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Working with DC sweeps and OP

MicheleA

Hi all,

I know it's not appealing to write one's first post in an RF focused group talking about DC but please bare with me :)

I am interested in plotting some MOS DC parameters like gm, gmoverid, Cgg, gds as they vary across VGS and VDS.

Keeping VDS fixed for the moment, I would expect to be able to DC sweep VGS and get a curve of - say - gm Vs. VGS. This is not the case. I actually get only one point, which is the point corresponding to the "static" value of VGS, in the design variables list.

Of course I could set up an equation in the Calculator for the derivative of Id but I think that "somewhere" the tool should calculate those parameters for me...And also because for some trickier parameters like Cgg the whole process would be more complicated.

The alternative to that is to run a parametric analysis, but this is way less effective timewise.

 Does anybody know of a way of overcoming this?

Thank you in advance,

 Michele

Parents

  • Hi Andrew,

    thanks for your reply. I think you were pretty clear and I should have implemented everything, but it isn't working yet.

    To recap, I have a very simple schematic with a transistor and two dc sources: I created an ASCII file named "save.scs" containing one single line of text:

    save M1:oppoint     [I tried save M1:gm and didn't work either]

    In the libraries setup, I added a path to this file [this is different from where the other libraries reside]

    ran the DC sweep

    plotted OP("/M1", "gm") from the Calc

    ==> I get the buffer evaluated to a number

    In order to select gm, I select the "op" choice in the Calc, then select an instance in the schematic view, and choose the parameter gm from the drop down list.

    In the input.scs  file, I do see the include directive for "save.scs" so I don't really know what could be wrong.

    I would have added some screenshots inline in the post, but I couldn't find my way through. I have attached a zip with 3 screenshots showing the testbench, the simulation windows, and the (incredibly complex) save.scs file :)

     Thanks for helping!

     Michele

    P.S. in te netlist the last line says:

    saveOptions options save=al currents=all subcktprobelvl=2

    is it conflicting with something?

    tb_MOS.zip
Reply

  • Hi Andrew,

    thanks for your reply. I think you were pretty clear and I should have implemented everything, but it isn't working yet.

    To recap, I have a very simple schematic with a transistor and two dc sources: I created an ASCII file named "save.scs" containing one single line of text:

    save M1:oppoint     [I tried save M1:gm and didn't work either]

    In the libraries setup, I added a path to this file [this is different from where the other libraries reside]

    ran the DC sweep

    plotted OP("/M1", "gm") from the Calc

    ==> I get the buffer evaluated to a number

    In order to select gm, I select the "op" choice in the Calc, then select an instance in the schematic view, and choose the parameter gm from the drop down list.

    In the input.scs  file, I do see the include directive for "save.scs" so I don't really know what could be wrong.

    I would have added some screenshots inline in the post, but I couldn't find my way through. I have attached a zip with 3 screenshots showing the testbench, the simulation windows, and the (incredibly complex) save.scs file :)

     Thanks for helping!

     Michele

    P.S. in te netlist the last line says:

    saveOptions options save=al currents=all subcktprobelvl=2

    is it conflicting with something?

    tb_MOS.zip
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