Abstract: In a voltage supply circuit for voltage-dependent capacitor diode tuning, a d.c. voltage is produced by a.c.-d.c. conversion of an a.c. voltage which is obtained by amplifying an a.c. signal from an oscillator with the aid of a transistor amplifier of the series-feedback type having an amplifying transistor having an amplification factor depending on a variable reactance coupled to an emitter of the amplifying transistor. The variable reactance controls the amount of negative feedback to obtain the corresponding gain of the amplifier. The degree of amplification decreases as the amount of the negative feedback increases, so that the output d.c. voltage will be stable especially in a low voltage region.