gm/id with modern cadence virtuoso

There must be about 3 million posts on this forum about gm/id (OK, I might be exaggerating a little), so a bit surprised you couldn't find something that worked. I suspect the older instructions you were talking about were suggesting you create a file to save the gm because if you do a DC sweep, then the gm is not saved for a transistor by default. The OP("/NM0" "gm") you've used will only get the value from a DC operating point, and if you did a dc sweep (as opposed to a sweep of a DC operating point) then it won't work.

Your expression also won't work because the denominator is incorrect (in all versions of the Virtuoso) - the error makes that pretty clear. It would have been:

OP("/NM0" "gm")/IDC("/NM0/D")

or:

OP("/NM0" "gm")/OP("/NM0" "id")

probably. However, if you're doing a DC sweep, this won't work as I said before - you'd just get a single value and not a swept value. If you've managed to plot the gm and the drain current in the graph, you should be able to right mouse->send to calculator for each and get the expressions for each of them and then use that in your calculation (I don't know how you've managed to save the gm because you didn't explain how you'd set up the sweep and what you'd saved).

Andrew.

Hello Andrew, I have shown bellow my schematics and my DC schematics swwep window,afterwards i have  send to calculator my gm plot and got its expression in the ADEL window.

Then i tried to divide current expression with this expression and the two other version you have shown in the reply shown bellow , all three of them gives me eval error.

Where did i go wrong doing gm/id?

Thanks

The first can't work for the reason I described before (clearly you weren't paying attention, because I suggested you send both signals to the calculator to get the expressions). You've presumably set up the saving of operating points via Outputs->To be saved->Select OP Parameters (otherwise the OS function wouldn't be there - this means "Operating point for Sweep" (where Sweep is DC sweep), so you would need to use either:

OS("/NM0" "gm") / IS("/NM0/D")

or

OS("/NM0" "gm") / OS("/NM0" "id")

Your last expression I'd expect to work (just not give you a sweep, so probably not what you want) unless you're not saving the drain current from the transistor. The OP("/NM0" "id") may not work because maybe the drain current in the operating point for that device is not called "id" (you'd have to check the available parameter names). If that's the reason my second expression above may also not work (you'd have had to ensure that the "id" parameter or equivalent is saved via Outputs->To be saved->Select OP Parameters).

BTW, whilst the expression sent from the waveform to the calculator included the path to the results directory, that's not strictly necessary as without this it will pick up the current result database anyway.

Andrew.

Thank you very much Andrew,It worked :-)