The Voltage-Amplifier Stage



The voltage-amplifier stage (or VAS) has often been regarded as the most critical part of a power amplifier, since it not only provides all the voltage gain but also must give the full output voltage swing. (The input stage may give substantial transconductance gain, but the output is in the form of a current.) However, as is not uncommon in audio, all is not quite as it appears. A well-designed VAS will contribute relatively little to the overall distortion total of an amplifier, and if even the simplest steps are taken to linearize it further, its contribution sinks out of sight.

As a starting point, Figure 5.1 shows the distortion plot of a model amplifier with a Class-A output (± 15 V rails, +, ...

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