An output buffer of a source driver is disclosed. The output buffer includes a buffer input, a buffer output, a differential input stage, a bias current source, an output stage, a compensation capacitor, and a comparator. The output stage and the comparator are both operated between an analog supply voltage (AVDD) and a half analog supply voltage (HAVDD), or both operated between the half analog supply voltage (HAVDD) and a ground voltage. The comparator compares an input signal with an output signal and outputs a control signal to the bias current source according to the compared result.