How do I make my PMOS circuit characteristic look like this in LTPSICE?

Discussion in 'Homework Help' started by Kai Chiang, Aug 31, 2016.

  1. Kai Chiang

    Thread Starter New Member

    Oct 27, 2015
    12
    0
    This is my PMOS circuit.
    upload_2016-8-31_21-36-13.png

    This graph obtained using LTSPICE. The graph is Isd VS Vsd
    upload_2016-8-31_21-37-3.png

    upload_2016-8-31_21-39-23.png

    I am trying to make it look like the one above but when I sweep it from vsd 0 to -3 with a increment value of -0.01 , it becomes one straight line.This is what I mean.
    upload_2016-8-31_21-41-24.png

    Did I do anything wrong? I am still quite new to LTspice :(
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,968
    387
    Connect the ground symbol to the positive connection of vsd instead of the negative connection.
     
  3. Kai Chiang

    Thread Starter New Member

    Oct 27, 2015
    12
    0
    Hi I have changed it but it is still the same.
    upload_2016-9-1_15-59-37.png
     
  4. Kai Chiang

    Thread Starter New Member

    Oct 27, 2015
    12
    0
    I would also like to check if circuit connection is correct in LTSPICE? (I believe it is correct)
    co
    I am using this circuit for PMOS parameter extraction
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Your polarity is reversed for Vsd.
    You have the minus connected to the drain and then you have a minus for the parameter which gives a plus voltage on the drain.

    Below is an example showing the plot as you wanted.


    upload_2016-9-1_9-30-45.png
     
    Last edited: Sep 1, 2016
  6. Kai Chiang

    Thread Starter New Member

    Oct 27, 2015
    12
    0
    Hi. Thank you very much for your help.
    I am wondering in my original circuit , when I set the voltage to negative , wont it reverse the polarity of vsd and vsg? but it doesn't produce the result i want.
    upload_2016-9-2_16-24-47.png
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,809
    1,105
    Your circuit exactly as in post #1 will give you the wanted result if you left-click the horizontal axis of the plot and set the Quantity Plotted value to -Vsd.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    I showed you polarities that work in post #5.
    Why then did you invert the sources in post #6 , which inverts the polarities? :confused:

    If you specify a positive voltage for a source then the (+) output will be positive with respect to the (-) output
    If you specify a negative voltage for a source then the (+) output will be negative with respect to the (-) output.
     
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