A power amplifier includes a power transistor configured to amplify an input radio-frequency (RF) signal, and a bias circuit configured to provide a bias current to the power transistor, and in a first power mode, detect a first signal corresponding to a first level or more in the input RF signal and generate the bias current corresponding to the first signal, or detect a second signal corresponding to a second level or less in the input RF signal and generate the bias current corresponding to the second signal, and in a second power mode, detect a third signal corresponding to a third level or more in the input RF signal and generate the bias current corresponding to the third signal, or detect a fourth signal corresponding to a fourth level or less in the input RF signal and generate the bias current corresponding to the fourth signal.