A pulse width modulated signal amplifier includes a first input terminal supplied with a rectangular wave signal as a carrier, a second input terminal supplied with a modulating signal, such as an audio signal, an integrator supplied with both the rectangular wave signal and the modulating signal, a high gain amplifier receiving the output of the integrator, a low pass filter receiving the output of the high gain amplifier and producing a demodulated signal corresponding to the modulating signal and which is supplied to an output terminal, and a negative feedback circuit connected between the output of the high gain amplifier and the input of the integrator.