An output buffer with an improved output deviation and a source driver of a flat panel display which employs the output buffer wherein the output buffer includes a first input terminal to which a first differential input signal is applied, a second input terminal to which a second differential input signal is applied, an output terminal that generates an output signal based on the second differential input signal and feeds back the output signal to the first input terminal as the first input signal, a first power supply terminal to which a first power supply voltage is applied, a second power supply terminal to which a second power supply voltage is applied, and an amplification unit that amplifies a difference between the first differential input signal and the second differential input signal, pulls up the output signal to the first power supply voltage or pulls down the output signal to the second power supply voltage, and includes a plurality of transistors.