An output buffer and a source driver using the same are provided. The output buffer includes an input stage module, a first output stage module, a second output stage module, and a first control module. The input stage module generates a first bias signal via a first connection terminal according to a driving signal and a output signal. The first output stage module generates the output signal in response to the first bias signal via an output terminal of the output buffer. The second output stage module generates a second bias signal in response to the first bias signal via a second connection terminal, and controls a first switch in the second output stage module. The first control module selectively connects a first current source to the output terminal of the output buffer or to the second connection terminal of the second output stage module according to an indication signal.