Diff Amp Biasing Problem

Discussion in 'General Electronics Chat' started by mr_ivoryz, Jan 18, 2012.

  1. mr_ivoryz

    Thread Starter New Member

    Nov 19, 2011
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    I am trying to figure out why there is 2.55mA running through the drain of M1, it should not be there....also I noticed that I have a "gate" current which should not be there also....


    I am using the The 50nm MOSFET models found at http://cmosedu.com/cmos1/cmosedu_models.txt

    The currents out of the Source of M1 & M2 are negative, due to LTspice's definition, but they are flowing the correct way.


    Does anybody know what I am doing wrong? Is their something wrong with my circuit design/DC Biasing?
     
    Last edited: Jan 18, 2012
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Check the voltages at the source connections of M1 and M2. If the 20A μcurrent source I2 that feeds them is pulling them below ground (substrate) potential, you might get leakage into the substrate connections of the FETs.
    Oh dear, my mistake, brain fart, completely wrong, the source potentials are OK - so I've no clue: sorry.
     
    Last edited: Jan 18, 2012
  3. mr_ivoryz

    Thread Starter New Member

    Nov 19, 2011
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    Yeah, I thought they were ok too...I still cant figure this out, if anyone else has any input, please let me know!
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Add LTspice simulation file
     
  5. mr_ivoryz

    Thread Starter New Member

    Nov 19, 2011
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  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    This strange behavior is a results of you MOSFET model.
    Simply reduce the supply voltage to 3.3V and then everything returns to normal. So for 5V Vdd we have some strange breakdown between drain and substrate (body).
     
  7. mr_ivoryz

    Thread Starter New Member

    Nov 19, 2011
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    Thanks for the help, Ive been trying to figure this out for a little while now....I should of played around with the supply voltage more, I never went lower than 5V.

    I got one more question, is there a better MOSFET model that I should be using? If so, where can I get it?
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    It all depends on what circuit you want to simulate.
     
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