PURPOSE: To realize a gain switching amplifier circuit suitable for IC processing by keeping a current flowing to a load resistance constant to make the bias of an output voltage constant.

CONSTITUTION: A bias adjustment transistor(TR) Q20, bias adjustment resistors RE 20-22, bias changeover switches SW 20-22 and base bias resistors RB 20, 21 are provided newly to a conventional gain amplifier circuit. Through the constitution above, when a changeover switch SW 10 is closed as before, the SW 21, 22 are closed and the SW 20, 22 and SW 20, 21 are closed respectively when gain changeover switches SW 11, 12 are turned on as before. Thus, with the SW 10 closed, a current flowing to a load resistor RC is the sum of currents flowing to an emitter feedback resistor RE 10 and bias adjustment resistors RE 21, 22 and with only the SW 11, 20, 22 closed, the current is the sum of currents flowing to RE 11, 20, 22. Moreover, with only SW 12, 20, 21 closed, the current is the sum of currents flowing to RE 12, 20, 21.