PURPOSE: To avoid the effect of the pre-stage by providing an impedance conversion circuit such as a voltage follower circuit between an output of at least a filter circuit and the input of the next filter circuit.

CONSTITUTION: The voltage follower circuit 305 is inserted between the output of a CR filter 302 and the input of the SCF 306 to apply the impedance conversion. The voltage follower circuit 305 consists of six N-channel MOS transistors (TRs) 406∼411, P-channel MOS TRs 402∼405 and a current source 401. Thus, the circuit is immune to the pre-stage circuit and even if components of the CR filter 302 have variance, since the characteristic of the SCF 306 is unchanged and ten the stable characteristic is offered even when the stability due to manufacturing accuracy or secular change in the components is deteriorated more or less.