PURPOSE: To make the DC potential of an output terminal independent of the gain change and to reduce the power consumption by connecting four transistors(TRs) between each collector of a differential amplifier TR amplifying differentially input signals and a load resistor so as to control the sum of the AC voltages with a control voltage.

CONSTITUTION: NPN TRs 9, 10 form a differential TR pairs amplifying input signals differentially and a constant current source 7 for differential pair connects between emitters connected in common and a main power supply 1. Emitters of 4 NPN TRs 11-14 connect in common to a collector of the TR 9 and emitters of four PNP TRs 15-18 connect in common to a collector of the TR 10. Moreover, bases of TRs 11, 12, 17, 18 and bases of TRs 13-16 are connected in common and a control power supply 23 whose voltage is made variable is inserted between the bases connected in common. Thus, the DC potential of the output terminal is independent of the gain change and the power consumption is less.