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Cgs and Cgs vs Vgs

anhnha

 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.

Parents

  • 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.

Reply

  • 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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