An operational amplifier includes an output node; an output stage, comprising a plurality of output current paths and a plurality of control nodes, wherein the plurality of control nodes are respectively coupled to the plurality of output current paths, and the plurality of output current paths are coupled to the output node and respectively coupled to a plurality of power supply sources providing different voltages; and a selecting unit, configured to couple an internal output node of the operational amplifier to one of the plurality of control nodes of the output stage.