Amplification systems and methods using fixed and modulated power supply voltages and buck-boost control

• US 6,285,251 B1
• Filed: 08/17/2000
• Issued: 09/04/2001
• Est. Priority Date: 04/02/1998
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Camp, William O. Jr., Dent, Paul Wilkinson
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Nguyen, Khanh Van
• Application Number:
  US09/640,408
• Time in Patent Office: 383 Days
• Field of Search: 330/124 R, 330/127, 330/129, 330/297, 330/136
• US Class Current: 330/127
• CPC Class Codes: H02M 3/1582 Buck-boost converters H02M3/1584 takes precedence
• CPC Class Codes: H03F 1/0227 using supply converters
• CPC Class Codes: H03F 1/0288 using a main and one or several auxiliary peaking amplifiers whereby the load is connected to the main amplifier using an impedance inverter, e.g. Doherty amplifiers
• CPC Class Codes: H03F 1/0294 using vector summing of two or more constant amplitude phase-modulated signals
• CPC Class Codes: H03F 2200/198 A hybrid coupler being used as coupling circuit between stages of an amplifier circuit
• CPC Class Codes: H03F 2200/324 An amplitude modulator or demodulator being used in the amplifier circuit
• CPC Class Codes: H03F 2200/331 Sigma delta modulation being used in an amplifying circuit
• CPC Class Codes: H03F 2200/336 A I/Q, i.e. phase quadrature, modulator or demodulator being used in an amplifying circuit
• CPC Class Codes: H03F 2200/432 Two or more amplifiers of different type are coupled in parallel at the input or output, e.g. a class D and a linear amplifier, a class B and a class A amplifier
• CPC Class Codes: H03F 2200/504 the supply voltage or current being continuously controlled by a controlling signal, e.g. the controlling signal of a transistor implemented as variable resistor in a supply path for, an IC-block showed amplifier
• CPC Class Codes: H03F 2200/516 Some amplifier stages of an amplifier use supply voltages of different value
• CPC Class Codes: H03F 2200/537 A transformer being used as coupling element between two amplifying stages
• CPC Class Codes: H03F 2200/541 Transformer coupled at the output of an amplifier
• CPC Class Codes: H03F 2200/543 A transmission line being used as coupling element between two amplifying stages
• CPC Class Codes: H03F 2203/21142 Output signals of a plurality of power amplifiers are parallel combined to a common output
• CPC Class Codes: H03F 3/211 using a combination of several amplifiers H03F3/60 takes precedence
• CPC Class Codes: H03F 3/602 Combinations of several amplifiers
• CPC Class Codes: H03G 3/004 Control by varying the supply voltage
• CPC Class Codes: H03G 3/3042 in modulators, frequency-changers, transmitters or power amplifiers transmission power control in bidirectional transmission systems H04W52/04

Abstract

A signal of varying amplitude in a first saturated power amplifier that operates from a fixed voltage power supply to produce a first output signal of constant amplitude. The signal of varying amplitude also is amplified in a second saturated power amplifier that operates from a modulated voltage power supply, to produce a second output signal of amplitude that depends on the signal of varying amplitude, a modulated power supply voltage, and on an invert/noninvert control signal. The first and second output signals are combined into a load. The modulated voltage power supply is modulated while generating the invert/noninvert control signal, such that the combined first and second output signals into the load amplify the signal of varying amplitude. The signal of varying amplitude also may be of varying amplitude and varying phase.

Claims

1. A method of amplifying a signal of varying amplitude comprising:

amplifying the signal of varying amplitude in a first saturated power amplifier that operates from a fixed voltage power supply to produce a first output signal of constant amplitude;

amplifying the signal of varying amplitude in a second saturated power amplifier that operates from a modulated voltage power supply to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and on an invert/noninvert control signal;

combining the first and second output signals into a load; and

modulating the modulated voltage power supply while generating the invert/noninvert control signal such that the combined first and second output signals into the load amplify the signal of varying amplitude.

2. The method according to claim 1 wherein the signal of varying amplitude is of varying amplitude and varying phase.

3. The method according to claim 1 wherein the modulating the modulated voltage power supply comprises modulating the modulated voltage power supply while generating the invert/noninvert control signal to invert the second output signal relative to the signal of varying amplitude when an instantaneous amplitude of the signal of varying amplitude is less than an amplitude that is produced in the load by the first saturated amplifier and to noninvert the second output signal relative to the signal of varying amplitude when the instantaneous amplitude of the signal is greater than the amplitude that is produced in the load by the first saturated amplifier, such that the combined first and second output signals into the load amplify the signal of varying amplitude.

4. The method according to claim 1 wherein the amplifying the signal using a second saturated power amplifier comprises amplifying the signal of varying amplitude using a second saturated power amplifier that operates from a modulated voltage power supply and that uses bilateral amplification devices that draw current from the modulated voltage power supply and that supply current to the modulated voltage power supply, to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and the invert/noninvert control signal.

5. The method according to claim 1 wherein the amplifying the signal in a first saturated power amplifier comprises amplifying the signal of varying amplitude in a first saturated power amplifier that operates from a fixed voltage power supply, that draws current from the fixed voltage power supply and that does not supply current to the fixed voltage power supply, to produce a first output signal of constant amplitude.

6. The method according to claim 1 wherein the combining the first and second output signals into a load comprises serially combining the first and second output signals with the load.

7. The method according to claim 6 wherein the serially combining the first and second output signals with the load comprises connecting one of the first and second output signals directly to the load and connecting the other of the first and second output signals to the load via an isolation transformer.

8. The method according to claim 6 wherein the serially combining the first and second output signals with the load comprises connecting the respective first and second output signals to the load via respective first and second quarter wave lines.

9. The method according to claim 1 wherein the amplifying the signal using a second saturated power amplifier comprises amplifying the signal of varying amplitude using a second saturated power amplifier that operates from a modulated voltage power supply and that draws current from the modulated voltage power supply when the invert/noninvert control signal indicates noninvert and that supplies current to the modulated voltage power supply when the invert/noninvert control signal indicates invert, to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and the invert/noninvert control signal.

10. The method according to claim 1:

wherein the signal of varying amplitude includes an in-phase component and a quadrature-phase component;

wherein the amplifying the signal of varying amplitude in a first saturated power amplifier comprises amplifying the in-phase component of the signal of varying amplitude in a first saturated power amplifier that operates from a first fixed voltage power supply to produce a first output signal of constant amplitude;

wherein the amplifying the signal of varying amplitude in a second saturated power amplifier comprises amplifying the in-phase component of the signal of varying amplitude in a second saturated power amplifier that operates from a first modulated voltage power supply to produce a second output signal of amplitude that depends on the in-phase component of the signal of varying amplitude, the first modulated power supply voltage and on a first invert/noninvert control signal;

the method further comprising;

amplifying the quadrature-phase component of the signal of varying amplitude in a third saturated power amplifier that operates from a second fixed voltage power supply to produce a third output signal of constant amplitude; and

amplifying the quadrature-phase component of the signal of varying amplitude in a fourth saturated power amplifier that operates from a second modulated voltage power supply to produce a fourth output signal of amplitude that depends on the quadrature-phase component of the signal of varying amplitude, the second modulated power supply voltage and on a second invert/noninvert control signal;

wherein the combining comprises combining the first, second, third and fourth output signals into a load; and

wherein the modulating comprises modulating the first and second modulated voltage power supplies while generating the first and second invert/noninvert control signals such that the combined first, second, third and fourth output signals into the load amplify the signal of varying amplitude.

11. A system for amplifying a signal of varying amplitude, comprising:

a fixed voltage power supply;

a modulated voltage power supply;

a first saturated power amplifier that is responsive to the signal of varying amplitude and that is powered by the fixed voltage power supply to produce a first output signal of constant amplitude;

a second saturated power amplifier that is responsive to the signal of varying amplitude and to an invert/noninvert control signal and that is powered by the modulated voltage power supply to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and on the invert/noninvert control signal;

a coupler that couples the first and second output signals to a load; and

a controller that modulates the modulated voltage power supply while generating the invert/noninvert control signal.

12. The system according to claim 11 wherein the signal of varying amplitude is of varying amplitude and varying phase.

13. The system according to claim 11 wherein the controller modulates the modulated voltage power supply while generating the invert/noninvert control signal to invert the second output signal relative to the signal of varying amplitude when an instantaneous amplitude of the signal of varying amplitude is less than an amplitude that is produced in the load by the first saturated amplifier and to noninvert the second output signal relative to the signal of varying amplitude when the instantaneous amplitude of the signal is greater than the amplitude that is produced in the load by the first saturated amplifier.

14. The system according to claim 11 wherein the second saturated power amplifier comprises bilateral amplification devices that draw current from the modulated voltage power supply and that supply current to the modulated voltage power supply.

15. The system according to claim 11 wherein the first saturated power amplifier draws current from the fixed voltage power supply and does not supply current to the fixed voltage power supply.

16. The system according to claim 11 wherein the coupler serially couples the first and second output signals with the load.

17. The system according to claim 11 wherein the coupler comprises a direct connection of one of the first and second output signals to the load and an isolation transformer that connects the other of the first and second output signals to the load.

18. The system according to claim 11 wherein the coupler comprises a first quarter wavelength line that connects the first output signal to the load and a second quarter wave line that connects the second output signal to the load.

19. The system according to claim 11 wherein the modulated voltage power supply comprises a bidirectional Direct Current (DC) power conversion circuit that transfers DC power in a forward direction from a first terminal at a first voltage to a second terminal at a second voltage, wherein the first voltage is higher than the second voltage, relative to a common voltage at a common terminal and in a reverse direction from the second terminal at the second voltage to the first terminal at the first voltage, the bidirectional DC power conversion circuit comprising:

an inductor having first and second inductor ends;

a first switch that is connected between the first terminal and the first inductor end to define a node therebetween;

a second switch that is connected between the node and the common terminal;

the second inductor end being connected to the second terminal; and

a switch controller that simultaneously closes the first switch and opens the second switch, that simultaneously opens the first switch and closes the second switch and that maintains the first switch closed and the second switch open for a percentage of time that approximates a ratio of the second voltage to the first voltage.

20. The system according to claim 11:

wherein the signal of varying amplitude includes an in-phase component and a quadrature-phase component;

wherein the first saturated power amplifier is responsive to the in-phase component of the signal of varying amplitude and is powered by the fixed voltage power supply to produce the first output signal of constant amplitude;

wherein the second saturated power amplifier is responsive to the in-phase component of the signal of varying amplitude and to the invert/noninvert control signal and is powered by the modulated voltage power supply to produce the second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and on the invert/noninvert control signal;

the system further comprising;

a third saturated power amplifier that is responsive to the quadrature-phase component of the signal of varying amplitude and that is powered by a second fixed voltage power supply to produce a third output signal of constant amplitude; and

a fourth saturated power amplifier that is responsive to the quadrature-phase component of the signal of varying amplitude and that is powered by a second modulated voltage power supply to produce a fourth output signal of amplitude that depends on the quadrature-phase component of the signal of varying amplitude, the second modulated power supply voltage and on a second invert/noninvert control signal;

wherein the coupler couples the first, second, third and fourth output signals into a load; and

wherein the controller modulates the first and second modulated voltage power supplies while generating the first and second invert/noninvert control signals such that the combined first, second, third and fourth output signals into the load amplify the signal of varying amplitude.

21. A system for amplifying a signal of varying amplitude, comprising:

first means for amplifying the signal of varying amplitude from a fixed voltage power supply to produce a first output signal of constant amplitude;

second means for amplifying the signal of varying amplitude from a modulated voltage power supply to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and on an invert/noninvert control signal;

means for combining the first and second output signals into a load; and

means for modulating the modulated voltage power supply while generating the invert/noninvert control signal such that the combined first and second output signals into the load amplify the signal of varying amplitude.

22. The system according to claim 21 wherein the signal of varying amplitude is of varying amplitude and varying phase.

23. The system according to claim 21 wherein the means for modulating comprises means for modulating the modulated voltage power supply while generating the invert/noninvert control signal to invert the second output signal relative to the signal of varying amplitude when an instantaneous amplitude of the signal of varying amplitude is less than an amplitude that is produced in the load by the first saturated amplifier and to noninvert the second output signal relative to the signal of varying amplitude when the instantaneous amplitude of the signal is greater than the amplitude that is produced in the load by the first saturated amplifier, such that the combined first and second output signals into the load amplify the signal of varying amplitude.

24. The system according to claim 21 wherein the second means for amplifying comprises second means for amplifying the signal of varying amplitude from a modulated voltage power supply and that uses bilateral amplification devices that draw current from the modulated voltage power supply and that supply current to the modulated voltage power supply, to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and the invert/noninvert control signal.

25. The system according to claim 21 wherein the first means for amplifying comprises first means for amplifying the signal of varying amplitude in a first saturated power amplifier that operates from a fixed voltage power supply, that draws current from the fixed voltage power supply and that does not supply current to the fixed voltage power supply, to produce a first output signal of constant amplitude.

26. The system according to claim 21 wherein the means for combining comprises means for serially combining the first and second output signals with the load.

27. The system according to claim 26 wherein the means for serially combining comprises means for connecting one of the first and second output signals directly to the load and means for connecting the other of the first and second output signals to the load via an isolation transformer.

28. The system according to claim 26 wherein the means for serially combining comprises means for connecting the respective first and second output signals to the load via respective first and second quarter wave lines.

29. The system according to claim 21 wherein the second means for amplifying comprises second means for amplifying the signal of varying amplitude using a second saturated power amplifier that operates from a modulated voltage power supply and that draws current from the modulated voltage power supply when the invert/noninvert control signal indicates noninvert and that supplies current to the modulated voltage power supply when the invert/noninvert control signal indicates invert, to produce a second output signal of amplitude that depends on the signal of varying amplitude, the modulated power supply voltage and on an invert/noninvert control signal.

30. The system according to claim 21:

wherein the signal of varying amplitude includes an in-phase component and a quadrature-phase component;

wherein the first means for amplifying comprises first means for amplifying the in-phase component of the signal of varying amplitude from a first fixed voltage power supply to produce a first output signal of constant amplitude;

wherein the second means for amplifying comprises second means for amplifying the in-phase component of the signal of varying amplitude from a first modulated voltage power supply to produce a second output signal of amplitude that depends on the in-phase component of the signal of varying amplitude, the first modulated power supply voltage and on a first invert/noninvert control signal;

the system further comprising;

third means for amplifying the quadrature-phase component of the signal of varying amplitude from a second fixed voltage power supply to produce a third output signal of constant amplitude; and

fourth means for amplifying the quadrature-phase component of the signal of varying amplitude from a second modulated voltage power supply to produce a fourth output signal of amplitude that depends on the quadrature-phase component of the signal of varying amplitude, the second modulated power supply voltage and on a second invert/noninvert control signal;

wherein the means for combining comprises means for combining the first, second, third and fourth output signals into a load; and

wherein the means for modulating comprises means for modulating the first and second modulated voltage power supplies while generating the first and second invert/noninvert control signals such that the combined first, second, third and fourth output signals into the load amplify the signal of varying amplitude.

31. A method of amplifying a signal, comprising:

generating a first input signal, a second input signal, an invert/noninvert control signal and a variable power supply voltage from the signal;

amplifying the first input signal using a fixed power supply voltage, to produce a first output signal;

selectively invertingly and noninvertingly amplifying the second input signal using the variable power supply voltage in response to the invert/noninvert control signal, to produce a second output signal; and

coupling the first and second output signals into a load to thereby amplify the signal.

32. The method according to claim 31 wherein the selectively invertingly and noninvertingly amplifying the second input signal comprises selectively drawing current from the modulated voltage power supply and selectively supplying current to the modulated voltage power supply.

33. The method according to claim 32 wherein the amplifying the first input signal comprises amplifying the first input signal using a fixed power supply voltage by drawing current from the fixed power supply voltage without supplying current to the fixed power supply voltage.

34. The method according to claim 31 wherein the coupling comprises coupling one of the first and second output signals directly to the load and coupling the other of the first and second output signals to the load via an isolation transformer.

35. The method according to claim 31 wherein the coupling comprises coupling the respective first and second output signals to the load via respective first and second quarter wave lines.

36. The method according to claim 31:

wherein the signal includes an in-phase component and a quadrature-phase component;

wherein the generating comprises generating a first in-phase input signal, a second in-phase input signal, a third quadrature-phase input signal and a fourth quadrature-phase input signal, a first invert/noninvert control signal, a second invert/noninvert control signal, a first variable power supply voltage and a second variable power supply voltage from the in-phase component and the quadrature-phase component;

wherein the amplifying the first input signal comprises amplifying the first in-phase input signal using a first fixed power supply voltage, to produce the first output signal;

wherein the selectively invertingly and noninvertingly amplifying the second input signal comprises selectively invertingly and noninvertingly amplifying the second in-phase input signal using the first variable power supply voltage in response to the first invert/noninvert control signal, to produce the second output signal;

the method further comprising;

amplifying the third quadrature-phase input signal using a second fixed power supply voltage, to produce a third output signal; and

selectively invertingly and noninvertingly amplifying the fourth quadrature-phase input signal using the second variable power supply voltage in response to the second invert/noninvert control signal, to produce a fourth output signal; and

wherein the coupling step comprises coupling the first, second, third and fourth output signals into a load to thereby amplify the signal.

37. A system for amplifying a signal, comprising:

means for generating a first input signal, a second input signal, an invert/noninvert control signal and a variable power supply voltage from the signal;

means for amplifying the first input signal using a fixed power supply voltage, to produce a first output signal;

means for selectively invertingly and noninvertingly amplifying the second input signal using the variable power supply voltage in response to the invert/noninvert control signal, to produce a second output signal; and

means for coupling the first and second output signals into a load to thereby amplify the signal.

38. The system according to claim 37 wherein the means for selectively invertingly and noninvertingly amplifying the second input signal comprises means for selectively drawing current from the variable power supply voltage and for selectively supplying current to the variable power supply voltage.

39. The system according to claim 38 wherein the means for amplifying comprises means for amplifying the first input signal using a fixed power supply voltage by drawing current from the fixed power supply voltage without supplying current to the fixed power supply voltage.

40. The system according to claim 37 wherein the means for coupling comprises means for coupling one of the first and second output signals directly to the load and means for coupling the other of the first and second output signals to the load via an isolation transformer.

41. The system according to claim 37 wherein the means for coupling comprises means for coupling the respective first and second output signals to the load via respective first and second quarter wave lines.

42. The system according to claim 37:

wherein the signal includes an in-phase component and a quadrature-phase component;

wherein the means for generating comprises means for generating a first in-phase input signal, a second in-phase input signal, a third quadrature-phase input signal and a fourth quadrature-phase input signal, a first invert/noninvert control signal, a second invert/noninvert control signal, a first variable power supply voltage and a second variable power supply voltage from the in-phase component and the quadrature-phase component;

wherein the means for amplifying the first input signal comprises means for amplifying the first in-phase input signal using a first fixed power supply voltage, to produce the first output signal;

wherein the means for selectively invertingly and noninvertingly amplifying the second input signal comprises means for selectively invertingly and noninvertingly amplifying the second in-phase input signal using the first variable power supply voltage in response to the first invert/noninvert control signal, to produce the second output signal;

the system further comprising;

means for amplifying the third quadrature-phase input signal using a second fixed power supply voltage, to produce a third output signal; and

means for selectively invertingly and noninvertingly amplifying the fourth quadrature-phase input signal using the second variable power supply voltage in response to the second invert/noninvert control signal, to produce a fourth output signal; and

wherein the means for coupling comprises means for coupling the first, second, third and fourth output signals into a load to thereby amplify the signal.

43. A system for amplifying a signal, comprising:

a signal generator that generates a first input signal, a second input signal, an invert/noninvert control signal and a variable power supply voltage from the signal;

a first power amplifier that amplifies the first input signal using a fixed power supply voltage, to produce a first output signal;

a second power amplifier that selectively invertingly and noninvertingly amplifies the second input signal using the variable power supply voltage in response to the invert/noninvert control signal, to produce a second output signal; and

a coupler that couples the first and second output signals into a load to thereby amplify the signal.

44. The system according to claim 43 wherein the second power amplifier selectively draws current from the modulated voltage power supply and selectively supplies current to the modulated voltage power supply.

45. The system according to claim 44 wherein the first power amplifier draws current from the fixed power supply voltage but does not supply current to the fixed power supply voltage.

46. The system according to claim 43 wherein the coupler comprises a direct connector of one of the first and second output signals to the load and an isolation transformer that connects the other of the first and second output signals to the load.

47. The system according to claim 43 wherein the coupler comprises a first quarter wave line that couples the first output signal to the load and a second quarter wave line that couples the second output signal to the load.

48. The system according to claim 43:

wherein the signal includes an in-phase component and a quadrature-phase component;

wherein the signal generator generates a first in-phase input signal, a second in-phase input signal, a third quadrature-phase input signal and a fourth quadrature-phase input signal, a first invert/noninvert control signal, a second invert/noninvert control signal, a first variable power supply voltage and a second variable power supply voltage from the in-phase component and the quadrature-phase component;

wherein the first power amplifier amplifies the first in-phase input signal using a first fixed power supply voltage, to produce the first output signal;

wherein the second power amplifier selectively invertingly and noninvertingly amplifies the second in-phase input signal using the first variable power supply voltage in response to the first invert/noninvert control signal, to produce the second output signal;

the system further comprising;

a third power amplifier that amplifies the third quadrature-phase input signal using a second fixed power supply voltage, to produce a third output signal; and

a fourth power amplifier that selectively invertingly and noninvertingly amplifies the fourth quadrature-phase input signal using the second variable power supply voltage in response to the second invert/noninvert control signal, to produce a fourth output signal; and

wherein the coupler couples the first, second, third and fourth output signals into a load to thereby amplify the signal.

49. The system according to claim 43 wherein the signal generator comprises a bidirectional Direct Current (DC) power conversion circuit that transfers DC power in a forward direction from a first terminal at a first voltage to a second terminal at a second voltage, wherein the first voltage is higher than the second voltage relative to a common voltage at a common terminal, and in a reverse direction from the second terminal at the second voltage to the first terminal at the first voltage, the bidirectional DC power conversion circuit comprising:

an inductor having first and second inductor ends;

a first switch that is connected between the first terminal and the first inductor end to define a node therebetween;

a second switch that is connected between the node and the common terminal;

the second inductor end being connected to the second terminal; and

a switch controller that simultaneously closes the first switch and opens the second switch, that simultaneously opens the first switch and closes the second switch and that maintains the first switch closed and the second switch open for a percentage of time that approximates a ratio of the second voltage to the first voltage.

50. A method of amplifying a signal, comprising:

generating first and second input signals and first and second variable power supply voltages from the signal;

amplifing the first input signal using the first variable power supply voltage, to produce a first output signal;

amplifying the second input signal using the second variable power supply voltage, to produce a second output signal; and

coupling the first and second output signals into a load to thereby amplify the signal.

51. The method according to claim 50 wherein the coupling comprises coupling one of the first and second output signals directly to the load and coupling the other of the first and second output signals to the load via an isolation transformer.

52. The method according to claim 50 wherein the coupling comprises coupling the respective first and second output signals to the load via respective first and second quarter wave lines.

53. A system for amplifying a signal, comprising:

means for generating first and second input signals and first and second variable power supply voltages from the signal;

first means for amplifying the first input signal using the first variable power supply voltage, to produce a first output signal;

second means for amplifying the second input signal using the second variable power supply voltage, to produce a second output signal; and

means for coupling the first and second output signals into a load to thereby amplify the signal.

54. The system according to claim 53 wherein the means for coupling comprises means for coupling one of the first and second output signals directly to the load and means for coupling the other of the first and second output signals to the load via an isolation transformer.

55. The system according to claim 53 wherein the means for coupling comprises means for coupling the respective first and second output signals to the load via respective first and second quarter wave lines.

56. A system for amplifying a signal, comprising:

a signal generator that generates first and second input signals and first and second variable power supply voltages from the signal;

a first power amplifier that amplifies the first input signal using the first variable power supply voltage, to produce a first output signal;

a second power amplifier that selectively amplifies the second input signal using the second variable power supply voltage, to produce a second output signal; and

a coupler that couples the first and second output signals into a load to thereby amplify the signal.

57. The system according to claim 56 wherein the coupler comprises a direct connector of one of the first and second output signals to the load and an isolation transformer that connects the other of the first and second output signals to the load.

58. The system according to claim 56 wherein the coupler comprises a first quarter wave line that couples the first output signal to the load and a second quarter wave line that couples the second output signal to the load.