An amplifier including a signal input terminal, at least one signal output terminal, a first and a second cascode amplifier circuits, a capacitor and a loading circuit. The signal input terminal receives an input signal. The first cascode amplifier circuit includes a first and a second input terminals and a first and a second output terminals. The first input terminal coupled to the signal input terminal receives the input signal. The second cascode amplifier circuit includes a third and a fourth input terminals and a third output terminal. The third input terminal is coupled to the first output terminal, and the third output terminal is coupled to the second input terminal. Two terminals of the capacitor are coupled to the fourth input terminal and the first output terminal respectively. A terminal of the loading circuit is coupled to the third output terminal, and another terminal of the loading circuit is coupled to the second output terminal. At least one of two terminals of the loading circuit is further coupled to the at least one signal output terminal.