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