A unity-gain buffer includes an operational amplifier, a control stage, and an auxiliary output stage. The operational amplifier includes a non-inverting input terminal, an output terminal, and an inverting input terminal. The control stage is connected between the non-inverting input terminal and the output terminal of the unity-gain buffer. The auxiliary output stage is connected between the control stage and the output terminal of the unity-gain buffer. According to an input voltage at the input terminal of the unity-gain buffer, the control stage generates a first driving current, the auxiliary output stage generates a second driving current, and the operational amplifier generates a third driving current, so that an overall driving current outputted from the output terminal of the unity-gain buffer is equal to the sum of the first driving current, the second driving current and the third driving current.