A source driver including a sensing circuit and an operational amplifier is provided. The sensing circuit senses pixel information of an organic light-emitting diode (OLED) pixel circuit. The operational amplifier includes an amplifier circuit and an offset voltage storing and reducing circuit. An input terminal of the amplifier circuit is coupled to the sensing circuit. The amplifier circuit includes a first gain circuit and a second gain circuit. An output terminal of the offset voltage storing and reducing circuit is coupled to a coupling terminal of the first gain circuit. An input terminal of the offset voltage storing and reducing circuit is coupled to an output terminal of the second gain circuit. The offset voltage storing and reducing circuit stores and reduces an offset voltage of the first gain circuit.