An amplifier includes first and second amplifier circuits. The first amplifier circuit amplifies positive-phase input signal to obtain a first positive-phase signal and amplifies negative-phase input signal to obtain a first negative-phase signal. The second amplifier circuit includes first and second amplifier units. The first amplifier unit amplifies positive-phase input signal to obtain a second positive-phase signal and amplifies negative-phase input signal to obtain a second negative-phase signal. The second amplifier unit amplifies positive-phase input signal to obtain a third positive-phase signal and amplifies negative-phase input signal to obtain a third negative-phase signal. The positive-phase output signal is sum of first and second positive-phase signals while the negative-phase output signal is sum of first and second negative-phase signals. Or the positive-phase output signal is sum of first positive-phase signal and third negative-phase signal while the negative-phase output signal is sum of first negative-phase signal and third positive-phase signal.