An operational amplifier includes a differential amplifier including an active load, a current mirror including a first branch and a second branch, a first switch connected between a first power source and an output node and switched in response to a voltage of a first output terminal of the differential amplifier, a first bias circuit to control an amount of a reference current flowing in the first branch in response to a voltage of a second output terminal of the differential amplifier, a second bias circuit to control a voltage of the second branch in which a mirror current flows, in response to a voltage of the first output terminal, a second switch connected between the output node and a second power source and switched in response to a voltage of the second branch, and a capacitor connected between the output node and the first output terminal.