An amplifier includes a biasing section, first and second differential amplifying sections and an output section. The biasing section outputs first and second bias currents based on first and second power source voltages. The first differential amplifying section outputs a first amplified voltage based on the first bias current. The second differential amplifying section outputs a second amplified voltage based on the second bias current. The output section outputs the second power source voltage based on the first amplified voltage and the first power source voltage, and outputs the first power source voltage based on the second amplified voltage and the second power source voltage. Therefore, a variation of the threshold voltage is compensated to enhance display quality.