PURPOSE: To reduce the change of output impedance due to the influence of a load current by controlling the operating current of a first transistor(TR) almost at a constant level even when the input/ output of the current due to the load of the differential operation by third and fourth TRs occurs.

CONSTITUTION: When the signal voltage of an input terminal Pi is increased, the current flows from an output terminal Po to the load. Thereby, the emitter current of the TR 1 i.e., the collector current Ic is increased. Assuming the collector saturation current of the TR 1 as Is, the voltage Vbe between the base and the emitter of the TR 1 can be expressed in Vbe=VTLn(Ic/Is), therefore, Vbe can be increased by increasing Ic. Meanwhile, since no load is applied to the TR 3, the collector current remains unchanged, and the voltage between the base and the emitter can be almost kept constant. Therefore, the change Vbe of the voltage between the base and the emitter of the TR 1 generated by the load current is inputted to the bases of the TRs 5, 6 comprising a differential pair, and the collector current of the TR 6 is controlled so as to reduce Vbe. As a result, it is possible to control the voltage between the base and the emitter of the TR 1 almost at the constant level.