Loop Gain Analysis 2 Pole system

[mirror_pole]

Hey guys,

I have a question concerning second order feedback systems. I read that such systems are always stable and that the feedback affects the poles that they move closer towards each other, but always stay at LHP.

But how about the zeros? I mean additional feedback also creates zeros and if i have a RHP zero my system could become unstable. Do the authors ignore this fact or did i missunderstand it?

 

[danadakk]

Hey guys,

I have a question concerning second order feedback systems. I read that such systems are always stable and that the feedback affects the poles that they move closer towards each other, but always stay at LHP.

But how about the zeros? I mean additional feedback also creates zeros and if i have a RHP zero my system could become unstable. Do the authors ignore this fact or did i missunderstand it?

That is true theoretically if system is pure second order, because phase shift of
180 degrees can never be reached, only in limits. This precludes the presence of
negative resistance in system (if my memory serves me).

But in practical terms, throw in a little input C or output C load, or imperfect passive
components, like L and C, whole other situation. Then we have to design for
phase margin as we now have a 3'rd or greater order system. To keep our amplifier
from becoming an oscillator.


Regards, Dana.