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Why gm is not zero even when iDS is a constant DC current?

BaaB

I am trying to plot transconductance gm of M0 in the picture below. However, what I am supprised here is why gm is not zero when iDS is a constant DC current (1uA).

gm = diDS/dvGS

So according to this, gm should be zero when iDS is a constant.

As seen from the picture below, gm is not zero.

Could anyone explain why and how gm is calculated by the simulator?

Thank you.

Parents
  • BaaB

    Thank you very much for the help.
    I think I misunderstood how transconductance is calculated by DC analysis.
    I thought that firstly it would plot large signal iD = f(vGS, vDS, vBS) and then derivative the curve, evaluate the value of the derivatived function at operating point to get gm.
    For transconductance formula, I think this is what you referred to.
    I am familiar with these formulations too.

    I would like to see the formula for gm in Virtuoso® Simulator Components and Device Models Reference. (cdnshelp)
    I tried to find it there but couldn't locate it. There is a transconductance parameter formula but it is a totally different parameter.
    If possible, could you tell me where in the manual should I find it?

Reply
  • BaaB

    Thank you very much for the help.
    I think I misunderstood how transconductance is calculated by DC analysis.
    I thought that firstly it would plot large signal iD = f(vGS, vDS, vBS) and then derivative the curve, evaluate the value of the derivatived function at operating point to get gm.
    For transconductance formula, I think this is what you referred to.
    I am familiar with these formulations too.

    I would like to see the formula for gm in Virtuoso® Simulator Components and Device Models Reference. (cdnshelp)
    I tried to find it there but couldn't locate it. There is a transconductance parameter formula but it is a totally different parameter.
    If possible, could you tell me where in the manual should I find it?

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