PURPOSE: To obtain excellent distortion characteristics by comparing the center potential of an output voltage generated on both ends of an integration capacitor with a prescribed bias potential and controlling current flowing into a pair of current circuits based upon the compared result.

CONSTITUTION: The integration capacitor CI2 is connected between both the output terminals (collectors of transistors(TRs) Q3, Q4) of a cascade type voltage/current conversion amplifier to differentially extract the output voltage. Thereby, distortion due to the non-linearity of the output characteristics of the TRs Q3, Q4 can be removed. On the other hand, the center potential of the output voltage is compared with the prescribed bias voltage VBI to control the positive pole side current of the amplifier. Thereby, the center potential of the output voltage is stabilized. Consequently, excellent distortion characteristics can be obtained independently of the non-linearity of the output characteristics of the TRs Q3, Q4.