PURPOSE: To compress and expand the signals in a small circuit scale by outputting the signal selected by a switch means as a rectified current in response to the signal level and converting the input signal into a current signal corresponding to the rectified current.

CONSTITUTION: A differential amplifier circuit consisting of the NPN transistor TR QN1 and QN2 works as an expanding circuit when an action changeover switch 4 is switched toward a terminal EXP of the expanding action side. Under such conditions, the differential amplifier circuit outputs the input current signal as a voltage signal and the switch 4 selects an input signal or an output signal. Then, a rectifying circuit 2 rectifies the selected signal and outputs this signal as a rectified current in response to the signal level. Furthermore, a gain cell 1 converts the input signal VIN into a current signal I0 corresponding to the rectified current IRECT and inputs the signal I0 to an I/V converting amplifier 3. Thus, an expanding action is carried out and a compressing action is carried out when the switch 4 is switched to the COMP side. In such a constitution, the circuit scale can be reduced.