Patent ID: 8237508

Claim:
A power amplifier comprising: an amplifier transistor having a collector; a bias circuit supplying bias current to the amplifier transistor; and a collector voltage terminal connected to the collector of the amplifier transistor, wherein the bias circuit includes: a reference voltage terminal into which a reference voltage is input, a power terminal connected to a power source, a transistor having a control terminal connected to the reference voltage terminal, a first terminal connected to the power terminal, and a second terminal that is grounded, the transistor supplying a bias current corresponding to the reference voltage to the amplifier transistor, a variable capacitor connected between the first terminal of the transistor and a grounding point, and a logic circuit controlling capacitances of the variable capacitor, and, when a collector voltage supplied to the collector voltage terminal is higher than a prescribed voltage, the logic circuit increases the capacitance of the variable capacitor to be larger than when the collector voltage is not higher than the prescribed voltage.