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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Cgs, Cgg, CGd etc. in cadence https://designers-guide.org/forum/YaBB.pl?num=1087411058 Message started by modern_analog on Jun 16th, 2004, 11:37am |
Title: Cgs, Cgg, CGd etc. in cadence Post by modern_analog on Jun 16th, 2004, 11:37am Do the values of caps which we get from cadence .op carry any physical significance. ie is Cgg = Cgs + Cgd + Cgb. In this case, should I take the modulus of the values. I feel it is not the case, but just want to re-confirm, since some of the caps are negative (due to derivative of Q being negative) If this is not the case, how can we have cadence give us the capacitance at any node and its constituents. For instance if cadence says node n1 has 1pf of cap, and node n1 has 4 transistors connected to it, how can we figure out which transistor is contributing how much cap and what are the constituents (Cgs, Cgd etc.) of that transistor. We used to have captab option in hspice. thanks |
Title: Re: Cgs, Cgg, CGd etc. in cadence- Post by Andrew Beckett on Jun 16th, 2004, 2:26pm I assume you're talking about bsim3v3 models here. Here's a note that I wrote some years ago on this: Quote:
Using the above note, you can translate from the partial derivatives spectre outputs to be in line with the bsim3v3 specification, into the more traditional capacitance terms. Note that spectre can also output a captab. See "spectre -h info". You'll see that you can do what=captab to output a capacitance table. Andrew. |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by superwormsz on Sep 2nd, 2004, 6:53am I have a question vdsat=vgs-vth???? |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by Andrew Beckett on Sep 2nd, 2004, 10:55am superwormsz, That doesn't appear to be a question... Andrew. |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by 101questions on Sep 2nd, 2004, 11:17am Andrew, Thanks for the clarifications. What about channel to bulk capacitance? Is it counted as a part of cjs, cjd? |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by Andrew Beckett on Sep 2nd, 2004, 9:02pm No, that would have been (traditionally) cgb. I just noticed a typo in my note above - I think it should say that the mapping should be: Quote:
(cgb was wrongly saying it mapped to -cbd). So if you want to find the channel-bulk capacitance, look at -cbg. At least I think that's correct ;) Andrew. |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by 101questions on Sep 2nd, 2004, 11:12pm Andrew, I'm puzzled. Cgb cap is between gate and bulk, right? Channel to bulk cap represents capacitance of depletion layer beneath the channel, if the channel exists, that is in strong inversion. If we change the bulk potential of mos in strong inversion, while gate, drain and source are are constant, we charge cjs, cjd, bulk-to-channel cap, and that's it. Cgb in strong inversion is almost zero, at least in active area. Am I missing something? |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by Andrew Beckett on Sep 3rd, 2004, 9:15am You're probably right. It's years since I've really thought about this, so I'll have to spend some time checking things. Won't be for a few days, given other commitments, as well as being out of the office. Sorry... Andrew. |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by modern_analog on Sep 17th, 2004, 9:12am This absolutely makes no sense... I did a simple experiment. I took a common source transistor and put a voltage source with a resistor (R1) at the gate and a resistor (R2) at the drain. pole at the gate of the transistor is 1/(R*C) where C is Cgs (not Csg) + (gain)*Cgd (not Cdg) At the drain, I got a zero at gm/(Cdg) and pole at 1/(R*Cdb+Cdg) Note the transistor was in saturation. It seems Cgd = overlap of the gate, drain ~ 0 but Cdg is very high. In fact, I read somewhere that Cdg is proportional to Cgs(or Cgg to be more exact). So, my question is a) Is the above correct, ie gate cap is split between a big Cgs and very small Cgd. b) How is Cdg calculated. It seems to be a big factor c) In all the above Cdd ~ 0 because I had a postive Cds cancel a negative Cdg. Strange? That basically means, I should not consider Cdd in my equations. ps: In all my statements above, I could have made a mistake with the correct cap values since they are so small. These were representative values Cdb=-24f Cdd=1f Cdg=-244f Cds=267f Cgd=.7f Cgg=766f Cgs=-800f Csg = -365f |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by Andrew Beckett on Sep 19th, 2004, 3:10pm You also have captab in spectre (see spectre -h info), as well as the explanations I gave earlier. Andrew. |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by Geoffrey_Coram on Sep 24th, 2004, 11:22am It should also be noted that some of the peculiarities arise from the BSIM3 implementation itself. Philips' MOS Model 11 is supposed to do a better job, in terms of making the capacitance model reciprocal. (With a nice non-reciprocal capacitor, one could power the world simply by driving the capacitor in a cycle and extracting energy ... :) ) |
Title: Re: Cgs, Cgg, CGd etc. in cadence Post by cricket_fan on Apr 8th, 2005, 12:09pm In order to look at Cgg vs a dc bias voltage , I have to do a dc simulation but it looks like I need to have an ac source for simulationg capacitance. I want to look at Cgg vs Vgb....in cadence. thanks MMN |
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