Power amplifier

1. A power amplifier comprising:

an amplifying element for amplifying a signal input to the amplifier,

a matching network for varying the reactance presented to the output of the amplifying element at the fundamental frequency of the input signal,

a switching arrangement for switching the matching network between first and second operating configurations, wherein in the first operating configuration, a net inductive reactance is presented to the output at the fundamental frequency and in the second operating configuration, a net capacitive reactance is presented to the output at the fundamental frequency,

wherein

the switching arrangement is configured to switch between the first operating configuration and second operating configuration dependent on the output power back-off of the amplifier; and

the switching arrangement is configured to switch from the first operating configuration to the second operating configuration when the output power back-off is at a first value and to switch from the second operating configuration to the first operating configuration when the output power back-off is at a second value that is higher than the first value.

2. A power amplifier according to claim 1 wherein the matching network is configured such that the resistance presented to the output of the amplifying element remains constant in both the first and second operating configurations.

3. A power amplifier according to claim 1 wherein the switching arrangement is operable to vary the reactance across a range of values in both the first and second operating configurations.

4. A power amplifier according to claim 3 , wherein the reactance is continuously variable in both the first and second operating configurations.

5. A power amplifier according to claim 3 , wherein the switching arrangement comprises a single element that is operable to switch between the first and second operating configurations and to vary the reactance within each of the first and second operating configurations.

6. A power amplifier according to claim 3 , wherein the switching arrangement includes a switching element for switching between the first and second operating configurations and a tuneable element for varying the reactance within the first and second operating configurations.

7. A power amplifier according to claim 5 or 6 wherein the element used to switch between the first and second operating configurations is a variable inductor or variable capacitor.

8. A power amplifier according to claim 1 , wherein the matching network includes an arrangement for varying the reactance presented to the output of the amplifying element at a second harmonic of the fundamental operating frequency of the input signal.

9. A power amplifier according to claim 8 , wherein the arrangement for varying the reactance presented at the second harmonic comprises an injection network for injecting a signal at the second harmonic of the input signal frequency into the matching network.

10. A power amplifier according to claim 8 , wherein the arrangement for varying the reactance presented at the second harmonic comprises a tuneable transmission line, the length of the transmission line being adjustable to set the magnitude of the second harmonic reactance.

11. A power amplifier according to claim 8 , wherein the arrangement for varying the reactance presented at the second harmonic comprises one or more stubs loaded with variable capacitors.

12. A power amplifier according to claim 8 , wherein the power amplifier is configured such that when the amplifier is in the first operating configuration, a net capacitive reactance is presented to the output of the amplifying element at the second harmonic frequency and when the power amplifier is in the second operating configuration, a net inductive reactance is presented to the output of the amplifying element at the second harmonic frequency.