PURPOSE: To reduce the load of an operational amplifier so as to prevent occurrence of an offset caused by a stray capacity by supplying a charging and discharging currents from a bias line against the stray capacity produced when a capacitor for deciding frequency band is made into an IC.

CONSTITUTION: An analog switch 13 is connected in the series to the connecting terminal on the feedback side to the input side of a capacitor 12 for blocking high band. In addition, another analog switch 14 is connected between the connecting point into which a stray capacity Cs' is inserted and earth. Then the output side of an operational amplifier (OP) 1 is fed back to the input side (-) of the OP 1 through an analog switch 5, the capacitor 12, and the switch 13. Therefore, it can be said that the switch 14 is inserted in parallel with the floating capacity Cs'. By supplying the charging and discharging currents to the stray capacity Cs' from a bias line, the load to the operational amplifier 1 is reduced and the capacity dead to the bias of the circuit is approximately established.