A differential amplifier having a positive input terminal, a negative input terminal, an output terminal, a bias terminal and a ground terminal is provided. The differential amplifier comprises a differential pair circuit and a current mirror circuit. Wherein, the differential pair circuit is coupled to the positive input terminal, the negative input terminal, the output terminal, and the bias terminal of the differential amplifier. The current mirror circuit receives a constant current from a current source, and mirrors the constant current to the differential pair circuit. The current mirror circuit further connects to the ground terminal of the differential amplifier, and the terminal of the current mirror circuit receiving the constant current is coupled to a first source/drain terminal of a first PMOS transistor. A second source/drain and a gate of the first PMOS transistor are connected to the bias terminal and the output terminal of the differential amplifier, respectively.