CMRR in open loop and closed loop configuration of opamp

[anubhavsingh]

For an opamp in open look configuration,

Here Ad should be equal to open loop gain of opamp and

Now consider the closed loop config of opamp (negative feedback used):

However, Ad is closed loop gain here (according to Sedra Smith) and

In terms of open loop gain,

A is open loop gain here.

So Adm is open loop gain in the first case and Adm is closed loop gain in the second case. How to know which gain to use in CMRR?

Reference: Microelectronic Circuits (Seventh Edition) by Sedra Smith

 

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[Baluncore]

How to know which gain to use in CMRR?

An op-amp is operated with negative feedback to maintain a low differential input voltage. An op-amp is NOT designed to be used as an open loop voltage comparator.

 

[alan123hk]

So Adm is open loop gain in the first case and Adm is closed loop gain in the second case. How to know which gain to use in CMRR?

Sorry, I am a little confused and incomprehensible about your question.

You have proposed two different circuits, one is an open-loop amplifier and the other is a closed-loop amplifier, and you mentioned their respective voltage gains. If I understand correctly, your question is whether you should use open-loop voltage gain or closed-loop voltage gain to calculate their respective CMRR.

If you think in accordance with the general logic, isn't this thing relatively simple?

So my understanding is of course that the CMRR of the open-loop amplifier should be calculated with the corresponding open-loop gains, and the CMRR of the closed-loop amplifier should be calculated with the corresponding closed-loop gains.

 

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[anubhavsingh]

Sorry, I am a little confused and incomprehensible about your question.

You have proposed two different circuits, one is an open-loop amplifier and the other is a closed-loop amplifier, and you mentioned their respective voltage gains. If I understand correctly, your question is whether you should use open-loop voltage gain or closed-loop voltage gain to calculate their respective CMRR.

If you think in accordance with the general logic, isn't this thing relatively simple?

So my understanding is of course that the CMRR of the open-loop amplifier should be calculated with the corresponding open-loop gains, and the CMRR of the closed-loop amplifier should be calculated with the corresponding closed-loop gains.

Yes, but why is that? Why can's the meaning of differential gain remains consistent whether the opamp is open loop or closed loop. Why should in closed loop configuration, the differential gain should not refer to the open loop gain of opamp as in open loop configuration? Just looks ambiguous to me.

 

[Tom.G]

Yes, but why is that? Why can's the meaning of differential gain remains consistent whether the opamp is open loop or closed loop. Why should in closed loop configuration, the differential gain should not refer to the open loop gain of opamp as in open loop configuration? Just looks ambiguous to me.

Try this site, it describes/explains some of the details you are asking about.
https://www.electronics-notes.com/a...erational-amplifier-op-amp/gain-equations.php

Cheers,
Tom

 

[alan123hk]

Yes, but why is that? Why can's the meaning of differential gain remains consistent whether the opamp is open loop or closed loop. Why should in closed loop configuration, the differential gain should not refer to the open loop gain of opamp as in open loop configuration? Just looks ambiguous to me.

To be honest, so far I don’t seem to be sure what the crux of the problem you want to discuss is.
But based on the CMRR-related content described in your first post, I believe the following article may be helpful to you. If you can fully understand the content of this article, including the equations mentioned, then your doubts may be solved.

https://www.analog.com/media/en/training-seminars/tutorials/MT-042.pdf