The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> loop gain
https://designers-guide.org/forum/YaBB.pl?num=1236696766

Message started by raja.cedt on Mar 10^th, 2009, 7:52am

Title: loop gain
Post by raja.cedt on Mar 10^th, 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.

Title: Re: loop gain
Post by raja.cedt on Mar 10^th, 2009, 8:02am

hi,
i forgot to attach fig,sorry.
Regards,
Rajasekhar.

Title: Re: loop gain
Post by buddypoor on Mar 10^th, 2009, 10:20am

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.

Title: Re: loop gain
Post by thechopper on Mar 10^th, 2009, 6:11pm

What buddyppor suggested is correct. In other words you can only picture the loop if you think of the small signal model for the transistor. Just draw that circuit and you will see the loop buddypoor was suggesting.

Regards
Tosei

Title: Re: loop gain
Post by raja.cedt on Mar 10^th, 2009, 10:16pm

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.

Title: Re: loop gain
Post by buddypoor on Mar 11^th, 2009, 1:27am

raja.cedt wrote on Mar 10^th, 2009, 10:16pm:

You can open the loop where you want, but never forget the negative sign at the summing point.
As you open (cut) the loop at one point, of course there are no different dimensiions/units at the point of cut. Therefore, loop gain always has no dimension and always is voltage or current gain.
Other examples are not easy to find because in most cases you can identify a closed loop by inspection. I will think about it.
Regards

Title: Re: loop gain
Post by buddypoor on Mar 11^th, 2009, 3:39am

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