Systems and methods providing loadline modulation of a power amplifier

1. A circuit comprising:

a power amplifier configured to provide amplified signals to a load;

an impedance matching network disposed between the power amplifier and the load, the impedance matching network comprising an adjustable impedance unit; and

a feedback loop comprising a rectifier, the rectifier being coupled with an output of the power amplifier, the feedback loop further comprising an impedance control circuit configured to receive a signal from the rectifier and to control the adjustable impedance unit in response to the signal from the rectifier, wherein the impedance control circuit comprises a weighting block configured to receive a rectified load voltage, and configured to generate a control signal for the adjustable impedance unit that is a function of the signal from the rectifier and the rectified load voltage.

2. The circuit of claim 1 , wherein the impedance control circuit comprises a comparator, wherein the comparator is configured to receive a reference voltage corresponding to a voltage swing level at the output of the power amplifier and configured to receive the signal from the rectifier.

3. The circuit of claim 1 , wherein the adjustable impedance unit comprises a variable capacitor coupled with the impedance control circuit.

4. The circuit of claim 3 , wherein the variable capacitor comprises one of a diode having a voltage-dependent capacitance or a trans-cap.

5. The circuit of claim 1 , wherein the power amplifier is configured to provide an amplified radio frequency (RF) signal to the load.

6. The circuit of claim 1 , wherein the signal from the rectifier comprises one of a voltage signal or a current signal.

7. The circuit of claim 1 , wherein the power amplifier comprises a metal oxide semiconductor (MOS) transistor, and wherein the impedance matching network is coupled with a drain of the MOS transistor.

8. The circuit of claim 1 , wherein a voltage at the output of the power amplifier comprises a differential voltage.

9. The circuit of claim 1 , wherein the load comprises an antenna.

10. A method performed at a radio frequency (RF) power amplifier (PA) system, the method comprising:

sampling a voltage swing at an output terminal of the PA, wherein the PA provides an amplified RF signal to a load;

generating a first control signal in response to the voltage swing;

applying the first control signal to a comparator to generate a second control signal, wherein the comparator receives a reference signal corresponding to a voltage swing threshold; and

adjusting an impedance of a matching network of the PA in response to the second control signal, wherein generating the first control signal comprises:

receiving a first rectified voltage from a first rectifier sampling the voltage swing;

receiving a second rectified voltage from a second rectifier sampling a voltage at the load; and

generating the first control signal as a function of the first rectified voltage and the second rectified voltage.

11. The method of claim 10 , wherein generating a first control signal comprises:

rectifying a voltage signal from the output terminal of the PA.

12. The method of claim 10 , wherein adjusting the impedance of the matching network comprises:

biasing a variable capacitor of the matching network in response to the second control signal.

13. The method of claim 10 , wherein sampling a voltage swing comprises:

sampling a differential voltage at the output terminal of the PA.

14. The method of claim 10 , further comprising:

receiving an RF signal at an input of the PA;

amplifying the RF signal by the PA; and

providing an amplified RF signal from the output terminal of the PA to the load.

15. A method performed in a radio frequency (RF) circuit having a power amplifier (PA), the method comprising:

providing an amplified RF signal from an output terminal of the PA to a load;

demodulating the amplified RF signal to generate an envelope signal;

processing the envelope signal to generate a control signal that is a function of the envelope signal; and

applying the control signal as a bias to an adjustable impedance element of a matching network, wherein the matching network is coupled between the output terminal of the PA and the load, further comprising applying a low pass filter to the control signal before applying the control signal as the bias to the adjustable impedance element.