US Patent 7317349 Class-D amplifier

A PWM circuit converts output data of a calculator to a pulse width modulation signal, and outputs it to a load (speaker) through a buffer amplifier and a low-pass filter. A digital low-pass filter has the same filter characteristic as a low-pass filter. An error calculator calculates the error Δ(z) between the input data and the output of the filter, and outputs it to the calculator. The output of the filter becomes a digital signal having substantially the same digitalized waveform as an analog signal applied to the load, and also no distortion contains in the digital signal. Accordingly, the output data Δ(z) of the error calculator becomes data corresponding to the distortion of the output signal. In the calculator, the data Δ(z) is subtracted from the input data, and the subtraction result is applied to the PWM circuit to reduce the distortion.