Calibrated compensation for an operational amplifier

• US 6,232,834 B1
• Filed: 02/09/2000
• Issued: 05/15/2001
• Est. Priority Date: 02/09/2000
• Status: Expired due to Term

• Current Assignee: Marvell Asia Pte Limited (Marvell Technology Group Limited)
• Original Assignee: Marvell International Limited (Marvell Technology Group Limited)
• Inventors: Zheng, Zhiliang
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Nguyen, Patricia T.
• Application Number:
  US09/501,557
• Time in Patent Office: 461 Days
• Field of Search: 330/2, 330/86, 330/107, 330/109, 330/292, 330/294
• US Class Current: 330/107
• CPC Class Codes: G01R 31/2837 Characterising or performance testing, e.g. of frequency response transient response G01R27/28
• CPC Class Codes: H03F 1/083 in transistor amplifiers H03F1/10 - H03F1/22 take precedence

Abstract

In this invention an integrated operational amplifier is calibrated to optimize phase margin. The calibration is done to correct changes caused by operating temp and supply voltages as well as process variations and aging that can affect the stability of the amplifier. A calibration circuit measures the response of the operational amplifier to a pulse input and controls a feedback impedance to produce an optimized phase margin. The output response to the pulse input is measured at two different times, at a first time close to the transition capturing the peak overshoot from an under damped amplifier and at second time later than the first measurement when the distortions from the under damped ringing have diminished. A quantizer circuit compares the two measure voltages and provides an input to control logic which selects the amount of reactance in the feedback of the operational amplifier. The calibration terminates when the dampening of the amplifier has reached the least significant bit of adjustment available to the calibration process.

Claims

1. An amplifier compensated for frequency response, comprising:

a) an operational amplifier with a variable feedback impedance,

b) said feedback impedance comprises a fixed real part and a variable reactive part,

c) output of said operational amplifier connected to a calibration circuit,

d) said calibration circuit measures an output response of said operational amplifier to an input pulse during calibration and sets control logic to adjust said reactive part of said feedback impedance to optimize phase margin of said operational amplifier,

wherein, said control logic is set during calibration by a comparison of a peak value of said output response and a later value of said output response taken after said peak value where said feedback impedance is adjusted until said peak value and said later value are within a minimum adjustment resolution of each other.

2. The amplifier of claim 1 wherein, during calibration said output response of said operational amplifier to said input pulse is detected and fed back to control said reactive part of said feedback impedance to optimize stability of said operational amplifier.

3. The amplifier of claim 1 wherein, said peak value and said later value are compared with a quantizer circuit that produces a digital signal depending whether said peak value or said later value is larger.

4. An amplifier compensated for frequency response, comprising:

a) an operational amplifier with a variable feedback impedance,

b) said feedback impedance comprises a fixed real part and a variable reactive part,

c) output of said operational amplifier connected to a calibration circuit,

d) said calibration circuit measures an output response of said operational amplifier to an input pulse during calibration and set control logic to adjust said reactive part of said feedback impedance to optimize phase margin of said operational amplifier,

wherein said variable reactive part is a plurality of capacitors having a plurality of values and selected by a plurality of switches to connect selected capacitors in parallel.

5. The amplifier of claim 4 wherein, switches of said plural of switches are selected by said control logic during calibration.

6. The amplifier of claim 4 wherein, said variable reactive part is a voltage sensitive capacitor which is varied by a voltage controlled by said control logic and resulting from comparison of said peak value and said later value during calibration.

7. A method of calibrating an operational amplifier for an optimum phase margin, comprising:

a) applying an input pulse to an operational amplifier during calibration,

b) measuring peak voltage of an output response of said operational amplifier to said input pulse,

c) measuring said output response for a second time at a time later than said peak voltage,

d) comparing measured peak voltage to measured voltage taken at said time later than the peak voltage,

e) producing a logic signal depending on results from comparison of said peak voltage to said voltage taken at said later time than the peak voltage,

f) using said logic signal to set control logic to adjust amount of feedback capacitance that is connected in a feedback path of said operational amplifier,

g) repeating process of applying said input pulse, measuring said output response and adjusting said feedback capacitance until a difference between said peak voltage and said voltage taken at said later time are within a minimum resolution of said comparison.

8. The method of claim 7 wherein, measuring said peak voltage is done at a time close to the transition of said output response in order to detect said peak voltage resulting from an under damped condition of said operational amplifier.

9. The method of claim 7 wherein, measuring said output response for said second time is delayed from said peak measurement to allow under damped distortions to be minimized.

10. The method of claim 7 wherein, comparison measured peak voltage with measured voltage at said later time is done with a quantizer circuit which produces a digital signal based on which two compared voltages are larger.

11. The method of claim 7 wherein, setting said control logic selects a plurality of switches that connect a plurality of capacitors in parallel to adjust said feedback reactance of said operational amplifier.

12. An amplifier compensated for frequency response, comprising:

an operational amplifier comprising a variable feedback impedance having a real part and a variable reactive part;

a calibration circuit to adjust the variable reactive part of said variable feedback impedance comprising;

a peak detector responsive to said operational amplifier;

a sample and hold circuit responsive to said operational amplifier;

a quantizer responsive to said peak detector and said sample and hold circuit; and

a control circuit responsive to said quantizer to adjust said variable reactive part of said variable feedback impedance.

13. An amplifier according to claim 12, wherein said variable reactive part of said variable feedback impedance comprises a plurality of selectable capacitances, wherein said control circuit selects a combination of at least one of said plurality of selectable capacitances.

14. An amplifier according to claim 12, wherein said operational amplifier comprises:

a buffer amplifier; and

an output amplifier coupled to an output of said buffer amplifier, said output amplifier comprising said variable feedback impedance.

15. An amplifier compensated for frequency response, comprising:

operational amplifier means for amplifying an input signal comprising a variable feedback impedance means having a real part and a variable reactive part;

calibration means for adjusting the variable reactive part of said variable feedback impedance means, wherein said calibration means comprises;

peak detecting means for measuring an output of said operational amplifier means in response to the input pulse at a first time;

sample and hold means for measuring the output of said operational amplifier means at a second time;

quantizer means for quantizing an output of said peak detector means and an output said sample and hold means; and

control means for adjusting the variable reactive part of said variable feedback impedance means in response to said quantizer.

16. An amplifier according to claim 15, wherein said variable reactive part of said variable feedback impedance means comprises a plurality of selectable capacitance means, wherein said control means selects a combination of at least one of said plurality of selectable capacitance means.

17. An amplifier according to claim 16, wherein said operational amplifier means comprises:

buffer amplifier means for amplifying the input signal; and

output amplifier means for amplifying an output of said buffer amplifier means, said output amplifier comprising said variable feedback impedance means.

18. An amplifier according to claim 15, wherein said calibration means controls the reactive part of said feedback impedance means to optimize stability of said operational amplifier means.

19. A method of calibrating an operational amplifier for optimum phase margin, comprising the steps of:

(a) applying an input pulse to the operational amplifier;

(b) measuring a peak output of the operational amplifier in response to the input pulse at a first time;

(c) measuring an output of the operational amplifier in response to the input pulse at a second time;

(h) quantizing results of step(b) and step(c)

(i) adjusting a variable reactive part of the variable feedback impedance of the operational amplifier in accordance with a result of step (d).

20. A method according to claim 19, wherein steps (b) through (e) are repeated until the difference between the peak measured in step (b) and the output measured in step (c) is less than a predetermined threshold.

21. A method according to claim 19, wherein in step (e) a capacitive component of the variable reactance is increased if the magnitude of the peak measured in step (b) is greater than the magnitude the output measured in step (c).

22. A method according to claim 19, wherein in step (e) the capacitive component of the variable reactance is decreased if the magnitude of the peak measured in step (b) is less than the magnitude the output measured in step (c).

23. A calibration circuit for an operational amplifier to adjust a variable reactive part of a variable impedance thereof to optimize phase margin. said calibration circuit comprising:

a peak detector responsive to the operational amplifier;

a sample and hold circuit responsive to the operational amplifier;

a quantizer responsive to said peak detector and said sample and hold circuit; and

a control circuit responsive to said quantizer to adjust the variable reactive part of the variable feedback impedance.

24. A calibration circuit according to claim 23, wherein the variable reactive part of the variable feedback impedance comprises a plurality of selectable capacitances, wherein said control circuit selects a combination of at least one of said plurality of selectable capacitances.

25. A calibration circuit for an operational amplifier to adjust a variable reactive part of a variable impedance thereof to optimize phase margin, said calibration circuit comprising:

peak detecting means for measuring an output of the operational amplifier means in response to the input pulse at a first time;

sample and hold means for measuring the output of said operational amplifier means at a second time;

quantizer means for quantizing an output of said peak detector means and an output of said sample and hold means; and

control means for adjusting the variable reactive part of said variable feedback impedance means in response to said quantizer.

26. An amplifier compensated for frequency response, comprising:

an operational amplifier comprising a variable feedback impedance having a real part and a variable reactive part;

a calibration circuit to adjust the variable reactive part of said variable feedback impedance to optimize phase margin of said operational amplifier in response to detecting an input pulse to said operational amplifier,

wherein said variable reactive part of said variable feedback impedance comprises a plurality of selectable capacitances, wherein said control circuit selects a combination of at least one of said plurality of selectable capacitances.

27. An amplifier compensated for frequency response, comprising:

operational amplifier means for amplifying an input signal comprising a variable feedback impedance means having a real part and a variable reactive part;

calibration means for adjusting the variable reactive part of said variable feedback impedance means for optimizing phase margin of said operational amplifier means in response to detecting an input pulse to said operational amplifier means,

wherein said variable reactive part of said variable feedback impedance means comprises a plurality of selectable capacitance means, wherein said control means selects a combination of at least one of said plurality of selectable capacitance means.

28. An amplifier compensated for frequency response, comprising:

an operational amplifier comprising a variable feedback impedance; and

a calibration circuit to adjust said variable feedback impedance comprising;

a peak detector responsive to said operational amplifier;

a sample and hold circuit responsive to said operational amplifier;

a quantizer responsive to said peak detector and said sample and hold circuit; and

a control circuit responsive to said quantizer to adjust said variable feedback impedance.

29. An amplifier compensated for frequency response, comprising:

operational amplifier means for amplifying an input signal comprising a variable feedback impedance means; and

calibration means for adjusting said variable feedback impedance means, wherein said calibration means comprises;

peak detecting means for measuring an output of said operational amplifier means in response to the input pulse at a first time;

sample and hold means for measuring the output of said operational amplifier means at a second time;

quantizer means for quantizing an output of said peak detector means and an output said sample and hold means; and

control means for adjusting said variable feedback impedance means in response to said quantizer.

30. A method of calibrating an operational amplifier for optimum phase margin, comprising the steps of:

(a) applying an input pulse to the operational amplifier;

(b) measuring a peak output of the operational amplifier in response to the input pulse at a first time;

(c) measuring an output of the operational amplifier in response to the input pulse at a second time;

(j) quantizing results of step(d) and step(c)

(k) adjusting a variable feedback impedance of the operational amplifier in accordance with a result of step (d).

31. A calibration circuit for an operational amplifier to adjust a variable impedance thereof to optimize phase margin, said calibration circuit comprising:

a peak detector responsive to the operational amplifier;

a sample and hold circuit responsive to the operational amplifier;

a quantizer responsive to said peak detector and said sample and hold circuit; and

a control circuit responsive to said quantizer to adjust the variable feedback impedance.

32. A calibration circuit for an operational amplifier to adjust a variable impedance thereof to optimize phase margin, said calibration circuit comprising:

peak detecting means for measuring an output of the operational amplifier means in response to the input pulse at a first time;

sample and hold means for measuring the output of said operational amplifier means at a second time;

quantizer means for quantizing an output of said peak detector means and an output of said sample and hold means; and

control means for adjusting said variable feedback impedance means in response to said quantizer.

33. An amplifier compensated for frequency responses, comprising:

an operational amplifier comprising a variable feedback impedance;

a calibration circuit to adjust said variable feedback impedance to optimize phase margin of said operational amplifier in response to detecting an input pulse to said operational amplifier,

wherein said variable feedback impedance comprises a plurality of selectable capacitances, wherein said control circuit selects a combination of at least one of said plurality of selectable capacitances.

34. An amplifier compensated for frequency response, comprising:

operational amplifier means for amplifying an input signal comprising a variable feedback impedance means;

calibration means for adjusting said variable feedback impedance means for optimizing phase margin of said operational amplifier means in response to detecting an input pulse to said operational amplifier means,

wherein said variable feedback impedance means comprises a plurality of selectable capacitance means, wherein said control means selects a combination of at least one of said plurality of selectable capacitance means.