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Simulators >> Circuit Simulators >> Cgs and Cgs vs Vgs
https://designers-guide.org/forum/YaBB.pl?num=1394849162

Message started by baab on Mar 14^th, 2014, 7:06pm

Title: Cgs and Cgs vs Vgs
Post by baab on Mar 14^th, 2014, 7:06pm

Hi.
I set up this circuit for simulation Cgs and Cgs as Vgs changes.

Below are the curves Cgs and Cgs vs Vgs.
(please see it in the second post. The forum doesn't allow to post two images in a single post)

I have some questions, hope you can help.

1. Why Cgs and Cgs are negative?
2. Why Cgs and Csg are not equal?
Thanks.

Title: Re: Cgs and Cgs vs Vgs
Post by baab on Mar 14^th, 2014, 7:07pm

And this is the curves Cgs and Cgs vs Vgs:

Title: Re: Cgs and Cgs vs Vgs
Post by aaron_do on Mar 16^th, 2014, 10:18pm

Hi baab,

I think this has been discussed more than once before, but you will need to do a search. I believe it is just something to do with the way capacitance is defined in the model which makes it a negative value. Also, Cgs and Csg may not be defined exactly symmetrically. For your purposes (matching I assume), I guess you're probably better off doing an AC or SP analysis to find the capacitance since all the capacitance will be lumped together...

regards,
Aaron

Title: Re: Cgs and Cgs vs Vgs
Post by baab on Mar 16^th, 2014, 10:38pm

Thanks, Aaron.

You are correct. Here is the link to the previous discussion.
http://www.designers-guide.org/Forum/YaBB.pl?num=1087411058

As I understand from the link, the capacitance between G and S will be -Csg in BSIM3V3 model.

I am going to post a new thread about LNA design procedure that is very confusing. Hope you could help.

Title: Re: Cgs and Cgs vs Vgs
Post by Andrew Beckett on Mar 18^th, 2014, 4:21pm

See also my response to your identical question http://www.cadence.com/Community/forums/p/28791/1332097.aspx#1332097 - I'm assuming that's you, because it's an enormous coincidence otherwise.

Andrew

Title: Re: Cgs and Cgs vs Vgs
Post by baab on Mar 20^th, 2014, 12:20am

Thanks. yes, it is me.