Power Amplifiers Topics Covered in Chapter : 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.

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.

31-1: Classes of Operation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  Fig 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.

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).

31-2: Class A Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig  Fig (a) shows a common-emitter class A amplifier circuit.  Fig (b) illustrates the dc load line.

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.

31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (a)  Fig (a) shows a class B push-pull amplifier.  The transistors, Q 1 and Q 2, conduct during opposite half-cycles of the input waveform.

31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (b)  The dc and ac load line for the circuit in Fig (a) is shown in Fig (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.

31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (c)  Fig (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.

31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (a)  Fig (a) shows how a typical class B push-pull amplifier would be biased.  This form of bias is called diode bias.

31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (b)  In the circuit of Fig (a), when V in is positive, Q 1 conducts and Q 2 cuts off (see Fig 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.,

31-3: Class B Push-Pull Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (c)  Fig (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.

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%.

31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (a)  Fig (a) shows a tuned class C amplifier.  The input coupling capacitor, base resistor, and base-emitter junction form a negative clamper.

31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (b)  Fig (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.

31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (c)  Fig (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.

31-4: Class C Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig (d)  Fig (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.