A capacitor includes a first transistor, a second transistor, and a control circuit. The first terminal of the first transistor is coupled to the first terminal of the capacitor. The first terminal of the second transistor is coupled to the second terminal of the capacitor. In a normal mode, the control circuit turns on the first transistor and the second transistor, the second terminal of the second transistor is coupled to the control terminal of the first transistor through the control circuit, and the control terminal of the second transistor is coupled to the second terminal of the first transistor through the control circuit. In a power saving mode, the control circuit turns off the first transistor and the second transistor.