FREQUENCY VARIABLE FILTER CIRCUIT

PROBLEM TO BE SOLVED: To provide a frequency variable filter circuit which can be achieved in simple circuit configuration and which can be made compact. SOLUTION: A frequency variable filter circuit 1 comprises: a parallel resonance circuit 20, wherein an inductor 12 and a variable capacitive element 14 (a second parallel capacitive element) are connected in parallel; an impedance adjusting circuit 21 including a plurality of variable capacitive elements 10, 11 and 13; and an impedance adjusting circuit 22 including two emitter-follower circuits 16 and 18. By adjusting a ratio between a capacitance value of the variable capacitive element 10 and a capacitance value of the variable capacitive element 11 (and the variable capacitive element 10 and the variable capacitive element 13), an impedance on the input side of the parallel resonance circuit 20 is adjusted to a desired value, and by adjusting an impedance on the output side of the parallel resonance circuit 20 to a desired value by the emitter-follower circuits 16 and 18, the frequency variable filter circuit that can be compact is configured in simple configuration. COPYRIGHT: (C)2010,JPO&INPIT