Amplifiers – Signal feedback – Phase shift means in loop path
Reexamination Certificate
2000-02-09
2001-05-15
Pascal, Robert (Department: 2817)
Amplifiers
Signal feedback
Phase shift means in loop path
C330S002000, C330S086000, C330S109000
Reexamination Certificate
active
06232834
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to integrated circuit analog amplifiers and more specifically calibrating the frequency response of operational amplifiers.
2. Description of Related Art
Operational amplifiers are found in a wide array of usages, and many of these operational amplifiers are integrated into a semiconductor chip. The ability to measure and compensate the integrated operational amplifiers are somewhat limited without using chip signal I/O and chip real estate. Process variations along with variations in temperature caused by differences in the environment in which the chips are placed can cause changes in the margin of stability of the operational amplifier. These factors along with the aging of the component parts of the operational amplifier can produce a situation which is detrimental to the stability of the amplifier by producing less phase margin or be detrimental to the operation of the amplifier by producing too much phase margin.
In U.S. Pat. No. 5,642,078 (Navabi et al.) an amplifier is disclosed that has frequency compensation using gain degeneration. The amplifier is compensated by dynamically varying the transconductance of a stage according with the gain of the output stage of the amplifier. In U.S. Pat. No. 5,117,200 (Scott, III) a wide bandwidth transconductance amplifier is stabilized over a wide range of output currents by using a compensation driver circuit to sense the output current and feed the information back through a compensation capacitor.
The stability of an amplifier can be measured by the gain margin or the phase margin of the amplifier. Where gain margin is the open loop gain required to make the amplifier unstable, and phase margin is the open loop phase shift required to make the closed loop amplifier unstable. Both these measurements of stability measurements would be difficult and laborious to make on an integrated circuit, and once made corrections would be as difficult. A way is needed to automatically make measurements on the integrated amplifier and correct for shifts in the characteristics of the amplifier that can be caused by differences in manuafacturing, changes in the environment such as temperature and aging.
SUMMARY OF THE INVENTION
In this invention a method an implementation is described for detecting the phase margin of an operational amplifier and making corrections at any time during the operational life of the amplifier. The method relies on the pulse response of the amplifier in which the output response is detected and a feedback impedance of the amplifier is modified to produce a response that is critically damped. Critically damped is defined as that characteristic of the amplifier which is the juncture between being over damped and under damped. Over damped is when there is too much phase shift in the response of the amplifier, and the amplifier takes too much time to settle out from the input pulse. Under damped is when there is too little phase shift m the amplifier resulting in less phase margin and ringing on the output response of the input pulse.
A pulse is applied to the input of the operational amplifier to be compensated, and the ringing, or the lack thereof, on the resulting output pulse is measured. Where ringing is the oscillatory condition at the end of a wave form transition which initially overshoots the end of the transition with a decaying like oscillation to a steady state value after a period of time. A feedback impedance to the operational amplifier is changed to decrease the tendency to ring by increasing the phase margin in an under damped condition, or increase the tendency to ring by decreasing the phase margin in the over damped condition.
In the preferred implementation of this invention a plurality of capacitors are connected in parallel by a plurality of switches. each capacitor being associated with one switch. The parallel combination of capacitors and their switches are connected in series with a resistor in a feedback loop on an operational amplifier. The capacitors are selected by control logic which is set to select more capacitance or less capacitance depending on the amount of dampening that exist on an output pules resulting from a pulse being applied to the input of the amplifier. A peak detector detect the overshot, or lack of overshoot, at a time t1 which is close to the end of the pulse transition on the output. A voltage detector samples and holds the value of the pulse at a time, t2, which is later than t1. A quantizer circuit produces a digital signal, a one or a zero, depending upon whether the peak detector at t1 reads a higher or lower voltage than the sample and hold voltage at t2. The digital signal from the quantizer circuit is fed to the control logic which then selects more capacitance if the peak voltage at t1 is higher than the sample and hold voltage at t2, or selects less capacitance if the peak voltage at t1 is less than the sample and hold voltage at t2.
Whereas the preferred implementation of this invention uses switches to select or deselect capacitance in the feedback of an operational amplifier to compensate the amplifier, other means such as using a variable capacitor driven from a digital to analog converter connected to the control logic could be used. The amplifier does not have to be wholly or partially integrated for this invention to be applicable. Any form of impedance in the feedback of the amplifier can be used where the reactive part of the impedance can be varied and controlled by the control logic. The control logic is also not necessary as long as some form of holding or latching the final value selecting the variable reactive part of the feedback impedance is used to maintain the value of the impedance until the next calibration.
REFERENCES:
patent: 3818336 (1974-06-01), Marshall
patent: 4164715 (1979-08-01), Thurmond
patent: 4481466 (1984-11-01), Roos et al.
patent: 4777430 (1988-10-01), Schabaner
patent: 4868516 (1989-09-01), Henderson et al.
patent: 5117200 (1992-05-01), Scott, III
patent: 5642078 (1997-06-01), Navabi et al.
Ackerman Stephen B.
Janofsky Eric
Marvell International Ltd.
Nguyen Patricia T.
Pascal Robert
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