An amplifier device and a duplexer circuit are provided. The amplifier device includes a first differential amplifier circuit and a controller. The first differential amplifier circuit includes first and second radio frequency (RF) input terminals, first and second transistors, first and second adjustable capacitor circuits, and first and second RF output terminals. The controller adjusts capacitance values of the first adjustable capacitor circuit of the first differential amplifier circuit and the second adjustable capacitor circuit of the first differential amplifier circuit according to at least one of a characteristic related to a first RF input signal of the first differential amplifier circuit, a characteristic related to the second RF input signal of the first differential amplifier circuit, a matching deviation between the first transistor and the second transistor of the first differential amplifier circuit, and a characteristic of the amplifier device.