Loop filter for class D amplifiers

A class D amplifier circuit, comprising:

• an adder, for generating a difference signal responsive to an input signal and a feedback signal;
• a pulse-width-modulator, for comparing the difference signal to a waveform having a switching frequency, and for generating a pulse-width-modulated output signal responsive to the result of the comparing;
• a power stage, for driving its output responsive to the pulse-width-modulated output signal;
• and
• a loop filter, coupled to the output of the power stage, for generating the feedback signal by applying a minimum-aliasing-error transfer function to a signal from the output of the power stage, the minimum-aliasing-error transfer function being a minimum of second order, and having a magnitude characteristic versus frequency that has a most negative slope for frequencies below the switching frequency that is flatter than the most negative slope of the real part of the minimum-aliasing-error transfer function at frequencies above the switching frequency.

A class D amplifier circuit, comprising:

• a pulse-width-modulator, for comparing a difference signal to a waveform having a switching frequency, and for generating a pulse-width-modulated output signal responsive to the result of the comparing;
• a power stage, for driving its output responsive to the pulse-width-modulated output signal;
• an LC filter, coupled to the output of the power stage, the LC filter having a transfer function, and having an output;
• and
• a loop filter, comprising:
• at least a first feedback loop path, coupled to the output of the power stage and producing a first feedback signal, and having a first order transfer function;
• a second feedback loop path and producing a second feedback signal, coupled to the output of the LC filter;
• and
• a multiplicative filter function, for applying a transfer function of at least first order to a sum including the first and second feedback signals and an input signal, to produce the difference signal.