Differential amplifier with gain linearization through transconductance compensation

• US 6,188,280 B1
• Filed: 08/27/1998
• Issued: 02/13/2001
• Est. Priority Date: 08/27/1998
• Status: Expired due to Term

• Current Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.), Hisys GmbH
• Original Assignee: Maxim Integrated Products, Inc. (Analog Devices, Inc.)
• Inventors: Filip, Jan
• Primary Examiner(s): Mottola, Steven J.
• Application Number:
  US09/141,874
• Time in Patent Office: 901 Days
• Field of Search: 330/252, 330/260, 330/311
• US Class Current: 330/252
• CPC Class Codes: H03F 1/3211 in differential amplifiers

Abstract

Disclosed is a differential amplifier including an emitter follower pair, a first differential pair, and a main differential amplifier. The emitter follower pair is operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal. The first differential pair is configured to feed a first differential current inversely to the emitter follower pair so that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair. The main differential amplifier is coupled to receive the shifted differential voltage signal and is configured to amplify the shifted differential voltage signal to generate an output voltage signal.

Claims

1. A differential amplifier, comprising:

an emitter follower pair operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal;

a first differential pair configured to feed a first differential current inversely to the emitter follower pair such that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair; and

a main differential amplifier, including a fifth transistor with an emitter, a sixth transistor with an emitter, and a first resistor and a second resistor coupled in series between the emitters of the fifth and sixth transistors, wherein the main differential amplifier is coupled to receive the shifted differential voltage signal and being configured to amplify the shifted differential voltage signal to generate an output voltage signal.

2. The differential amplifier as recited in claim 1, wherein the emitter follower pair includes a first transistor and a second transistor and wherein the first differential pair includes a third transistor and a fourth transistor, wherein transconductances of the first transistor and the second transistor changes to compensate for changes in transconductances of the fourth and third transistors, respectively.

3. The differential amplifier as recited in claim 2, wherein the first, second, third, fourth, fifth, and sixth transistors are bipolar junction transistors, each transistor having a base, a collector, and an emitter.

7. The differential amplifier as recited in claim 2, wherein the first, second, third, and fourth transistors are matching transistors.

8. The differential amplifier as recited in claim 2, wherein the fifth and sixth transistors are matching transistors.

9. The differential amplifier as recited in claim 2, wherein the first, second, third, fourth, fifth and sixth transistors are matching transistors.

4. A differential amplifier, comprising:

an emitter follower pair, including a first transistor and a second transistor, operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal;

a first differential pair, including a third transistor and a fourth transistor, configured to feed a first differential current inversely to the emitter follower pair such that a transconductances of the first transistor and the second transistor changes to compensate for changes in transconductances of the fourth and third transistors, respectively;

a main differential amplifier, including a fifth transistor with an emitter, a sixth transistor with an emitter, and a first resistor and a second resistor coupled in series between the emitters of the fifth and sixth transistors, wherein the main differential amplifier is coupled to receive the shifted differential voltage signal and being configured to amplify the shifted differential voltage signal to generate an output voltage signal;

wherein the first, second, third, fourth, fifth, and sixth transistors are bipolar junction transistors, each transistor having a base, a collector, and an emitter; and

wherein the emitters of the first and second transistors are coupled to the bases of the third and fourth transistors, respectively, wherein the third and fifth transistors are coupled in parallel at their bases and emitters, and wherein the fourth and sixth transistors are coupled in parallel at their bases and emitters.

5. The differential amplifier as recited in claim 4, wherein the collector of the fourth transistor is coupled to the emitter of the first transistor and the collector of the third transistor is coupled to the emitter of the second transistor such that collector currents of the third and fourth transistors are fed inversely to the second and first transistors, respectively.

6. The differential amplifier as recited in claim 5, wherein base-emitter voltages of first and second transistors change to compensate for changes in base-emitter voltages of the fourth and third transistors, respectively.

10. A differential amplifier, comprising:

an emitter follower pair, including a first transistor and a second transistor, operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal;

a first differential pair, including a third transistor and a fourth transistor, configured to feed a first differential current inversely to the emitter follower pair such that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair;

a main differential amplifier including;

a second differential pair including a fifth transistor and a sixth transistor, wherein the second differential pair is adapted to receive the shifted differential voltage to generate the output signal;

a first resistor and a second resistor coupled in series between the emitters of the fifth and sixth transistors, the first and second resistor being coupled to each other at a junction;

a first current source coupled to the junction for providing a current through the first and second resistors; and

a first load resistor and a second load resistor coupled to the collectors of the fifth and sixth transistors, respectively;

wherein the main differential amplifier is coupled to receive the shifted differential voltage signal and being configured to amplify the shifted differential voltage signal to generate an output voltage signal;

wherein the first, second, third, fourth, fifth, and sixth transistors are bipolar junction transistors, each transistor having a base, a collector, and an emitter;

wherein the emitters of the first and second transistors are coupled to the bases of the third and fourth transistors, respectively, wherein the third and fifth transistors are coupled in parallel at their bases and emitters, and wherein the fourth and sixth transistors are coupled in parallel at their bases and emitters; and

wherein the transconductances of the first transistor and the second transistor change to compensate for changes in transconductances of the fourth and third transistors, respectively.

11. The differential amplifier as recited in claim 10, wherein the first and second resistors are matching resistors and the first and second load resistors are matching resistors.

12. The differential amplifier as recited in claim 10, wherein the first and second resistors and the first and second load resistors are all matching resistors to provide a process-independent gain.

13. The differential amplifier as recited in claim 11, wherein the gain of the differential amplifier is determined in accordance with the values of the first resistor, the first load resistor, an emitter area of the sixth transistor, and an emitter area of the fourth transistor.

14. The differential amplifier as recited in claim 11, wherein the output is taken from the collectors of the fifth and sixth transistors.

16. The differential amplifier as recited in claim 10, wherein the collectors of the first and sixth transistors are coupled together and wherein the collectors of the second and fifth transistors are coupled together.

17. The differential amplifier as recited in claim 16, wherein the gain of the differential amplifier is determined in accordance with the values of the first resistor and the first load resistor.

18. The differential amplifier as recited in claim 16, wherein the gain of the differential amplifier is independent of the emitter areas of the transistors.

19. The differential amplifier as recited in claim 10, further comprising:

a secondary emitter follower pair coupled between the emitter follower pair and the second differential pair to further shift the shifted differential voltage from the emitter follower pair.

20. The differential amplifier as recited in claim 19, wherein the secondary emitter follower pair includes a seventh transistor and an eighth transistor, which are bipolar junction transistors having a base, a collector, and an emitter.

21. The differential amplifier as recited in claim 20, wherein the seventh and eighth transistor are matching transistors.

22. The differential amplifier as recited in claim 21, where in the bases of the seventh and eighth transistors are coupled to the emitters of the first and second transistors, respectively, and wherein the emitters of the seventh and eighth transistors are coupled to the base of the fifth and sixth transistors, respectively.

23. The differential amplifier as recited in claim 22, further comprising:

a second current source coupled to the emitter of the seventh transistor; and

a third current source coupled to the emitter of the eighth transistor, wherein the second and third current sources are operative to set collector currents for the seventh and eighth transistors, respectively.

24. The differential amplifier as recited in claim 19, wherein the collectors of the first and sixth transistors are coupled together and wherein the collectors of the second and fifth transistors are coupled together.

25. The differential amplifier as recited in claim 24, wherein the collector currents of the third and fourth transistors are fed back to the first and second load resistors, respectively.

26. The differential amplifier as recited in claim 24, wherein the gain of the differential amplifier is determined in accordance with the values of the first resistor and the first load resistor.

27. The differential amplifier as recited in claim 23, wherein the gain of the differential amplifier is independent of the emitter areas of the transistors.

15. The differential amplifier as recited in 10, wherein the output is taken from the collector of the fifth transistor.

28. A transconductance compensating circuit for a differential amplifier having a main differential pair, comprising:

an emitter follower pair operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a shifted differential voltage signal;

a first differential pair configured to feed a first differential current inversely to the emitter follower pair such that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair; and

wherein the main differential pair includes a second differential pair including a fifth transistor with an emitter, a sixth transistor with an emitter, wherein a first resistor and a second resistor are coupled in series between the emitters of the fifth and sixth transistors, and wherein the second differential pair is adapted to receive the shifted differential voltage signal to generate an output signal.

29. The differential amplifier as recited in claim 28, wherein the emitter follower pair includes a first transistor and a second transistor and wherein the first differential pair includes a third transistor and a fourth transistor, wherein transconductances of the first transistor and the second transistor changes to compensate for changes in transconductances of the fourth and third transistors, respectively.

30. The differential amplifier as recited in claim 29, wherein the first, second, third, fourth, fifth, and sixth transistors are bipolar junction transistors, each transistor having a base, a collector, and an emitter.

31. The differential amplifier as recited in claim 30, wherein the emitters of the first and second transistors are coupled to the bases of the third and fourth transistors, respectively, wherein the third and fifth transistors are coupled in parallel at their bases and emitters, and wherein the fourth and sixth transistors are coupled in parallel at their bases and emitters.

32. The differential amplifier as recited in claim 31, wherein the collector of the fourth transistor is coupled to the emitter of the first transistor and the collector of the third transistor is coupled to the emitter of the second transistor such that collector currents of the third and fourth transistors are fed inversely to the second and first transistors, respectively.

33. The differential amplifier as recited in claim 32, wherein base-emitter voltages of first and second transistors change to compensate for changes in base-emitter voltages of the fourth and third transistors, respectively.

34. The differential amplifier as recited in claim 29, wherein the first, second, third, and fourth transistors are matching transistors.

35. The differential amplifier as recited in claim 29, wherein the fifth and sixth transistors are matching transistors.

36. The differential amplifier as recited in claim 29, wherein the first, second, third, fourth, fifth and sixth transistors are matching transistors.

37. The differential amplifier as recited in claim 36, wherein the main differential amplifier further includes:

a first resistor and a second resistor coupled in series between the emitters of the fifth and sixth transistors, the first and second resistor being coupled to each other at a junction;

a first current source coupled to junction for providing a current through the first and second resistors; and

a first load resistor and a second load resistor coupled to the collectors of the fifth and sixth transistors, respectively.

38. The differential amplifier as recited in claim 37, wherein the first and second resistors are matching resistors and the first and second load resistors are matching resistors.

39. The differential amplifier as recited in claim 38, wherein the gain of the differential amplifier is determined in accordance with the values of the first resistor, the first load resistor, an emitter area of the sixth transistor, and an emitter area of the fourth transistor.

40. The differential amplifier as recited in claim 38, wherein the first and second resistors and the first and second load resistors are all matching transistors to provide a process-independent gain.

41. The differential amplifier as recited in claim 37, wherein the output is taken from the collectors of the fifth and sixth transistors.

43. The differential amplifier as recited in claim 37, wherein the collectors of the first and sixth transistors are coupled together and wherein the collectors of the second and fifth transistors are coupled together.

44. The differential amplifier as recited in claim 40, wherein the gain of the differential amplifier is determined in accordance with the values of the first resistor and the first load resistor.

45. The differential amplifier as recited in claim 40, wherein the gain of the differential amplifier is independent of the emitter areas of the transistors.

46. The differential amplifier as recited in claim 37, further comprising:

a secondary emitter follower pair coupled between the emitter follower pair and the second differential pair to further shift the shifted differential voltage from the emitter follower pair.

47. The differential amplifier as recited in claim 46, wherein the secondary emitter follower pair includes a seventh transistor and an eighth transistor, which are bipolar junction transistors having a base, a collector, and an emitter.

48. The differential amplifier as recited in claim 47, wherein the seventh and eighth transistor are matching transistors.

49. The differential amplifier as recited in claim 48, wherein the bases of the seventh and eighth transistors are coupled to the emitters of the first and second transistors, respectively, and wherein the emitters of the seventh and eighth transistors are coupled to the base of the fifth and sixth transistors, respectively.

50. The differential amplifier as recited in claim 49, further comprising:

a second current source coupled to the emitter of the seventh transistor; and

a third current source coupled to the emitter of the eighth transistor, wherein the second and third current sources are operative to set collector currents for the seventh and eighth transistors, respectively.

51. The differential amplifier as recited in claim 46, wherein the collectors of the first and sixth transistors are coupled together and wherein the collectors of the second and fifth transistors are coupled together.

52. The differential amplifier as recited in claim 51, wherein the collector currents of the third and fourth transistors are fed back to the first and second load resistors, respectively.

53. The differential amplifier as recited in claim 51, wherein the gain of the differential amplifier is determined in accordance with the values of the first resistor and the first load resistor.

54. The differential amplifier as recited in claim 49, wherein the gain of the differential amplifier is independent of the emitter areas of the transistors.

42. The differential amplifier as recited in 37, wherein the output is taken from the collector of the fifth transistor.

55. A method for compensating variations in transconductance of a differential amplifier, the method comprising:

receiving, by a pair of emitter follower transistors, a differential input voltage;

providing, by a differential pair of transistors, a compensating differential current inversely to the emitter follower transistors, wherein each of the differential pair of transistors has a first terminal, wherein a first resistor and a second resistor are coupled in series between the first terminals of the differential pair of transistors;

changing a transconductance of the emitter follower transistors inversely to compensate for a change in a transconductance of the differential pair transistors; and

shifting, by the emitter follower pair of transistors, the DC voltage level of the differential input voltage by a specified voltage in accordance with the change in the transconductance of the emitter follower pair transistors.

56. The method as recited in claim 55, further comprising:

amplifying the shifted differential input voltage; and

outputting the amplified voltage as an output signal.

57. The method as recited in claim 55, wherein the differential pair of transistors and the emitter follower pair transistors are matching transistors.

58. The method as recited in claim 55, wherein the differential amplifier includes a pair of matching transistors, wherein the differential pair of transistors, the emitter follower pair transistors, and the transistors of the differential amplifier are all matching transistors.

59. A transconductance compensating circuit for a differential amplifier having a main differential pair, comprising:

input means for receiving and shifting an input differential voltage signal to develop a shifted differential voltage signal; and

means for providing a first differential current inversely to the input means such that a transconductance of the receiving and shifting means changes inversely to compensate for a change in a transconductance of the differential current providing means, wherein the differential current providing means includes a third transistor and a fourth transistor, wherein the third transistor and the fourth transistor each has a first terminal, wherein a first resistor and a second resistor are coupled in series between the first terminals of the third transistor and the fourth transistor.

60. The differential amplifier as recited in claim 59, wherein the input means includes a first transistor and a second transistor and wherein the differential current providing means includes a third transistor and a fourth transistor, wherein transconductances of the first transistor and the second transistor changes to compensate for changes in transconductances of the fourth and third transistors, respectively.

61. The differential amplifier as recited in claim 60, wherein the main differential amplifier includes a second differential pair including a fifth transistor and a sixth transistor, the second differential pair, wherein the second differential pair is adapted to receive the shifted differential voltage signal to generate the output signal.

62. The differential amplifier as recited in claim 60, wherein the first, second, third, and fourth transistors are matching transistors.

63. The differential amplifier as recited in claim 61, wherein the fifth and sixth transistors are matching transistors.

64. The differential amplifier as recited in claim 61, wherein the first, second, third, fourth, fifth, and sixth transistors are all matching transistors.

65. A differential amplifier, comprising:

an emitter follower pair operative to receive an input differential voltage signal for shifting the input differential voltage signal to develop a first shifted differential voltage signal;

a first differential pair, including a third transistor and a fourth transistor, configured to feed a first differential current inversely to the emitter follower pair such that a transconductance of the emitter follower pair changes inversely to compensate for a change in a transconductance of the first differential pair;

a second emitter follower pair coupled to receive the first shifted differential voltage signal for shifting the shifted differential voltage signal to develop a second shifted differential voltage signal; and

a main differential amplifier, including a second differential pair which includes a fifth transistor and a sixth transistor, coupled to receive the second shifted differential voltage signal and being configured to amplify the second shifted differential voltage signal to generate an output voltage signal.