PURPOSE: To stabilize the output signal of a waveform shaping part by inputting the output of a subtracting circuit to the control input terminal of a voltage gain control amplifying circuit and then controlling the gain of the amplifier.

CONSTITUTION: An input signal to an input terminal 1 is amplified by a voltage variable gain amplifier 2, whose output is supplied to an upper envelope circuit 5 and a lower envelope circuit 6. The circuit 5 detects the upper envelope of the output signal of the amplifier 2, and the circuit 6 detects the lower envelope. The upper envelope waveform and lower envelope waveform inputted to a subtracting circuit 13 are processed by subtraction, and the difference signal is supplied to the control input terminal 22 of the voltage gain amplifier 2 through a low-pass filter 19. Namely, the signal obtained by cutting the high frequency component of the output of the subtracting circuit 13 is applied to the terminal 22 of the amplifier 2. Consequently, the gain of the amplifier 2 varies with the voltage applied to the terminal 22; the gain is low when the input voltage is low and high when the voltage is high.