Digital audio power amplifiers using Sigma Delta Modulation - Linearity problems in the class-D power stage

A completely digital audio power amplifier uses binary (two-level) signals up to the power stage. Often MOSFETs are used as power devices, as they can provide a high output power together with a fast switching speed. The generation of accurate, rectangular pulses in the analogue power amplifier circuit is a problem, especially at pulses with an extremely short duration, as they may come from high resolution Sigma-Delta Modulation. In this paper some of the typical pulse signal waveform distortions introduced by the realistic switching power stage are shown by simple circuit models. Additionally it will be explained, how certain errors can affect the linearity of pulse signals from Sigma-Delta Modulation.