An amplifier includes: a first operational signal path arranged to generate a first pre-output signal according to an input signal under a first operating mode of the amplifier; a second operational signal path arranged to generate a second pre-output signal according to the input signal under a second operating mode of the amplifier; and an output stage, coupled to the first operational signal path and the second operational signal path, for outputting a first output signal according to the first pre-output signal under the first operating mode and outputting a second output signal according to the second pre-output signal under the second operating mode, wherein the first operational signal path and the second operational signal path are arranged to share at least one common circuit element for generating the first pre-output signal and the second pre-output signal under the first operating mode and the second operating mode, respectively.