Power amplifier with stabilising network

1. A power amplifier circuit comprising:

a transistor for receiving a signal to be amplified at an input and for outputting an amplified signal at an output;

an envelope modulated power supply connected to the transistor output for providing a modulated power supply to the transistor;

a resonant network including a capacitance of the transistor and an inductance of a supply feed of the modulated power supply which resonant network presents a supply impedance to the transistor, wherein at certain frequencies across a range of operational frequencies the supply impedance presented to the transistor by the resonant network increases;

a resistive element connected at the transistor output, such that the supply impedance presented to the transistor is the combined impedance of the first resonant network and the resistive element, such that the supply impedance presented to the transistor is stable across a range of operational frequencies; and

a reactive element connected in series with said resistive element, wherein said reactive element comprises a capacitive element or an inductive element in series with a capacitive element.

2. The power amplifier circuit according to claim 1 wherein said resistive element comprises a resistor.

3. The power amplifier circuit according to claim 1 wherein said inductive element comprises a conductive element of said circuit.

4. The power amplifier according to claim 3 wherein said conductive element of said circuit comprises a part of a conductive track or a bond wire.

5. The power amplifier according to claim 1 wherein said inductive element comprises an inductor.

6. The power amplifier according to claim 1 wherein said inductive element includes the inductance of the supply feed of the modulated supply.

7. The power amplifier according to claim 1 wherein said capacitive element comprises a capacitor.

8. A radio frequency (RF) power stage comprising:

a transistor for receiving a signal to be amplified at an input and for outputting an amplified signal at an output;

an envelope modulated power supply connected to the transistor output for providing a modulated power supply to the transistor;

a resonant network including a capacitance of the transistor and an inductance of a supply feed of the modulated power supply which resonant network presents a supply impedance to the transistor, wherein at certain frequencies across a range of operational frequencies the supply impedance presented to the transistor by the resonant network increases;

a resistive element connected at the transistor output, such that the supply impedance presented to the transistor is the combined impedance of the first resonant network and the resistive element, such that the supply impedance presented to the transistor is stable across a range of operational frequencies; and

a reactive element connected in series with said resistive element, wherein said reactive element comprises a capacitive element or an inductive element in series with a capacitive element.

9. The RF power stage according to claim 8 wherein said resistive element comprises a resistor.

10. The RF power stage according to claim 8 wherein said inductive element comprises a conductive element of said circuit.

11. The RF power stage according to claim 10 wherein said conductive element of said circuit comprises a part of a conductive track or a bond wire.

12. The RF power stage according to claim 8 wherein said inductive element comprises an inductor.

13. The RF power stage according to claim 8 wherein said inductive element includes the inductance of the supply feed of the modulated supply.

14. The RF power stage according to claim 8 wherein said capacitive element comprises a capacitor.