A power amplifier for use in a PWM signal includes a pulse width modulation signal generating circuit proportional to an amplitude of an input signal, an amplitude modulating circuit for amplitude-modulating the pulse width modulating signal in accordance with the level of the input signal and a power amplifying stage driven by the pulse width modulation signal which was amplitude-modulated. The amplitude modulating circuit or modulator is comprised of, by way of example, a pair of level shifting circuits (21) and (22), each being supplied with the input signal, a pair of switching circuits (23) and (24) for switching the respective outputs of these level shifting circuits by the pulse width modulation signal and an adder (25) for adding the outputs from these switching circuits, wherein this adder can produce at its output a pulse width modulation signal which is amplitude-modulated and the power amplifying stage utilizing bipolar transistors can efficiently be driven by this pulse width modulation signal.