PURPOSE: To expand the dynamic range of the input signal level and also to attain the design taking the dynamic range of the input signal into account by providing a series circuit of plural resistors between input terminals and adding a detecting circuit detecting signal extracted from the series circuit.

CONSTITUTION: Captions 1, 2 and 3 are differential amplifiers each having a differential input and a differential output, an input signal Vin is given to an inverting input of the differential amplifier 1 of the 1st stage through a capacitor 9 and the differential output of the differential amplifier 1 of the 1st stage becomes the differential input to the differential amplifier 2 of the 2nd stage. Only the AC component is obtained through a capacitor from a signal extracted from a connecting point between resistors R1 and R2 connected between the non-inverting input and the inverting input of the differential amplifier 1 of the 1st stage and a signal extracted from the inverting output of each differential amplifier and the AC component is inputted to the detector, where the input is converted respectively to an output current corresponding to each signal level. Through the circuit constitution above, the detecting range for the input signal level is expanded to a signal level decided by the attenuation depending on the resistance ratio of the resistors R1 and R2 and by the saturation level of the detector.