PURPOSE: To attain the operation with high performance, by changing a bias in response to an input voltage or an output voltage, performing the operation of high efficiency and low loss at low frequencies and eliminating pulsive distortion through blunting the change in a collector voltage at high frequencies.

CONSTITUTION: When an input signal frequency is low, a differentiation output through a diode D4 and a Capacitor C1 is small and only an output detected from a diode D3 is applied to the base of the 2nd transistor (TR)Q2. Thus, the rate of change in a collector voltage Vc is small and the generation of a pulsive distortion is almost out of a problem. The higher the input signal frequency is, the larger the collector-to-emitter voltage of the 1st TRQ1 becomes, and since the capacitance between the collector and the base of the 1st TRQ1 is small even if the collector voltage Vc is changed rapidly, no pulsive distortion is generated in an output voltage e0. No pulsive distortion is generated even at a negative half cycle of the input signal with the similar operation.