ABSTRACT
The input stage of an amplifier performs the critical duty of subtracting the feedback signal from the input, to generate the error signal that drives the output. It is almost invariably a differential transconductance stage; a voltage-difference input results in a current output that is essentially insensitive to the voltage at the output port. Power amplifier design can live with a restricted common-mode range in the input stage that would be unusable in an op-amp, and this gives the designer great scope. Complexity in itself is not a serious disadvantage as the small-signal stages of the typical amplifier are of almost negligible cost compared with mains transformers, heat-sinks. Two established methods to produce a linear input transconductance stage are the cross-quad and the cascomp configurations. The distortion plot for a model amplifier designed so that the distortion from all other sources is negligible compared with that from the carefully balanced input stage.
