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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.

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  • awesome1

    Your test circuit is not correct.  

    As others have said, the transconductance (gm) is a small-signal parameter that is extracted at any operating point.

    gm should not be zero under constant DC bias--in fact, DC bias is needed to extract gm.

    You should do a small-signal extraction of parameters. There are several ways to do that.

Reply
  • awesome1

    Your test circuit is not correct.  

    As others have said, the transconductance (gm) is a small-signal parameter that is extracted at any operating point.

    gm should not be zero under constant DC bias--in fact, DC bias is needed to extract gm.

    You should do a small-signal extraction of parameters. There are several ways to do that.

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