Power amplifier and operating method thereof

A power amplifier includes a first amplifier and a second amplifier which include large and small size elements. An RF input signal is amplified by the first and second amplifiers. An output of the first amplifier is connected to an input of a first output matching circuit. An output of the second amplifier is connected to an input of a second output matching circuit. An output of the second output matching circuit is connected to an RF signal output terminal. In a high power state, the RF input signal is amplified by the first amplifier. In a low power state, the RF input signal is amplified by the second amplifier. In amplification with low power and high frequency, reactances of the second output matching circuit are set at predetermined values. In amplification with low power and low frequency, the reactances of the second output matching circuit are set at larger values.