An amplifier apparatus includes a differential input pair, a current source, and a load. The differential input pair includes first and second transistors, and an auxiliary transistor. A control terminal of the first transistor receives a reference voltage. A first terminal of the second transistor is coupled to the first terminal of the first transistor, and a control terminal of the second transistor receives an input voltage. A first terminal of the auxiliary transistor is coupled to a first terminal of the first transistor, a second terminal of the auxiliary transistor is coupled to a second terminal of the first transistor, a control terminal of the auxiliary transistor receives a control voltage, and a base terminal thereof receives a power supply voltage. The current source and load are respectively coupled to the first terminals and second terminals of the first and second transistors.