hi,
   How to find loop gain of regenerated transistor as shown in the fig(its simple transistor with regenerated by R).I don't know where to open and i tried to decouple feed back,but i couldn't make it.But i got answer from other mean like with out feed back output resistance is  1/Gds,with feedback (gm*R+1)/Gds,this indicate gm*R is loop gain.But i want to know actual procedure(like breaking).
Thanks,
Rajasekhar.

raja.cedt wrote on Mar 10^th, 2009, 7:52am:


Yes, it´s a bit tricky because you cannot open a loop if there is no loop at all !
However, you can build a loop.
Try to construct a typical control loop with a summing point (with a minus sign) at left: Vg-Vs.
And then starting with the ouput of the summing point close the loop via a block "gm" (with Id as output) and another block "Rs" with Vs as output.
As a result the loop gain is simply Aloop=-gmRs.  

hi  buddypoor and Tosei
   Thanks for your reply,this is my understanding from your reply,can u please verify the figure what i attached here. Here i opened at summing point and finally got voltage loop gain Gm*R.Correct me if any thing wrong.
   I feel  Loop gain is always voltage gain or current gain,it can't be trans conductance gain or trans resistance gain,is it correct?
   Can you please tell me is there any other circuit which are having this type of trick for finding loop gain,so that i can try them

Thanks,
Rajasekhar.

Hi rajasekhar !

In fact, it is not easy to find another example in electronics for a hidden loop.
However, in control systems you will find a lot of them.
Think, for example, of all kinds of thermal feedback or electrodynamic feedback in motor systems. In all cases you have to create a model of the system showing the loop from output to input. It is the normal procedure in control system design to crerate a block diagram reflecting the real situation - without the ability to "see" the loop in the original system.
Hope this helps.
Regards