What is Transistor Audio Power Amplifiers – Electronics Tutorial

What is Transistor Audio Power Amplifiers – Electronics Tutorial
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What is Transistor Audio Power Amplifiers – Electronics Tutorial

  • Page 1

    306 Principles of Electronics 12.1 Transistor Audio Power Amplifier 12.2 Small-Signal and Large-SignalAmplifiers 12.3 Output Power of Amplifier 12.4 Difference Between Voltage andPower Amplifiers 12.5 Performance Quantities of PowerAmplifiers 12.6 Classification of Power Amplifiers 12.7 Exp...

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    Transistor Audio Power Amplifiers 307Fig. 12.112.1 Transistor Audio Power AmplifierA transistor amplifier which raises the power level of thesignals that have audio frequency range is known as tran-sistor audio power amplifier.In general, the last stage of a multistage amplifier is thepower s...

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    308 Principles of Electronicssarily power amplifiers but it is safe to say that most are. In general, where amount of power involvedis 1W or more, the amplifier is termed as power amplifier.12.3 Output Power of AmplifierAn amplifier converts d.c. power drawn from d.c. supply VCC into a.c. o...

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    Transistor Audio Power Amplifiers 309Example 12.2. Determine the a.c. load power for the circuit shown in Fig. 12.3.Fig. 12.3Solution. The reading of a.c. voltmeter is 10.6V. Since a.c. voltmeters read r.m.s. voltage, wehave,A.C. output power, PO=22(10.6)200 ΩLLVR= = 561.8 mWExample 12.3....

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    310 Principles of Electronics(i) The transistor with high β ( >100) is used in the circuit. In other words, those transistors areemployed which have thin base.(ii) The input resistance Rin of the transistor is sought to be quite low as compared to thecollector load RC.(iii) A relatively...

  • Page 6

    Transistor Audio Power Amplifiers 311Solution.(i)P= V2/R∴a.c. output voltage, V =50 8PR=× = 20 V(ii)a.c. output current, I= V/R = 20/8 = 2.5 A12.5 Performance Quantities of Power AmplifiersAs mentioned previously, the prime objective for a power amplifier is to obtain maximum outpu...

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    312 Principles of Electronicstransistor case. The increased surface area allows heat to escape easily and keeps the case tempera-ture of the transistor within permissible limits.12.6 Classification of Power AmplifiersTransistor power amplifiers handle large signals. Many of them are drive...

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    Transistor Audio Power Amplifiers 313operation in terms of a.c. load line. Obviously, the operating point Q shall be located at collector cutoff voltage. It is easy to see that output from a class B amplifier is amplified half-wave rectification.In a class B amplifier, the negative half-cyc...

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    314 Principles of Electronics* Po= [(0.5 × 0.707) vce (p − p)] [(0.5 × 0.707) ic (p − p)]=()()8ce ppc ppvi−−×∴Collector η =()()8ce ppc ppCCCviVI−−×12.8. Maximum Collector Efficiency of Series-Fed Class AAmplifierFig. 12.6 (i) shows a **series – fed class A amplifier. ...

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    Transistor Audio Power Amplifiers 315Thus the maximum collector efficiency of a class A series-fed amplifier is 25%. In actual practice,the collector efficiency is far less than this value.Example 12.7. Calculate the (i) output power (ii) input power and (iii) collector efficiency of theampli...

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    316 Principles of Electronicsline is a vertical line rising from VCC as shown in Fig. 12.8 (ii). When signal is applied, the collectorcurrent will vary about the operating point Q.In order to get maximum a.c. power output (and hence maximum collector η), the peak value ofcollector current...

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    Transistor Audio Power Amplifiers 317∴Max. collector η =()100omaxdcPP×=22(1/ 2)CLCLIRIR′′ × 100 = 50%12.10 Important Points About Class A Power Amplifier(i) A *transformer coupled class A power amplifier has a maximum collector efficiency of 50%i.e., maximum of 50% d.c. supply pow...

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    318 Principles of ElectronicsTransformer turn ratio, n=10Zero signal collector current, IC= 100 mALoad as seen by the primary of the transformer isR′L= n2 RL = (10)2 × 100 = 10,000 Ω∴ Max. a.c. power output =()2211 10010,00022 1000CLIR′ =×= 50 WExample 12.10. A class A transf...

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    Transistor Audio Power Amplifiers 319Solution.Max. collector current change, ΔIC= 100 mAMax. collector-emitter voltage change isΔ VCE= 12 VLoudspeaker resistance, RL=5 Ω(i) Loudspeaker directly connected. Fig. 12.9 (i) shows the circuit of class A power amplifierwith loudspeaker directly...

  • Page 15

    320 Principles of ElectronicsTransformer secondary voltage=Primary voltagen = 12/4.9 = 2.47 VLoad current, IL =2.47 V5Ω = 0.49 APower transferred to the loudspeaker= I2L RL= (0.49)2 × 5 = 1.2 W = 1200 mWIt is clear that by employing transformer coupling, we have been able to tr...

  • Page 16

    Transistor Audio Power Amplifiers 321(iii)Zero signal VCE=30 552− + = 17.5 VSince the load is transformer coupled, VCC j 17.5 V.d.c. input power, Pdc= VCC IC = 17.5 V × 18 mA = 315 mWa.c. output voltage, Vce=30 522− = 8.84 Va.c. output current, Ic=35 122− = 12 mA∴ a.c....

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    322 Principles of ElectronicsHeat Sink*Almost the entire heat in a transistor is produced at the collector-base junction. If the temperature exceedsthe permissible limit, this junction is destroyed and the transistor is rendered useless.** Most of power is dissipated at the collector-base...

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    Transistor Audio Power Amplifiers 323It may be noted that the ability of any heat sink to transfer heat to the surroundings depends upon itsmaterial, volume, area, shape, contact between case and sink and movement of air around the sink.Finned aluminium heat sinks yield the best heat transfer...

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    324 Principles of Electronics(ii) If a heat sink is used with the above transistor, the value of θ is reduced to 60ºC/W. Find themaximum permissible power dissipation.Solution.(i) Without heat sinkTJ max= 90ºCTamb= 30ºCθ = 300ºC/W∴Ptotal=90 30300J maxambTT−−=θ = 0.2 W = 20...

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    Transistor Audio Power Amplifiers 325(i) Voltage amplification stage. The signals found in practice have extremely low voltage level(< 10 mV). Therefore, the voltage level of the weak signal is raised by two or more voltageamplifiers. Generally, RC coupling is employed for this purpose....

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    326 Principles of Electronicscycle of the signal. In this way, output voltage is a complete sine wave. At the same time, the circuitdelivers high output power to the load due to class B operation.12.17 Push-Pull AmplifierThe push-pull amplifier is a power amplifier and is frequently emplo...

  • Page 22

    Transistor Audio Power Amplifiers 327R′L=2122⎛⎞⎜⎟⎝⎠LNRNwhereN1 = Number of turns between either end of primary winding andcentre-tapN2 = Number of secondary turnsAdvantages(i) The efficiency of the circuit is quite high (j 75%) due to class B operation.(ii) A high a.c. output po...

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    328 Principles of ElectronicsFig. 12.15where Idc is the average current drawn from the supply VCC. Since the transistor is on for alternat-ing half-cycles, it effectively acts as a half-wave rectifier.∴Idc=()CsatIπ∴Pdc=()CCC satVIπ∴Max. collector η =()()()()0.25100=×πo maxCCC sat...

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    Transistor Audio Power Amplifiers 329(ii)D.C. input power, Pdc=()2CCC satCCCCLVIVVR=×ππ=121228×π× = 2.87 W(iii)Collector η =()225100100287omaxdcP.P.×=× = 78.4%Example 12.19. A class B push-pull amplifier with transformer coupled load uses twotransistors rated 10 W each. What is the ...

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    330 Principles of ElectronicsThis locates the second end-point of the a.c load line on the collector-emitter voltage axis. Byjoining these two points, the a.c. load line of the amplifier is constructed.12.19 Complementary-Symmetry AmplifierBy complementary symmetry is meant a principle of ...

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    Transistor Audio Power Amplifiers 3313. A class A power amplifier uses .......(i) two transistors (ii) three transistors(iii) one transistor (iv) none of the above4. The maximum efficiency of resistance loadedclass A power amplifier is ........(i) 78.5%(ii) 50%(iii) 30%(iv) 25%5. The maximum ...

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    332 Principles of Electronicsallowable collector voltage is ........(i)1 V(ii) 100 V(iii) 20 V(iv) 10 V23. When no signal is applied, the approximatecollector efficiency of class A power ampli-fier is ...........(i) 10%(ii)0%(iii) 25%(iv) 50%24. What will be the collector efficiency of apow...

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    Transistor Audio Power Amplifiers 333(ii) gives low power output(iii) requires a transformer(iv) none of the above39. If the gain versus frequency curve of a tran-sistor amplifier is not flat, then there is .........distortion.(i) amplitude(ii) intermodulation(iii) frequency(iv) none of the a...

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    334 Principles of Electronics6. A transistor has thermal resistance θ = 80°C/W. If the maximum junction temperature is 90°C and theambient temperature is 30°C, find the maximum permissible power dissipation.[750 mW]7. A power transistor dissipates 4 W. If TJ max = 90°C, find the maxi...