Digital correction of PWM switching amplifiers

Abstract

Pulsewidth modulated (PWM) signals for driving a switching audio amplifier can be synthesized in the digital domain with extremely high linearity and precision. However, nonidealities associated with the power stage degrade output performance. A method to digitally correct for these nonidealities, resulting in very low total harmonic distortion (THD) and high signal-to-noise ratio (SNR) performance, is presented. This method also provides excellent rejection of power supply noise which is otherwise absent in digital PWM amplifiers. To meet noise requirements for hi-fi audio, the feedback structure is a fourth-order structure which shapes the noise beyond the audio band. The method has been implemented on a bread board, and state-of-the-art performance was achieved. Total harmonic distortion of 85 dB and dynamic range of 100 dB was measured using Audio Precision test equipment.