Hi, everyone:

   I am a little loss at the control theory.
It says a system is stable if it has all poles in the left half plane. But if an opamp in closed loop has three poles (all in the left plane), we say it is unstable, because PM is negative.
  Is the stability mean different thing here? Or does it mean that the stability criterion means that if compensation can be applied to achieve stability, it still meets the criterion?

Judy

hi Judy,
          if the poles are in RHP then system is unstable,but if they are in LHP still there is a chance of unstable because it is necessary but not sufficient.Now come to 3 stage op amp,in this case based on poles location you have to decide whether it is stable or not,but most of the times you will get less phase margin(>30) so it wont give satisfactory transient response.

Hope this is useful.

Thanks,
Rajasekhar.

circuit_cook wrote on May 5^th, 2009, 7:44am:
Do you understand open-loop gain, closed-loop gain, GM and PM correctly ?

If no RHP pole in your closed loop gain is achieved by pole-zero cancellation, you have to consider internal stability.
Especially during startup of system or circuits, each block have time difference to reach steady state.
So RHP poles could not be canceled by RHP zeros during startup of system or circuits.

hi Judy,
          i think you need some minimum basics on stability.
1.For an open loop system to be stable you should have all LHP poles,otherwise system wont converge for steady input(This is called BIBO stability). In real working analog circuit you will not see RHP poles may be RHP zero(this is what i observed from my exp)
2.For an closed loop now poles will move with feedback, so as to examine the stability bode plot will help (this will take open loop information and feedback factor) otherwise if you are able to find you can roots then no need of bode plot.
3.But in circuits we wont consider BIBO stability because even though it is unstable you may limited by power supply or active device non linearity, so the main concern here is how good step response (this will come from the phase margin)  
4.Bode plot is exact in case of all pole system,if zero's are then use nyquist.........use this document..very clear one.
http://www.ee.iitm.ac.in/~nagendra/presentations/20090109vlsiconf/20090109vlsico...

hi pancho_hideboo,
                           from your previous ans i got this doubt, is it possible to cancel RH pole?could you please..i think answer is no.

hi pancho_hideboo,
                           ya but is it canceling there?in the magnitude point of view it is canceling,but from phase point of view how it is not canceling?Please don't mind if my concepts are clear.Thanks for spending time.

Thanks,
Rajasekhar.