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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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  • Andrew Beckett

    I had a quick look (it's the weekend so I didn't have time to spend too long looking at it) - there isn't much on exactly how gm is formulated. In the operating point parameters section of bsim4, for example, it references equation 12-32 (in the chapter on common mosfet equations) which just shows the derivative equation. Precisely how it's computed is probably dependent upon the model implementation, but it's equivalent to the derivative. You could potentially download the bsim4 source code (if it's bsim4) from Berkeley and take a look (I've looked at the code in the past, but I'll leave this as an exercise for you - I can't promise that the code is identical to our implementation because it's probably been optimised). I'm really not sure it matters that much though. The point is, it gives you the number that you can use in your calculations.

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  • Andrew Beckett

    I had a quick look (it's the weekend so I didn't have time to spend too long looking at it) - there isn't much on exactly how gm is formulated. In the operating point parameters section of bsim4, for example, it references equation 12-32 (in the chapter on common mosfet equations) which just shows the derivative equation. Precisely how it's computed is probably dependent upon the model implementation, but it's equivalent to the derivative. You could potentially download the bsim4 source code (if it's bsim4) from Berkeley and take a look (I've looked at the code in the past, but I'll leave this as an exercise for you - I can't promise that the code is identical to our implementation because it's probably been optimised). I'm really not sure it matters that much though. The point is, it gives you the number that you can use in your calculations.

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