Power amplifier

1. 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.

2. The power amplifier according to claim 1 , wherein,

when the collector voltage is higher than the prescribed voltage, the logic circuit outputs a high voltage,

when the collector voltage is not higher than the prescribed voltage, the logic circuit outputs a low voltage, and

the variable capacitor has an anode that is grounded and a cathode connected to an output of the logic circuit and connected to the first terminal of the transistor.