opamp main spec for stability

Kian Lin

Other Parts Discussed in Thread: TINA-TI

Hi all,
I find that in many practical applications, there are always occasions when one opamp is unstable but another opamp will operate normally even through the circuit is not good enough.
So I wonder if there are some important specs that very important for stability.
And would you kindly tell me how does the spec influence the stability of the opamp?
Thank you very much.

over 3 years ago

0 kai klaas69 over 3 years ago

Guru 140200 points

Hi Kian,

the keyword here is "phase margin". Running a phase stability analysis with TINA-TI or any other Spice simulator is the best way to find out whether your circuit is stable or not.

See here, for instance:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1169799/opa2197-about-opax197-pspice-model-rev-d

Also watch the TI's training video series on stability:

https://training.ti.com/node/1138805

Kai

+1 Raymond Zhang1 over 3 years ago

TI__Guru* 77541 points

Hi Kian,

Kian Lin said:
And would you kindly tell me how does the spec influence the stability of the opamp?

As Kai indicated, the keywords is Phase Margin. For certain application, the Gain Margin is also a key factor. There is no simple answer to your questions.

By looking at the op amp Open Loop gain and phase vs. frequency plot, we may be able to say with some certainty that an op amp is stable when driving a resistive load, but we have to check for the phase margin anyway in order to verify the feedback loop stability anyway, especially when an op amp is driving a complex load. The captured figure can be found in Figure 16 in OPA192.

I recommended to go through our Precision Op Amp Training videos. The input parasitic capacitance and output complex load in op amp can have influences and affect a stability of a closed loop feedback system. I am unable to place a few paragraph to summarize the complex topics.

Below is a part of slide that I presented sometimes ago in Chinese. English speakers are unable to read it.

If you have additional questions, please let us know.

Best,

Raymond

+1 Marek Lis over 3 years ago in reply to Raymond Zhang1

TI__Guru 69181 points

Adding to other comments, below please find the summary of different ways to determine stability - you should also run the small-signal transient analysis to confirm the stable operation.

0 Kian Lin over 3 years ago in reply to Marek Lis

TI__Expert 4240 points

Hi all,

Thanks for your reply. It's crystal clear to me. Thanks!

BRs

Kian

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opamp main spec for stability