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Find the MOSFET bias point

  1. Dec 27, 2013 #1
    1. The problem statement, all variables and given/known data

    Given the following circuit find the bias point (VDS,IDS).

    30b2pad.png

    2. Relevant equations



    3. The attempt at a solution

    The first thing I did was find the value of VG, the voltage from the gate of the mosfet to the -VDD source.

    VG = VDD * R2/(R1+R2)

    To find the current IDS I needed the voltage from the gate to the source, VGS, which is:

    VGS = VG - RsID

    Since we're operating in the saturation region I can replace ID by:
    VGS = VG - Rsk(VGS-VT)^2 (1+λVDS)

    where VDS is the voltage between the drain and the source.
    I also know that:
    VDS = 2VDD - (Rd+Rs)ID

    This is as far as I can get. I'm a little confused regarding what I should do next.

    If anyone could help I'd appreciate.

    Thanks.
    Daniel.
     
  2. jcsd
  3. Dec 27, 2013 #2

    rude man

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    Not a good start. Need to fix that first.
     
  4. Dec 27, 2013 #3
    Sorry, I meant to write 2VDD instead of only VDD.
     
  5. Dec 27, 2013 #4

    rude man

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    Better, but still not right.
     
  6. Dec 27, 2013 #5

    rude man

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    That equation is also wrong.

    You're having trouble with that "-V_DD".
     
  7. Dec 27, 2013 #6
    Thanks! I finally got it right.


    VG = VDD (R2-R1)/(R2+R1)
    VGS = VG - RsID + VDD

    ID = k(VGS-VT)2

    Replacing VGS in the expression (1) I managed to find the correct solution. I still have one doubt tho.
    The value of λ is very small, only 0.02 V-1. When the value is that low can I simply ignore it and solve the problem as if it was zero?
     
  8. Dec 27, 2013 #7

    rude man

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    OK now.
    Since you were given a number for lambda I would retain those values in your answer.
     
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