kane wrote on Jan 8th, 2013, 9:52pm:Hi, all.
I am designing an 10dB BPF. But I find the phase margin of the OP is good at point A but poor at point B.
If this BPF is stable?
I think it is stable ? But not very sure.
From your Open Loop AC signal ( called OL(s) )
Create a formula in your calculator: CL(s) = OL(s) / ( 1+OL(s) ) and probe the amplitude in dB20 of this complex expression. If you see any overshoot somewhere, you can consider your circuit is not robustly stable, and if you see a significant peaking you can consider it is unstable.
Then add the phase of CL(s): if you see the amplitude decrease while phase increases, it means you have complex poles with positive real part: it means your loop is totally unstable.
Now the question is: what do you mean by "is it stable?"
a 35deg phase margin circuit is stable "mathematically speaking" because it will converge at t=infinity, but will have lots of ringing.
In all cases, the best test to do is to perform a transient simulation with a step stimuli on the input.