Doherty amplifier circuit with phase-controlled load modulation

1. A Doherty amplifier circuit, comprising:

a symmetric Doherty amplifier comprising a main amplifier and a peaking amplifier of the same size as the main amplifier, the symmetric Doherty amplifier configured to operate at peak output power when the main amplifier and the peaking amplifier are each in saturation and at output-back-off (OBO) when the main amplifier is in saturation and the peaking amplifier is not in saturation; and

phase shift circuitry configured to shift the phase at an output of the peaking amplifier at OBO so that a load impedance seen by the main amplifier and efficiency of the symmetric Doherty amplifier both increase at OBO as a function of the phase shift at the peaking amplifier output.

2. The Doherty amplifier circuit according to claim 1 , wherein the phase shift circuitry comprises a first phase shifter coupled to an input of the main amplifier and a second phase shifter coupled to the output of the peaking amplifier, and wherein the first and second phase shifters are configured to impart the same phase shift.

3. The Doherty amplifier circuit according to claim 2 , further comprising a Doherty combiner configured to couple the main and peaking amplifiers to a load, wherein the phase shift circuitry further comprises a third phase shifter integrated in the Doherty combiner, and wherein the first phase shifter and the third phase shifter provide a combined phase shift approximately equal to a phase shift provided by the second phase shifter.

4. The Doherty amplifier circuit according to claim 1 , wherein the phase shift circuitry is configured to shift the phase at the output of the peaking amplifier by 10 to 60 degrees at OBO.

5. The Doherty amplifier circuit according to claim 1 , wherein the phase shift circuitry is configured to shift the phase at the output of the peaking amplifier at OBO over a range of degrees sufficient to move a maximum-efficiency operating point of the symmetric Doherty amplifier at OBO to any operating point between at least 6 dB OBO and 12 dB OBO.

6. The Doherty amplifier circuit according to claim 1 , wherein the phase shift circuitry is configured to shift the phase at the output of the peaking amplifier at OBO by lengthening a phasing line connected to the output of the peaking amplifier and lengthening a phasing line connected to an input of the main amplifier.

7. The Doherty amplifier circuit according to claim 6 , wherein the phase shift circuitry is configured to lengthen the phasing lines by 10 to 60 degrees at OBO.

8. A method of operating a Doherty amplifier circuit including a main amplifier and a peaking amplifier of the same size as the main amplifier, the method comprising:

operating the symmetric Doherty amplifier at peak output power when the main amplifier and the peaking amplifier are each in saturation and at output-back-off (OBO) when the main amplifier is in saturation and the peaking amplifier is not in saturation; and

shifting the phase at an output of the peaking amplifier at OBO so that a load impedance seen by the main amplifier and efficiency of the symmetric Doherty amplifier both increase at OBO as a function of the phase shift at the peaking amplifier output.

9. The method according to claim 8 , wherein shifting the phase at the output of the peaking amplifier at OBO comprises imparting the same phase shift at the output of the peaking amplifier and at an input of the main amplifier.

10. The method according to claim 9 , further comprising imparting the same phase shift to a phasing line of a Doherty combiner which couples the main and peaking amplifiers to a load.

11. The method according to claim 8 , wherein the phase at the output of the peaking amplifier is shifted by 10 to 60 degrees at OBO.

12. The method according to claim 8 , wherein the phase at the output of the peaking amplifier at OBO can be shifted over a range of degrees sufficient to move a maximum-efficiency operating point of the symmetric Doherty amplifier at OBO to any operating point between at least 6 dB OBO and 12 dB OBO.

13. The method according to claim 8 , wherein shifting the phase at the output of the peaking amplifier at OBO comprises:

lengthening a phasing line connected to the output of the peaking amplifier; and

lengthening a phasing line connected to an input of the main amplifier.

14. The method according to claim 13 , wherein the phasing lines are lengthened by 10 to 60 degrees at OBO.

15. A Doherty amplifier circuit, comprising:

a symmetric Doherty amplifier comprising a first amplifier of a first size and a second amplifier of a second size equal to the first size, the symmetric Doherty amplifier configured to operate at peak output power when the first amplifier and the second amplifier are each in saturation and at output-back-off (OBO) when the first amplifier is in saturation and the second amplifier is not in saturation; and

circuitry configured to dynamically control a voltage standing wave ratio (VSWR) seen by the first amplifier so that a maximum-efficiency operating point of the symmetric Doherty amplifier at OBO can move to any operating point between at least 6 dB OBO and 12 dB OBO by changing the VSWR seen by the first amplifier.

16. The Doherty amplifier circuit according to claim 15 , wherein the circuitry comprises a phase shifter configured to shift the phase at an output of the second amplifier at OBO so that the VSWR seen by the first amplifier increases at OBO.

17. The Doherty amplifier circuit according to claim 16 , wherein the phase shifter is configured to shift the phase at the output of the second amplifier at OBO by 10 to 60 degrees.

18. The Doherty amplifier circuit according to claim 16 , wherein the phase shifter is configured to shift the phase at the output of the second amplifier at OBO by lengthening a phasing line connected to the output of the second amplifier.

19. The Doherty amplifier circuit according to claim 18 , wherein the phase shifter is configured to lengthen the phasing line at OBO by 10 to 60 degrees.

20. A method of operating a Doherty amplifier circuit including a first amplifier of a first size and a second amplifier of a second size equal to the first size, the method comprising:

operating the symmetric Doherty amplifier at peak output power when the first amplifier and the second amplifier are each in saturation and at output-back-off (OBO) when the first amplifier is in saturation and the second amplifier is not in saturation; and

dynamically controlling a voltage standing wave ratio (VSWR) seen by the first amplifier so that a maximum-efficiency operating point of the symmetric Doherty amplifier at OBO can move to any operating point between at least 6 dB OBO and 12 dB OBO by changing the VSWR seen by the first amplifier.

21. The method according to claim 20 , wherein dynamically controlling the VSWR seen by the first amplifier comprises shifting the phase at an output of the second amplifier at OBO so that the VSWR seen by the first amplifier increases at OBO.

22. The method according to claim 21 , wherein the phase at the output of the second amplifier is shifted by 10 to 60 degrees at OBO.

23. The method according to claim 21 , wherein shifting the phase at the output of the second amplifier at OBO comprises lengthening a phasing line connected to the output of the second amplifier.

24. The method according to claim 23 , wherein the phasing line is lengthened by 10 to 60 degrees at OBO.

25. A Doherty amplifier circuit, comprising:

a Doherty amplifier comprising a main amplifier and a peaking amplifier, the Doherty amplifier configured to operate at peak output power when the main amplifier and the peaking amplifier are each in saturation and at output-back-off (OBO) when the main amplifier is in saturation and the peaking amplifier is not in saturation; and

phase shift circuitry configured to shift the phase at an output of the peaking amplifier at OBO so that a load impedance seen by the main amplifier and efficiency of the Doherty amplifier both increase at OBO as a function of the phase shift at the peaking amplifier output.