PWM Pre-Compensation Circuit for Class D Distortion Reduction
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Solution Overview
Problem
Pulse-width modulation in class D audio power amplifiers introduces significant non-linear distortion, especially at higher modulation indices, which complicates the pre-compensation process and only reduces distortion by about 10 dB using existing finite impulse response filters.
Innovation Solution
A secondary pulse-width modulator is used to pre-compensate for non-linear distortion by subtracting its output from twice the input signal, canceling out the error signal and reducing distortion effectively, with identical pulse-width modulators and noise shapers used in both stages to maintain accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If finite impulse response filters with shifting coefficients are used to pre-compensate non-linear distortion, then distortion reduction is achieved (about 10 dB), but device complexity becomes very high
Solution Approach 1:
The invention uses a copy of the pulse-width modulator (a second identical PWM) to generate the error signal, rather than using complex finite impulse response filters. This copy approach simplifies the pre-compensation means while achieving effective distortion reduction, as the second PWM replicates the same non-linear characteristics to create a compensating signal.
Solution Approach 2:
The invention introduces a subtracting means as an intermediary element that combines the output of the second PWM with the input signal to generate the pre-compensated output. This intermediary subtractor simplifies the overall structure compared to complex filtering approaches while effectively canceling distortion through signal subtraction.
2Loss of energy
If pulse-width modulation is used to achieve high power efficiency, then power losses are reduced, but non-linear distortion increases significantly
Solution Approach 1:
The invention applies preliminary action by pre-compensating the input signal before it enters the main pulse-width modulator. A second identical PWM is used to generate an error signal that predicts the distortion, which is then subtracted from the input signal in advance. This preliminary compensation ensures that when the main PWM processes the pre-compensated signal, the resulting distortion is significantly reduced while maintaining high power efficiency.
Data Source
AI summary
An arrangement for pulse-width modulating an analog or digital input signal is provided. The non-linear distortion generated in the pulse-width modulator is precompensated by applying a signal with reversed error to the pulse-width modulator. The signal with reversed error is generated by a further pulse-width modulator that receives the input signal and whose output signal is subtracted from twice the input signal. The arrangement may e.g. be used to drive class D audio amplifiers.