buddypoor wrote on Nov 6th, 2008, 2:08pm:I like to remind you on the Nyquist stability criterion in its most general form. This criterion allows us to prove the stability of a closed loop system also in case that the open loop is unstable (i. e. it has poles with a positive real part). There are a lot of examples that closing one specific loop can stabilize another loop which is unstable.
Regards
You are quite right. In fact I have designed such a system before, but it had slipped out of my mind. In that circuit, I had two feedback paths, one positive and one negative. I did run stability on both loops since I needed to verify that the negative path always had greater loop gain. Otherwise the positive feedback would become dominant, and I would have an oscillator.
I still think that it would be informative to run the three stability analyzes, though. Even though all are negative feedback, if one is flirting with instability, you should see it.