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Power Amplifiers Topics Covered in Chapter 31 31-1: Classes of Operation 31-2: Class A Amplifiers 31-3: Class B Push-Pull Amplifiers 31-4: Class C Amplifiers Chapter 31 © 2007 The McGraw-Hill Companies, Inc. All rights reserved.
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31-1: Classes of Operation The class of operation for an amplifier is defined by the percentage of the ac input cycle that produces an output current. The class of operation for an amplifier determines its power efficiency. The class also determines how much the input signal is distorted by the amplifier. The classes of transistor amplifiers are Class A Class B Class C McGraw-Hill© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
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31-1: Classes of Operation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-1 Fig. 31-1 illustrates class of operation for transistor amplifiers in terms of conduction angle. In Fig. (a) two cycles of sine wave input are shown. Fig. (b) shows that collector current, I C flows for 360° of the input in Class A. Fig. (c) shows that collector current, I C flows for 180° of the input in Class B. Fig. (d) shows that collector current, I C flows for 120° or less of the input in Class C.
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31-2: Class A Amplifiers In a class A amplifier, collector current flows for the full 360° of the ac input cycle. The signal amplitude of any Class A amplifier at the input should not be large enough to drive the amplifier into either cutoff or saturation. If the signal amplitude at the input is too large, either or both peaks of the output waveform will be clipped off (flattened).
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31-2: Class A Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-2 Fig. 31-2 (a) shows a common-emitter class A amplifier circuit. Fig. 31-2 (b) illustrates the dc load line.
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31-3: Class B Push-Pull Amplifiers The collector current, I C, of a transistor in a class B amplifier flows for 180° of the ac input cycle. The main disadvantage of class B operation is that two transistors must be used to get a linear reproduction of the input waveform being amplified. A class B push-pull amplifier uses two transistors to get a linear reproduction of the input waveform being amplified. A class B push-pull amplifier has medium efficiency.
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31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-5 (a) Fig. 31-5 (a) shows a class B push-pull amplifier. The transistors, Q 1 and Q 2, conduct during opposite half-cycles of the input waveform.
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31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-5 (b) The dc and ac load line for the circuit in Fig. 31-5 (a) is shown in Fig. 31- 5 (b). With no ac input signal, both transistors, Q 1 and Q 2, are cut off, and one- half of V CC appears across the collector-emitter region of each transistor.
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31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-5 (c) Fig. 31-5 (c) shows the problem with biasing the transistors exactly at cutoff. When V in crosses through zero, Q 1 and Q 2 are both cut off, resulting in a time when the output voltage does not follow the input voltage. This results in the undesirable effect known as crossover distortion.
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31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-6 (a) Fig. 31-6 (a) shows how a typical class B push-pull amplifier would be biased. This form of bias is called diode bias.
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31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-6 (b) In the circuit of Fig. 31-6 (a), when V in is positive, Q 1 conducts and Q 2 cuts off (see Fig. 31-6 b). Because Q 1 acts like an emitter follower, the ac signal voltage at the base and emitter are the same. The output coupling capacitor, C out, is charging during the positive alternation of V in. The charging current flows through R L and the collector- emitter region of Q 1.,
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31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-6 (c) Fig. 31-6 (c) shows the output when V in is negative; Q 2 conducts and Q 1 cuts off. Q 2 provides a discharge path for the output coupling capacitor, C out. The discharge path is through R L and the collector-emitter region of Q 2.
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31-4: Class C Amplifiers Class C amplifiers cannot be used in audio circuitry because of their high distortion. Class C amplifiers can be used as tuned rf amplifiers where the undesired harmonic frequencies can be filtered out. A class C amplifier is more efficient than either a class A or class B amplifier; its efficiency approaches 100%.
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31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-8 (a) Fig. 31-8 (a) shows a tuned class C amplifier. The input coupling capacitor, base resistor, and base-emitter junction form a negative clamper.
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31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-8 (b) Fig. 31-8 (b) illustrates the equivalent input circuit of the tuned class C amplifier. Because of the clamping action, only the positive peaks of the input signal drive the transistor, Q 1, into conduction. The R B C time constant is made long with respect to the period of the input waveform to provide the proper clamping action.
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31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-8 (c) Fig. 31-8 (c) shows the peak-to- peak output voltage from the tuned class C amplifier. The minimum voltage is zero and the maximum voltage is 2V CC. The peak-to-peak voltage available at the output equals 2V CC because the tank voltage adds to the positive value of V CC during the positive alternation of the output voltage.
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31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 31-8 (d) Fig. 31-8 (d) shows a graph of frequency versus voltage gain for the tuned class C amplifier. At the resonant frequency, f r, the impedance of the tuned LC circuit is maximum. The tank impedance, Z tank, is purely resistive at f r.
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