PURPOSE: To reduce characteristics that an operational amplifier requires, by advancing or delaying the timing of the input and output switching of the operational amplifier by a half period.

CONSTITUTION: Switches 1 and 2 on the input and output sides of an operational amplifier 3 inveriably have a half-period difference in the timing of switching. In other words, voltage changes of capacitors C2 and C3 have a half-period time difference. Therefore, a capacity load on the operational amplifier 3 is reduced, and the rising of the voltage change is speeded. Namely, the switch 1 connected to the input side of the operational amplifier 3 and the switch 2 connected to the output side are given a difference in the timing of switching all the time to reduce the capacity load on the operational amplifier 3 while reducing characteristics that the operational amplifier 3 requires.