Disclosed herein is a circuit for preventing an element from being damaged although output impedance of a final transistor is changed in a power amplifier. The power amplifier includes: a power stage amplifying a signal; a transformer connected to an output terminal of the power stage and coupling a signal output from the power stage; and a controller controlling a bias voltage from the power stage according to the coupled signal. Although output impedance is changed, damage to the power amplifier can be prevented. Also, the power amplifier can be automatically controlled to maintain performance thereof by sensing an operational state in which output impedance is normal.