A nonlinear feedback topology is used to reduce the deviations of a practical PWM output stage from ideal theoretical behaviour, the theoretical nonlinearity of the PWM process being corrected using predistortion. As the final output is analogue, an ADC is needed if the feedback is to be digital, and several problems arise from the practical limitations of commercially-available ADCs, including delay and addition of ultrasonic noise. We show how these problems can be minimised, and illustrate the performance of a digital PWM amplifier in which feedback results in a significant reduction of distortion throughout the audio range.
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