PURPOSE: To remove the shift of tuning frequency in a tuning circuit determining a tuning frequenc at its resistor and capacitor by switching the tuning frequency by turning on and off an operational amplifier and an analog switch connected to the operational amplifier.

CONSTITUTION: An input terminal 1 is connected to a resistor 2. The tuning circuit is formed by a resistor 2, resistors 3, 4, analog switch 5, capacitors 7, 8, resistor 9, and an operational amplifier 10, and outputs a tuned signal to an output terminal 11. The inversion input terminal of the operational amplifier 6 constituting a voltage follower is connected to a node between the resistors 3, 4 and the switch 5 is connected to a node between the inversion input terminal of the operational amplifier 6 and the output terminal. When the gate of the switch 5 is turned off, the impedance of the switch 5 and the inversion input of the operational amplifier 6 attain to high resistance. When the gate of the switch 5 is turned on, the potential of the node between the resistors 3, 4 coincides with the potential of the non-inversion input terminal i.e. the earth potential, so that the resistor 3 is directly earthed.