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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
  • Andrew Beckett

    You're looking in the wrong place. Look in b4ld.c and search for "Gm" (particularly "Gm ="). There are comments in here (not lots, but enough to give a clue). This also has some of the calculations of the derivatives too (it's far too much to work through all the details on a Saturday evening, especially when you really don't need to know this in order to design or to use a simulator...)

    The b4acld.c file is (I think, from a 30 second scan) simply setting up the data for the small signal model, not computing the small signal parameters. The b4ld.c is doing all the real calculations.

    Regards,

    Andrew.

Reply
  • Andrew Beckett

    You're looking in the wrong place. Look in b4ld.c and search for "Gm" (particularly "Gm ="). There are comments in here (not lots, but enough to give a clue). This also has some of the calculations of the derivatives too (it's far too much to work through all the details on a Saturday evening, especially when you really don't need to know this in order to design or to use a simulator...)

    The b4acld.c file is (I think, from a 30 second scan) simply setting up the data for the small signal model, not computing the small signal parameters. The b4ld.c is doing all the real calculations.

    Regards,

    Andrew.

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