Power amplifier

1. A power amplifier comprising:

an input matching circuit having an input terminal and an output terminal;

an amplifier transistor for amplifying an input signal received through the input matching circuit and output at the output terminal of the input matching circuit;

means for varying a collector voltage applied to the amplifier transistor;

a bias circuit for varying an idle current flowing in the amplifier transistor; and

a compensation circuit including a first capacitor and a variable capacitance connected to the first capacitor, wherein the fist capacitor and the variable capacitance are connected to the output terminal of the input matching circuit, the compensation circuit varying capacitance at the output terminal of the input matching circuit to maintain phase shift and input reflection of the power amplifier constant when the collector voltage and the idle current vary, and to prevent a decrease in efficiency of the power amplifier due to changes in output power of the amplifier transistor.

2. The power amplifier according to claim 1 , further comprising a resistance connected in series with the first capacitor.

3. The power amplifier according to claim 2 , wherein the resistance is a variable resistance.

4. The power amplifier according to claim 2 , wherein the resistance is a semiconductor layer.

5. The power amplifier according to claim 1 , further comprising an inductor connected in series to the first capacitor.

6. The power amplifier according to claim 1 , further comprising an attenuator for attenuating the input signal.

7. A power amplifier comprising:

an input matching circuit having an input terminal and an output terminal;

an amplifier transistor for amplifying an input signal received through the input matching circuit and output at the output terminal of the input matching circuit;

means for varying a collector voltage applied to the amplifier transistor;

a bias circuit for varying an idle current flowing in the amplifier transistor; and

a compensation circuit consisting of two independently controllable variable capacitors connected to the output terminal of the input matching circuit, the compensation circuit varying capacitance at the output terminal of the input matching circuit to maintain phase shift and input reflection of the power amplifier constant when the collector voltage and the idle current vary, and to prevent a decrease in efficiency of the power amplifier due to changes in output power of the amplifier transistor.

8. The power amplifier according to claim 7 , further comprising:

two wire bonding pads, each bonding pad being connected to one of the two variable capacitors; and

a ground terminal, wherein the ground terminal is connected to at least one of the wire bonding pads.

9. A power amplifier wherein:

an input matching circuit having an input terminal and an output terminal;

an amplifier transistor for amplifying an input signal received through the input matching circuit and output at the output terminal of the input matching circuit;

means for varying a collector voltage applied to the amplifier transistor;

a bias circuit for varying an idle current flowing in the amplifier transistor; and

a compensation circuit including a first diode which can be reverse biased and a second diode for connecting and disconnecting the first diode to and from the output terminal of the input matching circuit, the compensation circuit varying capacitance at the output terminal of the input matching circuit to maintain phase shift and input reflection of the power amplifier constant when the collector voltage and the idle current vary, and to prevent a decrease in efficiency of the power amplifier due to changes in output power of the amplifier transistor.

10. A power amplifier comprising:

an input matching circuit having an input terminal and an output terminal;

an amplifier transistor for amplifying an input signal received through the input matching circuit and output at the output terminal of the input matching circuit;

means for varying a collector voltage applied to the amplifier transistor;

a bias circuit for varying an idle current flowing in the amplifier transistor; and

a compensation circuit including a first capacitor and two diodes which may be connected to and disconnected from the output terminal of the output matching circuit, the compensation circuit varying capacitance at the output terminal of the input matching circuit by use of the two diodes to maintain phase shift and input reflection of the power amplifier constant when the collector voltage and the idle current vary, and to prevent a decrease in efficiency of the power amplifier due to changes in output power of the amplifier transistor.

11. A power amplifier comprising:

an input matching circuit having an input terminal and an output terminal;

an amplifier transistor for amplifying an input signal received through the input matching circuit and output at the output terminal of the input matching circuit;

means for varying a collector voltage applied to the amplifier transistor;

a bias circuit for varying an idle current flowing in the amplifier transistor; and a compensation circuit including a first capacitor, a field effect transistor (FET) having a gate and a drain, connecting and disconnecting the first capacitor to and from the output terminal of the input matching circuit, and a second capacitor connected between the gate and the drain of the FET, the compensation circuit varying capacitance at the output terminal of the input matching circuit to maintain phase shift and input reflection of the power amplifier constant when the collector voltage and the idle current vary, and to prevent a decrease in efficiency of the power amplifier due to changes in output power of the amplifier transistor.