An operational amplifier circuit is provided. The operational amplifier circuit includes a differential input stage circuit and a loading stage circuit. The differential input stage circuit includes a first current source, a first transistor, a second transistor, a third transistor, and a fourth transistor. The control terminal of the first transistor receives a first input signal. The control terminal of the second transistor receives a second input signal. The third transistor has a first terminal coupled to the second terminal of the first transistor, a second terminal coupled to the first current source, and a control terminal coupled to the control terminal of the second transistor. The fourth transistor has a first terminal coupled to the second terminal of the second transistor, a second terminal coupled to the first current source, and a control terminal coupled to the control terminal of the first transistor.