PURPOSE: To isolate the input and output electrically while the efficiency and the frequency characteristic are improved by applying PWM modulation to a full wave rectified waveform of an input signal so as to attain current amplification and rectifying and smoothing the result thereby using it as a power voltage of a class B push-pull amplifier.

CONSTITUTION: An input signal Si is subjected to full wave rectification by amplifiers 32, 34 and diodes 37, 38 via a terminal 30. The signal and a bias from a voltage divider comprising resistors 39∼41 are applied and the signal is subject to PWM modulation by a comparator 43 by using a triangle wave from a triangle wave generator 42 and subjected to current amplification by an FET 44. The signal is extracted from a secondary side of a transformer 45, rectified and smoothed by diodes 48∼51 and used as a power voltage of the class B push-pull amplifier 60. The push-pull amplifier 60 is driven by the input signal Si via the amplifier 32 and a transformer 70. Since the amplifier 60 has an excellent frequency characteristic and the collector-emitter voltage of transistors 63,64 is kept nearly constant regardless of the quantity of the input signal, excellent efficiency is attained.