HIGH FREQUENCY AMPLIFIER CIRCUIT

PURPOSE:To prevent a change of an antenna tuning circuit in case of a variation of an AGC voltage by adding an input capacity compensating circuit which corrects a variation of the input capacity of a dual gate MOSFET to which the AGC voltage is impressed. CONSTITUTION:The output of an antenna tuning circuit 1 consisting of a high frequency transformer T1 and a variable capacity VC1 is impressed to a gate G1 of a dual gate MOSFET10 through a coupling capacitor C1, and an AGC voltage Va is impressed to a gate G2 of the FET10 through a resistance R3. An input capacity compensating circuit 3 consisting of a varactor diode D, a capacitor C4, and a resistance R5 is connected between the gate G2, the capacity VC1, and a power source voltage Vdd. Values of the capacitor C4 and the diode D are so selected that the variation of a resultant capacity including a variation of the input capacity of the FET10 due to a variation of the voltage Va and that due to a variation of the voltage Va in the circuit 1 are equalized to each other by the diode D of the circuit 3. Thus, a change of the tuning frequency of the circuit 1 for the variation of the AGC voltage Va is avoided.