holddreams wrote on Sep 24th, 2008, 5:51am:Hi, all, I have a question here:
Instead of breaking the loop at point 2, how will it be when just breaking the loop at point 1 and the negative input of the opamp at the same time?
Is the frequency-phase the same?
Thanks.
Hi holddreams,
If you break the loop at point 1 and the negative input of opamp at the same time, this open-loop is a simple two stage sytem if you use one-stage opamp such as one stage folded-cascode. The dominant pole is opamp's output because its large output impedance.Please refer to the attachment.
Yes, just like other guys said, in this case, the ac output impedance is not the same situation with close-loop because in real application, the feedback will affect the output impedance. However I think, even output ac impedance is affected by feedback, Rout=Ro/(1+A0), here R0 is open loop output impedance, A0 is open loop gain, the dominant pole will not change and the second pole is pushed farther so that PM will be better. Right?