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Simulators >> Circuit Simulators >> Cdd & Css & Cgg in cadence OPT
https://designers-guide.org/forum/YaBB.pl?num=1270124796

Message started by casual on Apr 1^st, 2010, 5:26am

Title: Cdd & Css & Cgg in cadence OPT
Post by casual on Apr 1^st, 2010, 5:26am

what is the Cdd & Css? How are they derived? I obtained this values from cadence OPT command.

I know that Cgg=Cgs+Cgd+Cgb

Is Cdd=Cdg+Cdb? and Css=Csg+Csb??
Sound like not true from the value.

May I know why Cgd !=Cdg & Cgs !=Csg??

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by sv on Apr 2^nd, 2010, 10:45am

Cxy values reported by operating point information through spectre are derivied from partial derivatives of channel charge by terminal voltage as defined by BSIM equations. See section B.2.4 in this link { http://nikola.com/pdf/BSIM3APX-B.pdf } for the exact BSIM equations for the partial derivatives.

To be precise, this link { http://www.designers-guide.org/Forum/YaBB.pl?num=1087411058/1 } suggests that the partial derivates computed by BSIM model are further modified by spectre simulator before reporting as operating points. (I have never verified it.)

As you will find out, they are mathematical constructs and may or may not have any correspondence with actual physical capacitors mentioned in the textbooks.

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by casual on Apr 2^nd, 2010, 10:17pm

i didnt know BSIM in detail.

Could you explain in a simple way?

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by Geoffrey_Coram on Apr 5^th, 2010, 5:29am

MOS models have four "buckets" of charge; depending on the four terminal voltages (D, G, S, B) and hence the operating region, you get different amounts of charge in each bucket.

Cxy tells you how much the charge in bucket "x" (Qx) changes when you wiggle the voltage on terminal "y" (Vy).

You really can't expect that the change in Qd when wiggling Vg would be the same as the change in Qg when wiggling Vd.

However, charge is conserved, Qd+Qg+Qs+Qb=0, and there are only 3 independent branch voltages, so you get identities like Cgg=Cgs+Cgd+Cgb. Cdd=Cdg+Cdb+Cds and you can also write Cdd=Cgd+Csd+Cbd. (Note that Cxy = -dQx/dVy when x!=y.)

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by casual on Apr 5^th, 2010, 5:38am

Good and simple explanation. ;D
Now I got better understand about it.

one more query:
those Cdd, Cgg, Css extracted from cadence are only true for DC analysis (dc bias). Does these values vary a lot across freq? I just wonder how I could extract intrinsic cap values vs freq.

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by Geoffrey_Coram on Apr 5^th, 2010, 5:51am

supermoment wrote on Apr 5^th, 2010, 5:38am:

those Cdd, Cgg, Css extracted from cadence are only true for DC analysis (dc bias). Does these values vary a lot across freq?

They aren't "dc" values; they're mathematical expressions of the partial derivatives of mathematical equations where the voltages are the inputs. There is no frequency dependence of these mathematical expressions.

However, if you now place those "capacitances" along with the small-signal conductances into small-signal equivalent circuit, you will see frequency dependence of a (complicated) RC network. Note that the capacitances aren't 2-terminal capacitors -- and the conductances aren't 2-terminal resistors/conductors.

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by casual on Apr 5^th, 2010, 6:14am

Quote:

Note that the capacitances aren't 2-terminal capacitors -- and the conductances aren't 2-terminal resistors/conductors.

the above note make me a bit confuse. Do you mean they are freq dependent?

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by Geoffrey_Coram on Apr 5^th, 2010, 9:10am

A 2-terminal capacitor is one whose charge depends only on the voltage across it (across its two terminals). That's not what the capacitance is like in the MOS.

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by casual on Apr 5^th, 2010, 5:39pm

Got it. Thank you very much 8-)

Title: Re: Cdd & Css & Cgg in cadence OPT
Post by Mayank on Apr 21^st, 2010, 8:22am

Nice Explanation from Geoffrey_Coram, esp the first reply...Thanks...