shivamm wrote on Jun 7th, 2012, 2:34pm:1. After running stability analysis, I found that the system is unstable.
Even though ALL the inner loops are stable, the outer feedback loop is oscillating. Is there a way to find out what(component) is making the system unstable?
2. When I increase the resistance of the resistor(by a factor of 100) connecting the output of A3 to the input of A4, the system is stable and does not oscillate. I am looking for a better solution. Any suggestions?
At first, it is really no surprise that the loop is unstable - because of the various opamps and RC elements connected in a loop.
Nevertheless, if the magnitude ofthe loop gain is small enough it can be stable - as demonstrated by you (increased resistor value between A3 and A4). You ask for a "better solution",
Why do you think this is a bad solution?
It is quite normal in such a case either to reduce the loop gain (that`s what you did) or to increase the loop phase (for example by incorporating a PDT1/lead-lag controller).
But in any case - as I have suggested - check the loop gain in order to see what must be done to arive at the wanted phase margin.
And show us the simulation results!
Don`t forget to include an inductor at the output of A3. Otherwise your dc operating point is lost! I cannot understand why the former reply (wave) argues against it.