PURPOSE: To make extension on any route possible, by connecting a voltage control variable-capacity element to the position where the tuning frequency of an inter- stage sransformer can be adjusted, and also by varying the tuning frequency of the inter-stage transformer by supplying a control voltage from a control circuit to the variable-capacity element so as to switch band frequency.

CONSTITUTION: The inter-stage transformer ciucuit constituting an amplifier is contituted as follows. Trimmer capacitor C1 is connected to only primary winding P of interstage transformer T inserted into the collector circuit of pre-transistor Q1, thereby constituting the single-tuning circuit. In parallel to primary winding P where the tuning frequency of this circuit is adjusted, diode D as a voltage control variable-capacity element is connected via DC stopping capacitor C5. Further, power supply +B is supplied to the collector of transistor Q1 via primary winding P, and also applied to the base of transistor Q1 via secondary winding S after being voltage divided by bias resistors R1 and R2. In addition, bypass capacitors C3 and C4 are inserted into thesxe circuits