buddypoor
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Thank you to all contributors. As I´ve mentioned at the beginning – it is a question of more or less philosophical nature. My point of interest was only how to characterize results of ac simulations for unstable systems. Of course, we all know that in reality an opamp circuit with only pos. feedback will not function as an linear amplifier.
Here are some further comments from my side:
Sheldon: In the end, it may not be a question of right or wrong, rather a question of relevant or irrelevant.
From your assessment I would conclude that the result of an ac simulation is correct, but irrelevant as it assumes conditions which are not realistic (noise free, ideal constant supply)
Eugene: You say the AC analysis gives the correct gain. I assume you mean low frequency gain. One does not assess stability only by DC gain.
No, the ac analysis indicates stable operation also for higher frequencies, but the phase goes up with rising frequencies. This, of course, together with the 180 deg phase shift at low frequencies - in spite of feeding the plus terminal – looks a bit contradictory. By the way: Handmade calculations using classical formulas give the same result as AC analyses with simulation programs.
Eugene: I would bet that if you carefully examined the Nyquist plot you would see that the AC analysis indeed predicts instability when you reverse the sign of the feedback.
Yes, no surprise, the open loop response indicates instability !
Vivkr: I would like to add that you are computing your amp gain with a very simple model (ideal gain with no poles) while your simulation model is more realistic and latches.
No, as already mentioned, I have used a two-pole opamp model.
The above calculation as Sheldon pointed out is mathematically correct but irrelevant in practical terms.
Yes, agreed. ___________________________________________________
Finally, let me formulate something like a mechanical analogon: Put a small ball upon another ball (which may be larger). Under ideal conditions (no air movement, both centres of mass exact perpendicular, no other disturbances) the system will not loose balance. I guess a similar effect happens in the opamp with positive feedback.
Perhaps it is surprising, but the results of ac simulations of unstable opamp circuits are not completely useless. Presently, I am investigating a method to calculate phase and gain margins of feedback systems via ac analyses without opening the loop. And in this context play unstable feedback loops and their ac behaviour a certain role.
Thank you again.
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