PURPOSE: To attain temperature compensation, to eliminate a loss in an output of an amplifier and to vary the output in multi-stage by providing a comparison control means outputting a power voltage or an optimum amplifier current to the amplifier and controlling its output.

CONSTITUTION: An A/D converter 6, a comparison control section 8, a D/A converter 9 and a current control circuit 5 constitute a comparison control means. A current detector 2a detects the current of the amplifier 1, the compari son control section 8 compares an output of the A/D converter 6 and an output of a temperature detector 7 detecting the ambient temperature with data representing the relation between the current and the output of the amplifier 1 with respect to the ambient temperature stored in advance and outputs data corresponding to the optimum amplifier output. The current control circuit 5 converts an analog signal into the current of the amplifier 1 to control the output. Thus, the change in ambient temperature is compensated and the output is varied in multi-stage without losing the output of the amplifier.