ABSTRACT
This chapter deals with the vital matter of control of the quiescent conditions in a Class-B output stage. An optimised amplifier requires minimisation of output stage gain irregularities around the crossover point by holding the quiescent current Iq at its optimal value. VAS provides all the voltage gain, its collector impedance tends to be made high. Making a linear Voltage Amplifier Stage (VAS) is most easily done by applying a healthy amount of local negative feedback via the dominant pole Miller capacitor, and if VAS distortion needs further reduction, then the open loop gain of the VAS stage must be raised to increase this local feedback. The distortion signal may intrude into the input circuitry, the feedback path, or even the cables to the output terminals. The result is a kind of sawtooth on the distortion residual that is very distinctive, an extra distortion component which rises at 6 dB/octave with frequency.
