A power amplifier includes a first bias circuit including a first and third transistor, a first sub-bias circuit, and an amplifying circuit including a fourth transistor. In the first bias circuit, a second terminal of the first transistor and a second terminal of the first sub-bias circuit are grounded, a control terminal of the first transistor is connected to a control terminal of the first sub-bias circuit, a first terminal of the first sub-bias circuit is connected to a constant voltage terminal, a first terminal of the first transistor is connected to a second terminal of the third transistor, a first terminal of the third transistor is connected to a control terminal of the third transistor. The amplifying circuit amplifies an input signal power based on a first bias signal from the first bias circuit to a control terminal of the fourth transistor.