An amplifier including a first routing circuit, an input stage circuit, an output stage circuit, a second routing circuit, and a bias voltage generating circuit is provided. The bias voltage generating circuit generates a first bias voltage and a second bias voltage for respectively supplying a first tail current source and a second tail current source of the input stage circuit. During a first period, the first bias voltage is related to the voltage at a first input terminal of the amplifier, and the second bias voltage is related to the voltage at a second input terminal of the amplifier. During a second period, the first bias voltage is related to the voltage at the second input terminal of the amplifier, and the second bias voltage is related to the voltage at the first input terminal of the amplifier.