1 Output stages and power amplifiers Characteristics of npn BJT Low output resistance Efficient power delivery

2 Figure 14.1 Collector current waveforms for transistors operating in (a) class A, (b) class B, and (c) class AB amplifier stages. Classification of Output Stages sinusoidal input signal

3 Figure 14.2 An emitter follower (Q 1 ) biased with a constant current I supplied by transistor Q 2. Class A Output Stage transfer characteristic

4 Figure 14.4 Maximum signal waveforms in the class A output stage of Fig under the condition I = V CC /R L or, equivalently, R L = V CC /I. Signal Waveforms

5 Power-Conversion Efficiency

6 Figure 14.5 A class B output stage. Class B Output Stage transfer characteristic

7 Power-Conversion Efficiency

8 Figure 14.8 Power dissipation of the class B output stage versus amplitude of the output sinusoid. Power Dissipation

9 Figure 14.7 Illustrating how the dead band in the class B transfer characteristic results in crossover distortion. Crossover Distortion