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Cgs and Cgs vs Vgs

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.

anhnha over 11 years ago

Here are the curves Cgs and Cgs vs Vgs:
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Andrew Beckett over 11 years ago

If this is a bsim3v3 or a bsim4 model, they are not strictly capacitances - they are partial derivatives:
cgs (F) dQg_dVs
csg (F) dQs_dVg
Many years ago (over 15) spectre changed to use the native operating point parameters in the model rather than the traditional "SPICE" meanings of cgs etc, and I wrote a note describing this change (which published by a colleague at the time as this solution). This explains a bit more why they are negative and that the traditional "cgs" capacitance value would be found by taking -csg as reported by spectre.
Regards,
Andrew.
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anhnha over 11 years ago

Hi.
Thanks a lot for the explanation. Could you attach the solution? The link doesn't work for me.
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Andrew Beckett over 11 years ago

Try this instead. It contains the same information. However, in general you should be able to access Cadence Online Support if you're a customer on maintenance - if you are from a university, there is a mechanism via whoever is running the university programme at your university (it depends on the region you're from precisely how that works).
Andrew.
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anhnha over 11 years ago

Thank you.
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