PURPOSE: To control the fluctuation of the free-run frequency at no signal without losing a DC amplifier output current in the presence of a signal by providing a resistor to decrease an output current in the absence of signal to the DC amplifier.

CONSTITUTION: A resistor R 11 connects between an input node A receiving an output current 11 of a differential amplifier circuit 12 and a collector of a transistor(TR) Q3, and a resistor R 12 connects between an input node B receiving an output current 12 and a collector of a transistor(TR) Q4. Thus, the currents 13, 14 in the no signal are decreased in a DC amplifier 20 and a desired potential difference is given between bases of TRs Q1 and Q2. Thus, the output current (control current of a variable reactance circuit 14) of the DC amplifier 20 is not lost in the presence of a signal and the bias current of the variable reactance circuit 14 is decreased in the absence of the signal thereby suppressing the deviation of the free-run frequency.