@BOE : you are right to note that at ~153Mhz the phase margin has already crossed 0deg and it has a margin of -5deg, why do you say that since its is -5deg, its amplitude should be increasing, could you please give me a reference for this reasoning?
If things are not deducible from PM plot, then nyquist plot is only other one, but how do i find my oscillating freq. from it? all i know is that it goes in circles around that (-1,0) :)
@raja.cedt : there was no provision for me to attach more than one file in a single post, that's why i had no choice but zip it.
how do i find this natural freq. Wn of this system?
I checked the transient (conservative) simulation, the freq doesn't change much it hovers around that 153MHz through out, in fact the amplitude is also the same through out. thanks for you reference at razavi, i'm reading it.
@harpoon: b) you are right, the osc freq hovers around 153Mhz
c) my loop break point is correct and i'm doing an STB analysis which actually does middlebrook's, so loading issues on both side taken care off.
As you all pointed out its a large signal behaviour, can i reason like this, when the sudden jump happens at VIN_P, the VIN_N tries to catch up but over shoots, once overshot little bit, that is amplified by the gain and the reponse comes after the delay of the first stage, then the second stage amplifies and the reponse comes after the delay of the second stage, by this time it has overshot in the other direction
so can i say the amplitude will depend on the delay in the two stages and its gain?
thank you Boe, Raja and harpoon for your precious time in replying,
thanks,
Praveen