PURPOSE: To obtain a low-pass filter having a low cut-off frequency by connecting a capacity between the differential amplifying circuit output of an operational amplifier subjected to voltage follower connection and the output terminal of the operational amplifier.

CONSTITUTION: When the mutual conductance of a transistor TR Ml and the channel conductance are denoted as gm1 and gds1 and TRs M1 and M2 have the same size and TRs M3 and M4 have the same size, gM1=gm2 and gm3=gm4 are true with respect to mutual conductance, and gds1=gds2 and gds3=gds4 are true with respect to channel conductance, and a gain A1 of the differential amplifier is expressed with a formula I. An output resistance Ro is expressed with a formula II. Consequently, a cut-off frequency f-3dB is equal to gm4/2πCc. The cut-off frequency is determined by the mutual conductance gm4 of the TR M4 and a value CC of a capacity 6. Since the mutual conductance gm of the TR is 10-3∼10-4Ω and is proportional to (the drain current)1/2, the low cut-off frequency is attained with a small area.