Output Stages and Power Amplifiers

The amplifier circuits studied in previous chapters aim to achieve a high gain with desirable input and output impedance levels. However, many applications require circuits that can deliver a high power to the load. For example, the cellphone described in Chapter 1 must drive the antenna with 1 W of power. As another example, typical stereo systems deliver tens or hundreds of watts of audio power to speakers. Such circuits are called “power amplifiers” (PAs).

This chapter deals with circuits that can provide a high output power. We first reexamine circuits studied in previous chapters to understand their shortcomings for this task. Next, we introduce the “push-pull” stage and various modifications to improve its performance. The chapter outline is shown below.



The reader may wonder why the amplifier stages studied in previous chapters are not suited to high-power applications. Suppose we wish to deliver 1 W to an 8-Ω speaker. Approximating the signal with a sinusoid of peak amplitude VP, we express the power absorbed by the speaker as


where denotes the root mean square (rms) value of the sinusoid and RL represents ...

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