to the base of a transistor gives a peak collector current of1mA. Then zero signal collector current must be atleast equal to 1mA so that even ...

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    Transistor Biasing 195Fig. 9.4When VCE is too low (less than 0.5V for Ge transistors and 1V for Si transistors), the collector-base junction is not properly reverse biased. Therefore, the collector cannot attract the charge carri-ers emitted by the emitter and hence a greater portion of them...

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    196 Principles of ElectronicsFig. 9.5Thus, the maximum collector current allowed during any part of the signal is 2 mA. If thecollector current is allowed to rise above this value, VCE will fall below 1 V. Consequently, value ofβ will fall, resulting in unfaithful amplification. (ii) Dur...

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    Transistor Biasing 197This large variation in parameters is a characteristicof transistors. The major reason for these variationsis that transistor is a new device and manufacturingtechniques have not too much advanced. Forinstance, it has not been possible to control thebase width and it ma...

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    198 Principles of ElectronicsThe collector leakage current ICBO is strongly dependent on temperature. The flow of collectorcurrent produces heat within the transistor. This raises the transistor temperature and if no stabilisationis done, the collector leakage current ICBO also increases....

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    Transistor Biasing 1999.7 Methods of Transistor BiasingIn the transistor amplifier circuits drawn so far biasing was done with the aid of a battery VBB whichwas separate from the battery VCC used in the output circuit. However, in the interest of simplicityand economy, it is desirable that t...

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    200 Principles of ElectronicsIt may be noted that VCC is a fixed known quantity and IB is chosen at some suitable value. Hence,RB can always be found directly, and for this reason, this method is sometimes called fixed-bias method.Stability factor. As shown in Art. 9.6,Stability factor, ...

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    Transistor Biasing 201In the circuit shown in Fig. 9.7 (i), biasing is provided by a battery VBB (= 2V) in the base circuitwhich is separate from the battery VCC (= 9V) used in the output circuit. The same circuit is shown ina simplified way in Fig. 9.7 (ii). Here, we need show only the su...

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    202 Principles of ElectronicsOperating point Q. As it is a silicon transistor, therefore, VBE = 0.7V. Referring to Fig. 9.8 (i),it is clear that :IB RB + VBE= VCCorIB=(60.7) V530 kCCBEBVVR−−=Ω = 10 µA∴Collector current, IC=β IB = 100 × 10 = 1000 µA = 1 mA ...

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    Transistor Biasing 203Fig. 9.9VCC= IB RB + VBE + IE × 1or10 = 1000 IB + *0 + (IC + IB)or10 = 1000 IB + (β IB + IB)or10 = 1000 IB + (100 IB + IB)or10 = 1101 IB∴IB= 10/1101 = 0.0091 mAIC=β IB = 100 × 0.0091 = 0.91 mAIE= IC + IB = 0.91 + 0.0091 = 0.919 mAExample 9.7. Design base...

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    204 Principles of Electronics∴RB=(150.6) V0.02 mACCBEBVVI−−= = 720 kΩΩΩΩΩExample 9.8. A *base bias circuit in Fig. 9.11is subjected to an increase in temperature from 25°Cto 75°C. If β = 100 at 25°C and 150 at 75°C, deter-mine the percentage change in Q-point values (VCE...

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    Transistor Biasing 205It is clear that decrease in VBE increases IB. This will shift theQ-point (IC = βIB and VCE = VCC – IC RC). The effect of change inVBE is negligible if VCC >> VBE (VCC atleast 10 times greater thanVBE).(ii) Effect of ICBO. The reverse leakage current ICBO has t...

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    206 Principles of ElectronicsFig. 9.149.9 Emitter Bias CircuitFig. 9.15 shows the emitter bias circuit. This circuit differs from base-bias circuit in two importantrespects. First, it uses two separate d.c. voltage sources ; one positive (+ VCC) and the other negative(– VEE). Normally, t...

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    Transistor Biasing 207NowIC j IE and IC =βIB∴ IB j EIβ Putting IB = IE/β in the above equation, we have,VEE = EI⎛⎞⎜⎟β⎝⎠ RB + IE RE + VBEorVEE – VBE = IE (RB/β + RE)∴IE =/EEBEEBVVRR−+βSince IC j IE, we have,IC =/EEBEEBVVRR−+β(ii) Collector-emitter ...

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    208 Principles of ElectronicsIt is clear that IC is dependent on VBE and β, both of which change with temperature.If RE >> RB/β, then expression for IC becomes :IC = EEBEEVVR−This condition makes IC (j IE) independent of β.If VEE >> VBE, then IC becomes :IC (j IE)=EEEVRThis...

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    Transistor Biasing 209∴VCE= VC – VE = 11.7 – (– 2.4) = 14.1V % age change in IC=1.76 mA – 1.73 mA× 1001.73 mA = 1.7% (increase)% age change in VCE =14.1V – 14.6V10014.1V× = – 3.5% (decrease)9.11 Biasing with Collector Feedback ResistorIn this method, one end of RB is connect...

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    210 Principles of ElectronicsSuppose the temperature increases. This will increase collector leakage current and hence thetotal collector current. But as soon as collector current increases, VCE decreases due to greater dropacross RC. The result is that VCB decreases i.e. lesser voltage ...

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    Transistor Biasing 211RC=(128) V4k1mACCCECVVI−−==Ω Also IB=1mA0.01 mA100CI==βUsing the relation, RB=CCBEBCBVVI RI−− β=120.3 10040.01−−× 0.01 × = 770 kΩΩΩΩΩ(ii) Now β = 50, and other circuit values remain the same.∴VCC= VBE + ...

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    212 Principles of ElectronicsNowIB + IC j IC ; IE j IC and IB = CIβ∴VCC – IC RC – CIβ RB – VBE – IC RE = 0orIC (RE + BRβ + RC) = VCC – VBE∴IC = /CCBEEBCVVRRR−+β +Putting the given circuit values, we have,IC =12V – 0.7V1kΩ + 400 kΩ/100 + 4 kΩ=11.3V9kΩ = 1.26 mAVC...

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    Transistor Biasing 213emitter junction. This causes the base current and hence collector current flow in the zero signalconditions.Fig. 9.24Circuit analysis. Suppose that the current flowing through resistance R1 is I1. As base currentIB is very small, therefore, it can be assumed with re...

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    214 Principles of Electronics= IC RC + VCE + IC RE(ä IE j IC)= IC (RC + RE) + VCE∴VCE= VCC − IC (RC + RE)Stabilisation. In this circuit, excellent stabilisation is provided by RE. Consideration of eq. (i)reveals this fact.V2 = VBE + IC RESuppose the collector current IC increases du...

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    Transistor Biasing 215Fig. 9.25Operating point. For silicon transistor,VBE = 0.7 VVoltage across 5 kΩ isV2 =15 55105105CCV××=++ = 5 V∴ Emitter current, IE =250.74.3 V2k2kBEEVVR−−==ΩΩ = 2.15 mA∴ Collector current isIC j IE = 2.15 mA Collector-emitter voltage, VCE...

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    216 Principles of ElectronicsFig. 9.26Referring to Fig. 9.26 (ii), we have,E0 = IB R0 + VBE + IE RE = IB R0 + VBE + IC RE (ä IE j IC)= IB R0 + VBE + β IB RE = IB (R0 + β RE) + VBEorIB=00BEEEVRR−+β∴ Collector current, IC=()00BEBEEVIRRβ−β=+βDividing the numerator and de...

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    Transistor Biasing 217(ii) When VBE = 0.3 V,Collector current, IC=220.31kBEEVVR−−=Ω = 1.7 mAComments. From the above example, it is clear that although VBE varies by 300%, the value ofIC changes only by nearly 10%. This explains that in this method, IC is almost independent of tran...

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    218 Principles of Electronicsor0 =BCdIdI (R0 + RE) + RE∴BCdIdI=0EERRR−+...(i)Fig. 9.28The general expression for stability factor is Stability factor, S =11BCdIdIβ+−βPutting the value of dIB/dIC from eq. (i) into the expression for S, we have,S =0011–11EEEERRRRRRβ+β+=β⎛...

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    Transistor Biasing 219R0 can be obtained by making R2 very small. But with low value of R2, current drawn from VCC willbe large. This puts restriction on the choice of R0. Increasing the value of RE requires greater VCC inorder to maintain the same zero signal collector current. Due to these ...

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    220 Principles of ElectronicsIC =2BEEVVR− where V2 = 212CCVRRR×+andVCE = VCC – IC (RC + RE)However, by replacing the potential divider circuit by Thevenin’s equivalent circuit, the expres-sion for IC can be found more accurately. If not mentioned in the problem, any one of the two...

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    Transistor Biasing 2219.14 Design of Transistor Biasing Circuits(For low powered transistors)In practice, the following steps are taken to design transistor biasing and stabilisation circuits :Step 1. It is a common practice to take RE = 500 − 1000Ω. Greater the value of RE, better is th...

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    222 Principles of ElectronicsorVCC= IC RC + VCE + IC RE(ä IC j IE)or9 = 2 mA × 2.2 kΩ + 3 + 2 mA × RE∴RE= 94.432−− = 0.8 kΩ = 800 ΩΩΩΩΩVoltage across R2, V2 = VBE + VE = 0.3 + 2 mA × 0.8 kΩ= 0.3 + 1.6 = 1.9 V∴Resistance R2 = V2/I1 = 1.9 V/0.4 mA = 4.75 kΩ...

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    Transistor Biasing 223Fig. 9.33R0 =121210 55010515RRRR×==++ = 3.3 kΩApplying Kirchhoff’s voltage law to the base-emitter loop [See Fig. 9.33 (ii)],E0 = IB R0 + VBE + IE RESince IE j IC, therefore, IB = IE /β.∴E0 =0EBEEEIRVI R++β=0EBEERIVR⎛⎞ ++⎜⎟β⎝⎠∴IE =0050.73.32100...

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    224 Principles of ElectronicsFig. 9.34Example 9.29. Fig. 9.35 shows the potential divider method of biasing. What will happen if(i) resistance R2 is shorted(ii) resistance R2 is open-circuited(iii) resistance R1 is shorted(iv) resistance R1 is open ?Solution. (i) If resistance R2 is sho...

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    Transistor Biasing 2259.15 Mid-Point BiasingWhen an amplifier circuit is so designed thatoperating point Q lies at the centre of d.c. loadline, the amplifier is said to be midpoint bi-ased. When the amplifier is mid-point biased,the Q-point provides values of IC and VCE thatare one-half of t...

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    226 Principles of ElectronicsFig. 9.38Since VCE is nearly one-half of VCC, the amplifier circuit is midpoint biased.Note. We can determine whether or not the circuit is midpoint biased without drawing the dcload line. By definition, a circuit is midpoint biased when the Q-point value of VC...

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    Transistor Biasing 2279.16 Which Value of β to be used ?While analysing a biasing circuit, we have to refer to the specification sheet for the transistor toobtain the value of β. Normally, the transistor specification sheet lists a minimum value (βmin) andmaximum value (βmax) of β. In ...

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    228 Principles of ElectronicsFig. 9.41Solution. Such a problem should not pose any difficulty. We are to simply find the d.c. values.Note that capacitors behave as open to d.c. Applying Kirchhoff’s voltage law to the path passingthrough RB, VBE, RE and VEE, we have,– IB RB – VBE – ...

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    Transistor Biasing 229Solution.VEE = IB RB + VBEorIB = (9 – 0.7) V100 kΩEEBEBVVR−= = 0.083 mA∴IC =βIB = 45 × 0.083 mA = 3.73 mAAlsoVEE = IC RC + VCE∴VCE = VEE – IC RC = 9V – (3. 73 mA) (1.2 kΩ) = 4.52V∴ Operating point is 4.52V, 3.73 mA.Example 9.35. It is desired to desig...

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    230 Principles of Electronics9.18 Silicon Versus GermaniumAlthough both silicon and germanium are used in semiconductor devices, the present day trend is touse silicon. The main reasons for this are :(i) Smaller ICBO. At room temperature, a silicon crystal has fewer free electrons than a g...

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    Transistor Biasing 231= 4.1 µA = 0.0041 mAA 30°C rise in temperature would cause ICEO in silicon to increase 8 times. Now ICEO=8 × 0.0041 = 0.0328 mA∴ Zero signal collector current at 55°C= 2 + 0.0328 = 2.0328 mAPercentage change in zero signal coll...

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    232 Principles of Electronicscollector current increases and collector-emitter voltage decreases. Moreover, during the positivehalf cycle of the signal (i.e. from 0 to π rad.), the operating point moves from 20 µA to 20 + 10 = 30µA and then back again i.e. operating point follows the pa...

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    Transistor Biasing 233Solution. Since VB (i.e., base voltage w.r.t. ground) is zero, it means that there is no path forcurrent in the base circuit. The transistor will be biased off i.e., IC = 0 and IE = 0. Therefore, VC =10 V (ä IC RC = 0) and VE = 0. The obvious fault is that R1 is...

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    234 Principles of ElectronicsMULTIPLE-CHOICE QUESTIONS8. For faithful amplification by a transistor cir-cuit, the value of VCE should ........ for sili-con transistor.(i) not fall below 1 V(ii) be zero(iii) be 0.2 V(iv) none of the above9. The circuit that provides the best stabilisationof...

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    Transistor Biasing 235the current I1 flowing through R1 and R2should be equal to or greater than ........(i) 10 IB(ii)3 IB(iii)2 IB(iv)4 IB17. The leakage current in a silicon transistor isabout ......... the leakage current in a germa-nium transistor.(i) one hundredth (ii) one tenth(iii) one...

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    236 Principles of Electronics32. The stabilisation of operating point in po-tential divider method is provided by ........(i) RE consideration(ii) RC consideration(iii) VCC consideration(iv) none of the above33. The value of VBE ............(i) depends upon IC to moderate extent(ii) is almo...

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    Transistor Biasing 2377. Mention the steps that are taken to design the transistor biasing and stabilisation circuits.8. Write short notes on the following :(i) Operating point (ii) Stabilisation of operating pointProblems1. An npn silicon transistor has VCC = 5V and the collector l...

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    238 Principles of Electronics5. Find the value of IC for potential divider method if VCC = 9V, RE = 1kΩ, R1 = 39 kΩ, R2 = 10 kΩ,RC = 2.7 kΩ, VBE = 0.15 V and β = 90.[1.5mA]6. In an RC coupled amplifier, the battery voltage is 16V and collector load RC = 4 kΩ. It is required toset the...

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    Transistor Biasing 239Fig. 9.54Fig. 9.5513. To what value can RE in Fig. 9.55 be reduced without transistor going into saturation ?[639ΩΩΩΩΩ]14. When can the effect of β be neglected in the emitter bias circuit ?[When RE >> RB/βββββ]15. What is the minimum value of β in Fig....