PURPOSE: To always secure the satisfactory frequency/temperature characteristics by detecting the change of the ambient temperatures to compensate this change with output of a compensating digital code and then changing the gain of a DC amplifier to compensate the change of the working point against the input of the control voltage.

CONSTITUTION: A voltage controlled piezo-oscillator is obtained by connecting a piezo-oscillator 1 to a variable capacity diode 2. A temperature detector 10, the 1st A/D converter 8, a memory circuit 7, a D/A converter 6 and a digitally controlled DC amplifier 5 are connected in series to the one side of the diode 2. Thus a compensating circuit is obtained to the ambient temperatures. While VAFC is turned into a digital code by an A/D converter 9 and the gain of the amplifier 5 is controlled. Thus a compensating circuit is obtained to the input of VAFC. The change of the load capacity for compensation of temperature can be changed as shown by A∼C because the gain of the amplifier 5 can be controlled by VAFC. This secures the satisfactory frequency/temperature characteristics.