PURPOSE: To improve the voltage use rate and to obtain a satisfactory linearity by setting the collector current of a transistor TR, which has the emitter directly grounded, to an optimum value by a current detecting circuit and detecting the minute change of the collector current with a satisfactory sensitivity and comparing it with the reference voltage preliminarily set by a differential amplifying circuit and changing it to a current value corresponding to the difference to feed back this value to the base of the TR.

CONSTITUTION: A current detecting circuit 1 of small loss is interposed between a tuning circuit C2 and a low voltage power source, and a differential amplifying circuit 2 is connected to its detection output side, and the output side is connected to the base of a TR TR1. The passing current and the detection sensitivity of the current detecting circuit 1 and the reference voltage of the differential amplifying circuit 2 are set by a memory circuit 4 where data outputted from a control circuit 3 is stored. The output of the current detecting circuit 1 and the reference voltage are compared with each other by the differential amplifying circuit 2, and the output generated by this comparison is fed back to the base of the TR TR1.