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[SOLVED] Why do we need to check stability of open-loop OPAMP

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CHL

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Hello

Stability issue occurs only when there is a feedback in an amplifier.

If an OPAMP works in open-loop, there isn't the stability problem.

However, when we design an OPAMP, we consider the phase margin and gain margin although there isn't a feedback initially (we may include a feedback path after designing the amp).

What is the reason?

Is it for the general use of the OPAMP? (ex, any feedback loop can be added to make a feedback amp, such as a buffer by connecting the input and output directly)

Thanks
 

Yes - you are right. An amplifier without feedback cannot be unstable (however, a high-gain opamp is in saturation due to offset voltages - another problem).
But it is common practice to check if an amplifier will be stable - after feedback is applied. The most critical case is 100% feedback (closed-loop unity gain application).
Therefore, the stability margin of the open-loop gain Aol is determined because for 100% feedback the loop gain (which matters only for stability) is LG=Aol*1=Aol.
 
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