PURPOSE: To improve the linearity of a control voltage versus frequency characteristic by connecting a diode from a collector of one transistor(TR) to a base of other TR in forward polarity.

CONSTITUTION: When a voltage VB2 is larger than a voltage VBB1, a base-emitter voltage VBE1 of a TR Q1 is smaller than an on-voltage VD and the TR Q1 is turned off and a TR Q2 is turned on. Since a current source IV1 draws a current from an emitter of the TR Q2 via a capacitor CC in this case, an emitter voltage VE1 of the TR Q1 starts decreasing. When the base-emitter voltage VBE1 of the TR Q1 reaches on-voltage VD, the TR Q1 is turned on, a base voltage VB2 of the TR Q2 and the anode voltage of a diode D are decreased. Since a capacitor voltage VC is not immediately changed, the diode D and the TR Q2 are turned off.