PURPOSE: To decrease number of components and to reduce a DC offset by providing the 1st and 2nd capacitors and the 1st, 2nd and 3rd switches switching and connecting the capacitors between an inverting input terminal and an output terminal of an operational amplifier OP.

CONSTITUTION: A non-inverting input terminal of the operational amplifier OP is connected to a common potential point and the 1st and 2nd capacitors 2K3 and the switches S1, S2 and S3 are connected between the inverting input and the output terminals. One end of the 1st capacitor 2K3 is connected to a common potential and the other end is connected to the inverting input terminal of the operational amplifier via the 3rd switch S3. The 1st switch S1 changes over the common potential point and one end of the 1st capacitor 2K3 and connects it to one end of the 2nd capacitor 2K3. The other end of the 2nd capacitor 2K3 is connected selectively to the output terminal and the common potential by the 2nd switch S2.