An amplifier device includes an amplifying unit, a bias module and an impedance unit. A first end of the amplifying unit electronically connects to a voltage source. A second end of the amplifying unit receives an input signal. The first end of the amplifying unit outputs an output signal amplified by the amplifying unit. A third end of the amplifying unit connects to a first reference potential. The bias module electrically connects to the second end of the amplifying unit for providing a bias voltage to the amplifying unit. An impedance unit is electrically connects to the bias module. An impedance value of the impedance unit is variable. The bias module adjusts the amplifier's linearity according to a frequency value of the input signal, a voltage value of the voltage source or a temperature value of the amplifier device. The impedance is adjusted according to the above-mentioned values.