An operational amplifier includes an input stage, an output stage, an output enable switch, an internal capacitor, a coupling effect reduction circuit. The input stage provides an intermediate signal according to an input signal. The output stage, including an output node, provides a driving signal according to the intermediate signal. The output enable switch is turned on during an output enable period, having a start time point, to drive a load with the driving signal. The internal capacitor is coupled between the input stage and the output stage. The coupling effect reduction circuit, coupled between the internal capacitor and the output node or between the internal capacitor and the input stage, is turned off during an operational period starting from the start time point, to prevent coupling charge generated when the output enable switch is turned on from affecting operational voltage levels of the input stage.