A main amplifier generates an output signal S.sub.OUT according to a difference between first and second voltages VP and VN. A first gm amplifier is arranged as a differential input stage. A second, fully differential, gm amplifier amplifies a voltage difference between its non-inverting and inverting input terminals, and outputs a differential current signal I.sub.3N/I.sub.3P via its inverting and non-inverting output terminals. An integrator integrates a differential input current I.sub.4P/I.sub.4N input via its non-inverting and inverting input terminals, and samples and holds the signal every predetermined period, to generate a differential voltage signal. A first selector is arranged as an upstream stage of the second gm amplifier, and outputs the differential input signal without change or otherwise after swapping. A second selector is arranged as a downstream stage of the second gm amplifier, and outputs the signal I.sub.3N/I.sub.3P output from the second gm amplifier without change or otherwise after swapping.