An input stage of a differential amplifier includes a differential pair formed by an N-channel MOS transistor MN1 having a gate connected to an INM and an N-channel MOS transistor MN2 having a gate connected to an INP, both having sources connected to each other, a constant current source connected to the sources of the MN1 and MN2, and a variable current source connected to the sources of the MN1 and MN2. A subsequent-stage processing circuit having an intermediate stage and an output stage includes a phase compensation capacitor and outputs an output responsive to a change in the differential inputs by charging and discharging the phase compensation capacitor through the constant current source. The variable current source turns ON when the change reaches a level causing a parasitic capacitor at the sources of the differential pair to be discharged, and supplies a current for discharging the parasitic capacitor.