Analog Electronics. Lecture 6. Op amp Stability Analysis and Compensation. Muhammad Amir Yousaf. Lecture:. Stability analysis and compensation of op-amps. Op-amps Three gains: Open Loop Gain A ol Closed Loop Gain A cl Loop Gain A ol B Un-Stability Compensation. Op-amp Circuits.
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Op amp Stability Analysis and Compensation
Muhammad Amir Yousaf
Negative feedback is used to control the gain
The term AolB is very important in stability analysis and is called ‘Loop Gain’
As the Loop Gain is identical in both inverting and non inverting amplifier circuits, hence the stability analysis is identical.
Bode plots of loop gainis a tool to understand Stability:
Stability is determined by the loop gain,
when AolB = -1 = |1| ∠180o
instability or oscillation occurs
The open loop gain of even the simplest operational amplifiers will have at least two poles.
At some frequency, the phase of the amplifier\'s output = -180° compared to the phase of its input signal.
The amplifier will oscillate if it has a loop-gain of one at this frequency.
Phase Margin =ΦM
Phase margin is a measure of the difference in the actual phase shift and the theoretical 180° at gain 1 or 0dB crossover point.
Gain Margin = AM
The gain margin is a measure of the difference of actual gain (dB) and 0dB at the 180° phase crossover point.
For Stable operation of system:
ΦM > 45o or AM > 2 (6dB)
The phase margin is very small, 20o
So the system is nearly stable
A designer probably doesn’t want a 20° phase margin because the system overshoots and rings badly.
Increasing the loop gain to (K+C) shifts the magnitude plot up. If the pole locations are kept constant, the phase margin reduces to zero and the circuit will oscillate.
The result is a phase margin of ≈ 45°, depending on the proximity of still higher poles.
Feedback factor B is equal to inverse of
Closed loop gain Acl so technique is called Gain compensation.
As long as the application can stand the higher gain, gain
compensation is the best type of compensation to use.
It consists of putting a zero (inverse of a pole) in the loop transfer function to cancel out one of the poles.
The best place to locate the zero is on top of the second pole, since this cancels the negative phase shift caused by the second pole.
Slides by ‘Pearson Education’ for Electronic Devices by Floyd
‘Op.amp for every one’ by Ron Mancini
’StabilityAnalysis for volatge feedback op-amps’, Application Notes byTexas Instruments (TI)
’Feedback amplifiersanalysistool’ by TI
‘Feedback, Op Amps and Compensation’ Application Note 9415 by Intersil
Modified by Muhammad Amir Yousaf