System and method for current-mode amplitude modulation

• US 6,785,521 B2
• Filed: 03/21/2001
• Issued: 08/31/2004
• Est. Priority Date: 03/21/2001
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Hadjichristos, Aristotele, Pehlke, David R.
• Primary Examiner(s): Tran, Pablo N.
• Application Number:
  US09/813,593
• Publication Number:
  US 20020137480A1
• Time in Patent Office: 1,259 Days
• Field of Search: 455/127.1, 455/110, 455/73, 455/126, 455/127.2, 455/127.3, 455/68, 455/71, 455/108, 455/109, 455/232.1, 455/343.1, 455/525, 325/295-298, 325/300, 325/301, 330/10, 330/136, 330/296
• US Class Current: 455/108
• CPC Class Codes: H03F 1/0261 with control of the polarisation voltage or current, e.g. gliding Class A
• 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: H04B 1/04 Circuits
• CPC Class Codes: H04B 2001/0491 with frequency synthesizers, frequency converters or modulators

Abstract

An amplifier circuit includes a power amplifier biased for saturated mode operation, and a controllable current source to provide supply current to the power amplifier. The controllable current source effects desired amplitude modulation of the output signal from the power amplifier by modulating the supply current it provides responsive to an amplitude information signal.

Claims

1. A power amplifier circuit comprising:

a power amplifier biased for saturated mode operation to generate an RF output signal by amplifying a constant-envelope RF input signal; and

a current source responsive to an amplitude information signal to modulate a supply current of said power amplifier to impart desired amplitude modulation to said RF output signal.

2. The power amplifier circuit of claim 1 wherein said current source comprises a closed-loop control circuit to linearly vary said supply current responsive to said amplitude information signal.

3. The power amplifier circuit of claim 2 wherein said closed-loop control circuit comprises:

a current sensor to generate a feedback signal proportional to said supply current of said power amplifier;

a pass transistor responsive to a control signal to control said supply current of said power amplifier; and

a control amplifier circuit to generate said control signal responsive to said feedback signal and said amplitude information signal.

4. The power amplifier circuit of claim 3 wherein said control amplifier circuit comprises an operational amplifier, said operational amplifier comprising:

a first input to receive said feedback signal;

a second input to receive said amplitude information signal; and

an output to provide said control signal used to control said pass transistor.

5. The power amplifier circuit of claim 4 wherein said first input comprises an inverting input of said operational amplifier, such that said feedback signal operates as a negative feedback signal to said operational amplifier.

6. The power amplifier circuit of claim 3 wherein said current sensor comprises a resistor disposed in series in a supply current path carrying said supply current of said power amplifier.

7. The power amplifier circuit of claim 3 further comprising:

at least two sense resistors; and

a switch circuit responsive to a selection signal to selectively switch a selected one of said at least two sense resistors into a supply current path carrying said supply current of said power amplifier.

8. The power amplifier circuit of claim 7 further comprising selection logic to generate said selection signal based on a magnitude of said supply current of said power amplifier.

9. The power amplifier circuit of claim 3 wherein said pass transistor comprises a field-effect transistor (FET) device disposed in a supply current path carrying said supply current of said power amplifier.

10. The power amplifier circuit of claim 3 wherein said control amplifier circuit comprises an operational amplifier linearly responsive to said amplitude information signal.

11. The power amplifier circuit of claim 3 further comprising a feedback signal amplifier disposed in a feedback loop of said control amplifier to provide amplification of said feedback signal.

12. The power amplifier circuit of claim 11 wherein said feedback signal amplifier comprises an operational amplifier.

13. The power amplifier circuit of claim 1 wherein said current source comprises a current mirror to control said supply current of said power amplifier responsive to said amplitude information signal.

14. The power amplifier circuit of claim 13 wherein said current mirror comprises:

a first transistor circuit to generate a control current responsive to said amplitude information signal; and

a second transistor circuit coupled to said first transistor circuit and disposed in a supply path of said power amplifier to control said supply current of said power amplifier proportionate to said control current.

15. The power amplifier circuit of claim 13 wherein said current mirror comprises first and second matched transistor circuits with matched device geometries, wherein a scaling between said matched device geometries determines a current gain between said control current and said supply current.

16. The power amplifier circuit of claim 13 wherein said current mirror comprises:

a first field effect transistor (FET) device to carry a control current set by said amplitude information signal; and

a second FET device disposed in a supply path carrying said supply current and coupled to said first FET device to control said supply current of said power amplifier in proportion to said control current.

17. The power amplifier circuit of claim 13 wherein said current mirror comprises:

a first bipolar junction transistor (BJT) device to carry a control current set by said amplitude information signal; and

a second BJT device disposed in a supply path carrying said supply current and coupled to said first BJT device to control said supply current of said power amplifier in proportion to said control current.

18. The power amplifier circuit of claim 13 wherein said current mirror comprises:

an input circuit to generate a control voltage signal proportional to said amplitude information signal;

a reference current circuit to generate a reference current into a reference load responsive to said control voltage signal, said reference load providing a feedback voltage signal to said input circuit to maintain proportionality between said amplitude information signal and said reference current; and

an output current circuit to control said supply current to said power amplifier proportional to said reference current into said reference load.

19. The power amplifier circuit of claim 18 wherein said input circuit comprises a bipolar transistor comprising a base terminal coupled to said amplitude information signal, a collector terminal coupled to a supply voltage through a collector resistor, and an emitter terminal coupled to a signal ground through an emitter degeneration resistor, said emitter terminal further coupled to said feedback voltage signal from said reference load.

20. The power amplifier circuit of claim 18 wherein said reference current circuit comprises a first field-effect transistor comprising a gate terminal coupled to said control voltage signal, a source terminal coupled to a supply voltage, and a drain terminal coupled to said reference load to provide said reference current proportional to said control voltage signal to said reference load.

21. The power amplifier circuit of claim 20 wherein said output current circuit comprises a second field-effect transistor matched to said first field-effect transistor and comprising a gate terminal coupled to said control voltage signal, a source terminal coupled to said supply voltage, and a drain terminal to provide said supply current to said power amplifier proportional to said reference current into said reference load.

22. The power amplifier circuit of claim 1 wherein said power amplifier comprises a heterojunction bipolar transistor (HBT) device.

23. The power amplifier circuit of claim 1 wherein said power amplifier circuit comprises a portion of a RF transmitter in a mobile terminal for use in a wireless communication network.

24. The power amplifier circuit of claim 1 wherein said power amplifier circuit comprises a portion of a base station transmitter used in a base station forming part of a wireless communication network.

25. A RF transmitter comprising:

a baseband processor to generate a phase information signal and an amplitude information signal;

a modulator responsive to said phase information signal to generate a phase-modulated signal with a constant envelope;

a power amplifier circuit responsive to said phase-modulated signal to generate said RF output signal, said power amplifier circuit comprising;

a power amplifier biased for saturated mode operation to generate said RF output signal by amplifying said phase-modulated signal; and

a current source responsive to said amplitude information signal to modulate a supply current of said power amplifier to impart desired amplitude modulation to said RF output signal.

26. The RF transmitter of claim 25 wherein said current source comprises a closed-loop control circuit to linearly vary said supply current responsive to said amplitude information signal.

27. The RF transmitter of claim 26 wherein said closed-loop control circuit comprises:

a current sensor to generate a feedback signal proportional to said supply current of said power amplifier;

a pass transistor responsive to a control signal to control said supply current of said power amplifier; and

a control amplifier circuit to generate said control signal responsive to said feedback signal and said amplitude information signal.

28. The RF transmitter of claim 27 wherein said control amplifier circuit comprises an operational amplifier, said operational amplifier comprising:

a first input to receive said feedback signal;

a second input to receive said amplitude information signal; and

an output to provide said control signal used to control said pass transistor.

29. The RF transmitter of claim 28 wherein said control amplifier circuit comprises a second amplifier disposed in a feedback loop of said operational amplifier to amplify said feedback signal.

30. The RF transmitter of claim 28 wherein said first input comprises an inverting input of said operational amplifier, such that said feedback signal operates as a negative feedback signal to said operational amplifier.

31. The RF transmitter of claim 27 wherein said current sensor comprises a resistor disposed in series in a supply current path carrying said supply current of said power amplifier.

32. The RF transmitter of claim 27 further comprising:

at least two sense resistors; and

a switch circuit responsive to a selection signal to selectively switch a selected one of said two sense resistors into a supply current path carrying said supply current of said power amplifier.

33. The RF transmitter of claim 32 further comprising selection logic to generate said selection signal based on a magnitude of said supply current of said power amplifier.

34. The RF transmitter of claim 27 wherein said pass transistor comprises a field-effect transistor (FET) device disposed in a supply current path carrying said supply current of said power amplifier.

35. The RF transmitter of claim 27 wherein said control amplifier circuit of said closed-loop control circuit comprises an operational amplifier linearly responsive to said amplitude information signal.

36. The RF transmitter of claim 25 wherein said current source comprises a current mirror to control said supply current of said power amplifier responsive to said amplitude information signal.

37. The RF transmitter of claim 36 wherein said current mirror comprises:

a first transistor circuit to generate a control current responsive to said amplitude information signal; and

a second transistor circuit coupled to said first transistor circuit and disposed in a supply path of said power amplifier to control said supply current of said power amplifier proportionate to said control current.

38. The RF transmitter of claim 36 wherein said current mirror comprises first and second matched transistor circuits with matched device geometries, wherein a scaling between said matched device geometries determines a current gain between said control current and said supply current.

39. The RF transmitter of claim 36 wherein said current mirror comprises:

a first field effect transistor (FET) device to carry a control current set by said amplitude information signal; and

a second FET device disposed in a supply path carrying said supply current and coupled to said first FET device to control said supply current of said power amplifier in proportion to said control current.

40. The RF transmitter of claim 36 wherein said current mirror comprises:

a first bipolar junction transistor (BJT) device to carry a control current set by said amplitude information signal; and

a second BJT device disposed in a supply path carrying said supply current and coupled to said first BJT device to control said supply current of said power amplifier in proportion to said control current.

41. The RF transmitter of claim 36 wherein said current mirror comprises:

an input circuit to generate a control voltage signal proportional to said amplitude information signal;

a reference current circuit to generate a reference current into a reference load responsive to said control voltage signal, said reference load providing a feedback voltage signal to said input circuit to maintain proportionality between said amplitude information signal and said reference current; and

an output current circuit to control said supply current to said power amplifier proportional to said reference current into said reference load.

42. The RF transmitter of claim 25 wherein said RF transmitter comprises a portion of a mobile terminal used in a wireless communication network.

43. The RF transmitter of claim 42 wherein said baseband processor comprises digital signal processor to generate said phase and amplitude information signals based on input data and a predetermined modulation scheme.

44. The RF transmitter of claim 43 wherein said mobile terminal further comprises a user interface, said user interface comprising a microphone to generate an audio input signal.

45. The RF transmitter of claim 42 further comprising an analog-to-digital converter to generate at least a portion of said input data to said digital signal processor based on digitizing said audio input signal.

46. A method of modulating a transmit signal, the method comprising:

amplifying a constant-envelope signal with a power amplifier operating in a saturated mode to generate a transmit signal, said power amplifier having a supply current provided by a variable current source; and

varying said supply current provided by said current source to said power amplifier in response to an amplitude information signal to modulate the amplitude of said transmit signal.

47. The method of claim 46 wherein varying said supply current provided by said current source to said power amplifier in response to an amplitude information signal to modulate the amplitude of said transmit signal comprises:

generating a control voltage based on a difference between said amplitude information signal and a feedback signal that is proportional to a magnitude of said supply current provided to said power amplifier; and

controlling a pass transistor with said control voltage, said pass transistor being disposed in a supply current path carrying said supply current of said power amplifier.

48. The method of claim 47 further comprising generating said feedback signal from a sense resistor disposed in series in said supply current path of said power amplifier.

49. The method of claim 48 further comprising amplifying said feedback signal from said sense resistor before using said feedback signal to generate said control voltage.

50. The method of claim 46 wherein varying said supply current provided by said current source to said power amplifier in response to an amplitude information signal to modulate the amplitude of said transmit signal comprises:

providing said supply current to said power amplifier via a current mirror; and

controlling said current mirror to set a magnitude of said supply current based on said amplitude information signal.

51. A method of generating a RF transmit signal with both phase and amplitude modulation, the method comprising:

generating baseband phase and amplitude information signals;

modulating a RF carrier signal with said phase information signal to generate a constant-envelope phase-modulated signal;

amplifying said constant-envelope phase-modulated signal with a power amplifier biased for saturated mode operation to generate a RF transmit signal with phase modulation corresponding to said phase information signal; and

modulating a supply current of said power amplifier based on said amplitude information signal to impart amplitude modulation to said RF transmit signal corresponding to said amplitude information signal.

52. The method of claim 51 wherein modulating a supply current of said power amplifier based on said amplitude information signal to impart amplitude modulation to said RF transmit signal corresponding to said amplitude information signal comprises:

generating said supply current using a variable current source; and

varying a magnitude of said supply current provided by said variable current source based on said amplitude information signal.

53. The method of claim 51 wherein varying a magnitude of said supply current provided by said variable current source based on said amplitude information signal comprises:

generating a control voltage based on a difference between said amplitude information signal and a feedback signal that is proportional to a magnitude of said supply current provided to said power amplifier; and

controlling a pass transistor with said control voltage, said pass transistor being disposed in a supply current path carrying said supply current of said power amplifier.

54. The method of claim 53 further comprising generating said feedback signal from a sense resistor disposed in series in said supply current path of said power amplifier.

55. The method of claim 52 wherein varying a magnitude of said supply current provided by said variable current source based on said amplitude information signal comprises:

providing said supply current to said power amplifier via a current mirror; and

controlling said current mirror to set a magnitude of said supply current based on said amplitude information signal.

56. The method of claim 55 wherein controlling said current mirror to set a magnitude of said supply current based on said amplitude information signal comprises varying said supply current based on controlling a reference current of said current mirror based on said amplitude information signal, the magnitude of said reference current determining the corresponding magnitude of said supply current.

57. The method of claim 51 wherein generating baseband phase and amplitude information signals comprises:

digitizing an audio input signal to create digitized audio samples;

encoding said digitized audio samples to create a sequence of transmit symbols having both phase and amplitude information;

generating said amplitude information signal based on said amplitude information of said sequence of transmit symbols; and

generating said phase information signal based on said phase information of said sequence of transmit symbols, said phase information signal generated synchronous with said amplitude information signal.

58. The method of claim 51 wherein generating said amplitude information signal comprises:

generating said amplitude information signal as a digital signal corresponding to desired amplitude modulations; and

converting said digital signal into an analog signal to form an analog version of said amplitude information signal.

59. The method of claim 58 wherein modulating a supply current of said power amplifier based on said amplitude information signal to impart amplitude modulation to said RF transmit signal corresponding to said amplitude information signal comprises modulating said supply current of said power amplifier based on said analog version of said amplitude information signal.