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gmoverid in dc operating point is not equal to gm/id

rfham

Hi,

I just want to do the gm/id plot, by following circuit

.

and i included one scs file with "save NM0:all", i can got gm, id and gmoverid by dc simulation. use following 3 expression to get the gm/id plot.

found in weak inversion region,  why NM0:gmoverid is not same as getdata("NM0:gm"?result"dc") / getdata("NM0:id"?result"dc") ? thanks.

  • My guess, without doing any experiments to test it, is that it's because the gmoverid operating point parameter is actually gm/ids not gm/id (there are two outputs, one called id and one called ids).

    Andrew.

  • good point, thanks a lot Andrew.

  • Hi Andrew, what is the difference between ids and id?

    Furthermore, is the Vdsat number also given by id/gm on the dc opt point?  I notice that Vdsat is not equal to what some designers call V* = gm/id

  • Hi highspeedio,

    id is the current that flows in the drain terminal, whereas ids is a current source within the bsim 4 model. You can find an equivalent circuit model of this transistor in the Virtuoso Simulator Components and Device Models Reference.

    I think the relation between Vdsat and gm/ids only applies in saturation mode and only for the square law model. Since you have here a very complex transistor model, which varies it's region during dc sweep, you can't apply this formula.

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