PURPOSE: To increase the dynamic range and also to obtain an output signal of a low distortion factor by constituting plural stages of a diode load of a gain adjusting circuit and also constituting the amplifier characteristic of an amplifier circuit so as to provide the logarithmic characteristic.

CONSTITUTION: Transistors (TR)Q1, Q2 constituted as a differential pair conduct voltage-current conversion. Then a DC voltage E1 and a resistor R1 decide a bias voltage of the TRQ1. Thus, a voltage E1 and an input signal V1 are applied superimposingly to a base of the TRQ1. Further, n-sets of diodes Dn1 connected in series are activated as a load circuit of the TRQ1 and n-sets of diodes Dn2 are operated as a load circuit of the TRQ2 so as to form the amplifier characteristic into a logarithmic characteristic. The gain adjustment of a gain adjusting circuit 2 is conducted by current supply amount of the current control circuits 2a, 2b. Then in increasing/decreasing a current i13 by the control of a current control circuit 3a, the charging current to a capacitor C1 is controlled and the frequency characteristic of an output signal V011 is adjusted by the charging time and discharging time based on the capacitance of the capacitor C1.