hi--didn't the following from your post.

Now, I consider the possibility of having the inverting terminal of Gm3 at incremental ground. Output voltage is now function of both Va-Vb and Va+Vb.

I think there is only an open loop gain here because it is feedforward. Probably can do an AC test. Furthermore, there should no stability problems because the circuit is feedforward. No negative feedback, no stability problems.

Hello nrk1

Thank you for the response.
Gm3 appears in the loop gain expression only if there is some feedback to the positive terminal of the opamp as well. Otherwise(e.g. if used as a voltage follower), it is just a feedforward path.

I believe this statement is made in reference to the schematic in the first post. My understanding is that gm3 appears in loop gain for schematic shown in post 3 (Gm stages hooked up in red). Isn't that correct?

I probably did not make myself clear, but the question that I have in mind is the following. Consider an opamp with has differential gain and a significant common mode gain. One is interested in assessing its stability under unity gain feedback. One way in which I have seen it being measured is to excite the amplifier differentially and check the frequency response to get an idea of stability margins. The other way (which I usually do) is to insert iprobe in the feedback path and check for the stability margins. My understanding is that both the methods give different results as the former method fails to account for the common mode gain, but I am not quite sure if my understanding is indeed correct. Any thoughts on it would be appreciated.

Thanks and regards

Hello Daniel

Thank you for the reply. That was not exactly what I was looking for. Let us say we have a single ended opamp with both differential and common mode gains. Exciting the opamp with differential and common mode signals in open loop gives respective frequency response plots. What I have seen some people do is that they derive stability margins from the differential response. My understanding if that it should be done by connecting the opamp in unity gain configuration and perform stb analysis. In this gain, loop gain happens to be a weighted sum on both differential and common mode gains and it is from this plot, I believe stability margins should be derived. I wanted to confirm if my understanding is correct.

For a fully differential amplifier, I usually use differential stability probe from analogLib to arrive at stability margins.

Thanks and regards

Yes. I agree. I was wondering if it is possible to have a case where both common mode and differential mode frequency responses show adequate stability margins and yet the unity gain feedback configuration is unstable (Since loop gain in unity gain feedback is going to be a weighted sum of differential and common mode gains).

Loop is analyzed for unity feedback configuration when you have done frequency compensation by miller compensation. Because for a miller compensated feedback loop UGB(unity gain bandwidth) is the worst freq which could make the loop unstable. For a feed-forward configuration any freq can be bad, because your system has multiple poles inside UGB which is compensated by zeros. This is why feed-forward is used in application specific opamps and not on general purpose opamps. So analyze stability for the loop that you are going to use the opamp in.

Injecting current into a node and measuring voltage for the transfer function will only for a passive network, or when you are finding the output impedance, in which case you short/open the input. When you have control sources in a network you need to use the adjoint network. You can get it using Tellegen't theorem. For a passive network the adjoint network is the same. That is why it will work in that case.

Also opening the loop for finding won't give u an accurate loopgain. Use diffstb probe and analyze the stability of common mode and differential loop gain separately. Like you have told, if both are stable loops, their linear combination has to be a stable network.