What is Multistage Transistor Amplifiers – Electronics Tutorial

What is Multistage Transistor Amplifiers – Electronics Tutorial
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What is Multistage Transistor Amplifiers – Electronics Tutorial

  • Page 1

    280 Principles of Electronics11.1 Multistage TransistorAmplifier11.2 Role of Capacitors inTransistor Amplifiers11.3 Important Terms11.4 Properties of dB Gain11.5 RC Coupled TransistorAmplifier11.6 Transformer-Coupled Amplifier11.7 Direct-Coupled Amplifier11.8 Comparison of Different Typesof...

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    Multistage Transistor Amplifiers 281In a multistage amplifier, a number of single amplifiers are connected in *cascade arrangementi.e. output of first stage is connected to the input of the second stage through a suitable couplingdevice and so on. The purpose of coupling device (e.g. a capac...

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    282 Principles of Electronics(ii) It passes the a.c. signal from one stage to the next with little or no distortion.Fig. 11.22.As bypass capacitors. Like a cou-pling capacitor, a bypass capacitor alsoblocks d.c. and behaves as a short or wire(due to proper selection of capacitor size)to an...

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    Multistage Transistor Amplifiers 283The gain of a multistage amplifier is equal to the product of gains of individual stages. Forinstance, if G1, G2 and G3 are the individual voltage gains of a three-stage amplifier, then total voltagegain G is given by :*G = G1 × G2 × G3It is worthwhile to...

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    284 Principles of Electronics∴ Power gain = 10 log10 outinPdbPIf the two powers are developed in the same resistance or equal resistances, then,P1 =22ininVIRR=P2 =22outoutVIRR=∴Voltage gain in db =210102/10 log20 log/outoutininVRVVVR=Current gain in db =21010210 log20 logoutout...

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    Multistage Transistor Amplifiers 285Fig. 11.740 decibels phoneThe voltage gain of an amplifier changes with frequency. Referring to the frequency response inFig. 11.7, it is clear that for any frequency lying between f1 and f2, the gain is equal to or greater than70.7% of the maximum gain. T...

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    286 Principles of Electronics∴Gain = Antilog 4 = 104 = 10,000(ii)Power gain = 43 db = 4.3 bel∴ Power gain = Antilog 4.3 = 2 × 104 = 20,000Alternatively.10 log10 21PP= 43 dborlog10 21PP= 43/10 = 4.3∴21PP= (10)4.3 = 20,000In general, we have,21VV= (10)gain in db/2021PP= (...

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    Multistage Transistor Amplifiers 287Example 11.6. A certain amplifier has voltage gain of 15 db. If the input signal voltage is 0.8V,what is the output voltage ?Solution.db voltage gain = 20 log10 V2/V1or15 = 20 log10 V2/V1or15/20 = log10 V2/V1or0.75 = log10 V2/0.8Taking antilogs, we get,An...

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    288 Principles of Electronics∴P1 =240W40W40W==antilog 2.53163.16 × 10 = 126.5 mW(ii)Voltage gain in db = 20 log1021VVor 40 = 20 log10 21VV∴21VV= antilog 2 = 100NowV2 =2= 40W × 10 ΩPR = 20 V∴V1 =220V=100100V = 200 mVExample 11.10. In an amplifier, the maximum voltage gain is 200...

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    Multistage Transistor Amplifiers 289∴Increase in db power gain = 10 log10 1000 – 10 log10 100= 30 – 20 = 10 dbThis property also applies for the decrease in power gain.(ii) Each time the ordinary power gain increases (decreases) by a factor of 2, the db powergain increases (decreases) b...

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    290 Principles of ElectronicsOperation. When a.c. signal is applied to the base of the first transistor, it appears in the amplifiedform across its collector load RC. The amplified signal developed across RC is given to base of nextstage through coupling capacitor CC. The second stage do...

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    Multistage Transistor Amplifiers 291several hundred ohms whereas the input im-pedance of a speaker is only a few ohms.Hence, little power will be transferred to thespeaker.Applications.The RC coupled amplifiers have excel-lent audio fidelity over a wide range of fre-quency. Therefore, they a...

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    292 Principles of Electronics*10 kΩ || 100 Ω is essentially 100 Ω.○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○Fig. 11.11∴ Voltage gain of second stage=β × RC / Rin = 100 × 2/1...

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    Multistage Transistor Amplifiers 293Req=3310 3.6103.6CCRRRR×=++ = 2.65 kΩFig. 11.12The circuit shown in Fig. 11.13 (i) then reduces to the one shown in Fig. 11.13 (ii). Referring toFig. 11.13 (ii), we have,Fig. 11.13Voltage across R4, VB=44202.22.652.2CCeqVRRR×=×++ = 9.07 VThus the b...

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    294 Principles of ElectronicsFig. 11.14Voltage across R6 =656152.515 + 2.5CCVRRR×=×+ = 2.14 VVoltage across R8 = 2.14 − 0.7 = 1.44 VEmitter current in R8, IE=81.44 V1.44 V1kR=Ω = 1.44 mAr′e for second stage = 25 mV25 mV1.44 mAEI= = 17.4 ΩSimilarly, it can be shown that r′e...

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    Multistage Transistor Amplifiers 295pedance-changing properties of transformer, the low resistance of a stage (or load) can be reflected asa high load resistance to the previous stage.Transformer coupling is generally employed when the load is small. It is mostly used for poweramplification....

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    296 Principles of Electronicsmatter of fact, a single stage of properly designed transformer coupling can provide the gain of twostages of RC coupling.Disadvantages(i) It has a poor frequency response i.e.the gain varies considerably with frequency.(ii) The coupling transformers are bulky a...

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    Multistage Transistor Amplifiers 297Thus the load on the primary side is comparable to the output impedance of the transistor. Thisresults in maximum power transfer from transistor to the primary of transformer. This shows that lowvalue of load resistance (e.g. speaker) can be “stepped-up...

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    298 Principles of ElectronicsFig. 11.18Let n ( = NP /NS) be the required turn ratio. When no signal is applied, the transistor ‘sees’ a loadof RP (= 300 Ω) only. However, when a.c. signal is applied, the load RL in the secondary is reflectedin the primary as n2RL. Consequently, the ...

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    Multistage Transistor Amplifiers 299Example 11.20. In the above example, find the number of primary and secondary turns. Giventhat core section of the transformer is such that 1 turn gives an inductance of 10μH.Solution.We know that inductance of a coil is directly proportional to the squa...

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    300 Principles of ElectronicsThe weak signal is applied to the input of first transistor T1. Due to transistor action, an amplifiedoutput is obtained across the collector load RC of transistor T1. This voltage drives the base of thesecond transistor and amplified output is obtained across...

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    Multistage Transistor Amplifiers 301D.C. emitter current, IE2=264.48V10 kΩEVR= = 0.448 mAD.C. voltage at collector, VC2= VCC – IC2 R5(Q IE2 j IC2)= 12V – 0.448 mA × 10 kΩ = 7.52V(ii) Voltage gain To find voltage gain, we shall use the standard formula : total a.c. collector loaddivid...

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    302 Principles of Electronics1. A radio receiver has ......... of amplification.(i) one stage(ii) two stages(iii) three stages(iv) more than three stages2. RC coupling is used for .......... amplification.(i) voltage(ii) current(iii) power(iv) none of the above3. In an RC coupled amplifier,...

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    Multistage Transistor Amplifiers 30318. The ear is not sensitive to........(i) frequency distortion(ii) amplitude distortion(iii) frequency as well as amplitude distor-tion(iv) none of the above19. RC coupling is not used to amplify extremelylow frequencies because ........(i) there is consid...

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    304 Principles of Electronics35. RC coupling is generally confined to lowpower applications because of ........(i) large value of coupling capacitor(ii) low efficiency(iii) large number of components(iv) none of the above36. The number of stages that can be directlycoupled is limited becaus...

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    Multistage Transistor Amplifiers 305 6. A multistage amplifier consists of three stages ; the voltage gain of stages are 60, 100 and 160. Calculatethe overall gain in db.[119.64db]7. A multistage amplifier consists of three stages ; the voltage gains of the stages are 30, 50 and 60.Calculate ...