Radio-Frequency (RF) Amplifier Circuits and Related Techniques

• US 20110187437A1
• Filed: 02/03/2011
• Published: 08/04/2011
• Est. Priority Date: 02/03/2010
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Jurkov, Alexander S., Perreault, David J.
• Granted Patent: US 8,451,053 B2
• Time in Patent Office: Days
• US Class Current: 327/355
• CPC Class Codes: H03F 1/0294 using vector summing of two or more constant amplitude phase-modulated signals
• CPC Class Codes: H03F 1/56 Modifications of input or output impedances, not otherwise provided for
• CPC Class Codes: H03F 3/602 Combinations of several amplifiers

Abstract

A power combining and outphasing system and related techniques for simultaneously providing both wide-bandwidth linear amplification and high average efficiency is described. Providing linear amplification encompasses the ability to dynamically control an RF output power level over a wide range while still operating over a wide frequency bandwidth. The system and techniques described herein also operate to maintain high efficiency across a wide range of output power levels, such that a high average efficiency can be achieved for highly modulated output waveforms.

Claims

1. A system for delivering and controlling power from a number N greater than two radio-frequency (RF) power sources to a single output, the system comprising:

N RF power sources each having an output port, each of said N RF power sources configured to deliver RF power to its output port;

a lossless power combiner, said lossless power combiner comprising N input ports, 2N−

2 reactive branches having specified reactance values, and an output port configured to receive RF power, wherein each of the output ports of the N RF power sources are coupled to a corresponding one of the input ports of said lossless power combiner, and RF power delivered to the output port of said power combiner is controlled by adjusting the phases of the N radio-frequency power sources as a function of the reactance values of the 2N−

2 reactive branches.

2. The system of claim 1 wherein in response to adjusting the phases of the N radio-frequency power sources as a function of the reactance values of the 2N−

2 reactive branches, an effective loading impedance presented to each of said N RF power sources at the input ports of said power combiner are purely resistive at N output power levels.

3. The system of claim 2 wherein the largest magnitudes of the susceptive portion of the effective loading admittances seen by said N RF power sources are smaller than is achievable with any combination of Chireix and lossless in-phase power combining over a range of RF output power levels bounded by highest and lowest RF output power levels where said purely resistive loading of said radio-frequency power sources occurs.

4. A radio-frequency (RF) system comprising:

four or more RF signal sources each of said four or more RF signal sources having an output port; and

a reactive power combiner having four or more input ports and an output port, each reactive power combiner input port coupled to a corresponding one of the RF signal source output ports and wherein said reactive power combiner includes 2N−

2 reactive branches, each of the 2N−

2 reactive branches having a predetermined reactance value, wherein the power delivered to the reactive power combiner output port is controlled by adjusting a phase characteristic of each of said four or more RF signal sources as a function of the reactance values of the 2N−

2 reactive branches.

5. The RF system of claim 4 wherein each of said four or more RF signal sources comprise an RF amplifier having an RF input port and an RF output port coupled to a respective one of the input ports of said reactive power combiner.

6. The RF system of claim 4 wherein the number of input ports of said reactive power combiner matches the number of RF signal sources.

7. The RF system of claim 4 wherein each of said 2N−

2 reactive branches of said power combiner comprises at least one reactive element with first ones of the reactive elements having a first specified reactance magnitude at an operating frequency and second ones of the reactive elements have a second, different specified reactance magnitude at the operating frequency.

8. The RF system of claim 4 wherein each of said 2N−

2 reactive branches of said power combiner comprises at least one reactive element with first ones of the reactive elements having a first specified reactance at an operating frequency and second ones of the reactive elements have a second, opposite specified reactance at the operating frequency.

9. The RF system of claim 4 wherein the reactance values of each of said 2N−

2 reactive branches of said reactive power combiner are selected such that said reactive power combiner provides dominantly resistive loading of each of the RF signal sources coupled to the reactive power combiner input ports over a range of output power.

10. The RF system of claim 4 wherein the combination of said four or more RF signal sources and said reactive power combiner form a power amplifier circuit and wherein the RF system comprises a plurality of said power amplifier circuits.

11. The RF system of claim 10 further comprising:

a plurality of power amplifier circuits; and

an isolating combiner coupled between each pair of power amplifier circuits such that outphasing groups of said power amplifier circuits can be achieved.

12. The RF system of claim 11 further comprising a drain modulation circuit coupled to each of said power amplifier circuits.

13. The RF system of claim 12 wherein said drain modulation circuit comprises one or more of:

an adaptive bias circuit, a polar modulation circuit, a discrete drain switching circuit.

14. The RF system of claim 12 wherein said drain modulation circuit operates in accordance with an asymmetric multilevel outphasing technique.

15. The RF system of claim 12 wherein said drain modulation circuit implements gate-width switching of the power amplifiers to reduce losses at small output powers.

16. The RF system of claim 4 wherein said reactive power combiner comprises at least one of:

one or more distributed circuit elements;

one or more lumped circuit elements;

or

a combination of distributed and lumped circuit elements.

17. The system of claim 4 wherein the relative phases of each of said four or more RF signal sources are controlled such that the worst case effective immitance phase amplitude seen at any of the power combiner inputs is limited as output power varies over a specified ratio, said worst-case phase amplitudes satisfying at least one of:

less than 1 degree over a factor of 5 in power;

less than 5 degrees over a factor of ten in power;

less than 10 degrees over a factor of 20 in power; and

less than 30 degrees over a factor of 50 in power

18. The system of claim 4 wherein the relative phases of each of said four or more RF signal sources are controlled such that the worst case effective immitance phase amplitude seen at any of the power combiner inputs is limited as output power varies over a specified ratio, said worst-case phase amplitudes satisfying at least one of:

less than two degrees over a factor of ten in power;

five degrees over a factor of twenty in power; and

fifteen degrees over a factor of fifty in power.

19. The system of claim 4 wherein the relative phases of said RF signal sources are adjusted as a continuous function of the commanded output power or voltage amplitude.

20. A radio-frequency (RF) system comprising:

N greater than or equal to 8 RF signal sources each of said at least N RF signal sources having an output port; and

a reactive power combiner having at least eight input ports and an output port, each reactive power combiner input port coupled to a corresponding one of the RF signal source output ports and wherein said reactive power combiner includes 2N−

2 reactive branches, each of the 2N−

2 reactive branches having a predetermined reactance value, wherein the power delivered to the reactive power combiner output port is controlled by adjusting a phase characteristic of each of said N RF signal sources as a function of the reactance values of the 2N−

2 reactive branches.

21. The system of claim 20 wherein the relative phases of each of said N RF signal sources are controlled such that the worst case effective immitance phase amplitude seen at any of the power combiner inputs is limited as output power varies over a specified ratio, said worst-case phase amplitudes satisfying at least one of:

less than 1 degree over a factor of 5 in power;

less than 5 degrees over a factor of ten in power;

less than 10 degrees over a factor of 20 in power; and

less than 30 degrees over a factor of 50 in power.

22. The system of claim 20 wherein the relative phases of each of said N RF signal sources are controlled such that the worst case effective immitance phase amplitude seen at any of the power combiner inputs is limited as output power varies over a specified ratio, said worst-case phase amplitudes satisfying at least one of:

less than two degrees over a factor of ten in power;

five degrees over a factor of twenty in power; and

fifteen degrees over a factor of fifty in power.

23. A power amplifier system comprising:

N power amplifiers where N=2^M and M is an integer greater than one, each of the N power amplifiers having an input port and an output port;

a reactive combiner having an output port and N input ports, each of the N input ports of said reactive combiner coupled to an output of a respective one of the N power amplifiers, said reactive combiner including a plurality of reactive branches with each of the reactive branches having an impedance characteristic corresponding to one of M specified impedance levels; and

a control system configured to adjust an amplitude of a voltage at the output of the combiner by controlling the relative phase of the N power amplifiers as a function of all of the M specified impedance levels.

24. The system of claim 23 wherein the effective susceptances at the N inputs of the combiner are zero for at least N output voltage amplitudes.

25. The system of claim 24 wherein for the range of output voltage amplitudes between those for which said effective susceptances are zero, the effective susceptance amplitude at each of the combiner inputs is substantially the same.

26. The system of claim 24 wherein for the range of output voltage amplitudes between those for which said effective susceptances are zero, the effective immitance phase amplitude at each of the combiner inputs is substantially the same.

27. The system of claim 23 wherein said M specified impedance levels are selected as a function of a single design parameter and a specified load resistance.

28. The system of claim 23 wherein there are 2N−

2 of said reactive branches with 2^m branches having an impedance magnitude at the m^th=1 . . . M impedance level.

29. The system of claim 23 wherein the power combiner can be transformed into a network including a binary tree of reactances having complementary reactances at each bifurcation in the tree through application of zero or more of:

T-Delta transformations; and

topological duality transformations.

30. The system of claim 23 wherein M=3.

31. The system of claim 30 wherein the relative phases of each of said N RF signal sources are controlled such that the effective immitance phase amplitude seen at any of the power combiner inputs is limited as output power varies over a specified ratio, said phase amplitudes satisfying at least one of:

1 degree for 20;

1 power;

2 degrees for 50;

1 power; and

5 degrees for 100;

1 power.

32. The system of claim 23 wherein said control system is further configured to adjust an amplitude of a voltage at the output of the combiner by controlling one or more of:

the power supply voltages of said N power amplifiers; and

the RF input drive amplitudes of said N power amplifiers.