CHAPTER
10
Output Stage Design and Crossover Distortion
T
he output stage is in many ways the most important part of a power amplifier. It is surely the most difficult section in which to reduce distortion when all reasonable measures have been taken to reduce distortion in the input and VAS stages. There is also a lot more money tied up in the output stage. The output stage discussion was begun in Chapter 5. Concepts of crossover distortion were introduced and other classes and topologies were covered. Here we focus on the emitter follower (EF) output stage. The CFP output stage was discussed and was found to have some serious challenges and shortcomings, so it will not be discussed further. However, many concepts discussed here also apply to CFP output stages.
10.1 The Class AB Output Stage The class AB output stage is the workhorse of most audio power amplifiers. This stage, shown in simple form in Figure 10.1, was examined in some detail in Chapters 3 and 5, where a simple power amplifier was evolved to a fairly high-performing design of conventional topology. The output stage of Figure 10.1 is the classic Locanthi T circuit [1, 2], which will also be referred to here as a Triple EF or simply a Triple. This is my preferred BJT output stage, as it provides far higher performance than a simple Darlington output stage (a Double EF). It will also be assumed throughout this chapter that the output stage is being driven in voltage mode. This loosely means that the output impedance of the VAS is much lower than the input impedance of the output stage. Bear in mind that the output impedance of the VAS is often fairly low, especially at high frequencies, due to the shunt feedback created by Miller compensation. Assuming predriver and driver transistor beta of 100 and output transistor beta of 50, the current gain of the Triple is 500,000, meaning that its input impedance is about 1 MΩ even when driving a 2-Ω load. The voltage gain of the output stage is determined by the voltage divider formed by the output stage emitter follower output impedance and the loudspeaker load impedance. The output impedance of each half of the output stage is approximately the sum of the dynamic emitter resistance re’ and the external emitter resistance RE. The output impedance of the stage is thus approximately equal to RE when re’ = RE.
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