PURPOSE:To prevent deterioration in S/N in an excellent way with a simple circuit by using a gain variable circuit utilizing nonlinearity of an output voltage to an input voltage of a diode element. CONSTITUTION:An output signal subject to synchronizing detection or envelope detection is inputted to a signal input terminal 4a, branched into two, one is rectified by a rectifier 22, a low frequency component is extracted by an LPF 3a, the output voltage is amplified by an amplifier 7a with offset and an output voltage given with an offset is fed to a diode 6a via coil 8a. In this case, since the diode 6a has nonlinearity between the input voltage and the output current, the signal input/output gain of the diode 6 is fluctuated proportional to the bias voltage to the diode 6a of the output of the amplifier 7a. In applying the bias voltage to the diode 6a via coils 8a, 8b, the amplitude of the output voltage is proportional to the square of the amplitude of the input voltage 4a. Since the similar operation is applied to the input/output of the diode 6b, the output of an adder 10 is the synthesis of maximum ratio.
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