An amplifier circuit includes a modulation signal generating circuit, a driving stage circuit and an output stage circuit. The modulation signal generating circuit generates a pair of modulation signals according to a pair of differential input signals and a pair of clock signals. The pair of clock signals includes a first clock signal and a second clock signal having a phase difference therebetween. The driving stage circuit generates a pair of driving signals according to the pair of modulation signals. The output stage circuit generates a pair of amplified output signals according to the pair of driving signals.