differential amplifier

Discussion in 'Homework Help' started by ihaveaquestion, Dec 12, 2010.

  1. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
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    hi guys,

    need a little help with the attached circuit as im not sure why im able to get three different values of transconductance:

    here's my evaluation so far:

    - va = 1/λ, therefore λ=1/va leads to 1/10 = λ=0.1
    - since we are given that Iss = 0.32, we know that since the transistors are matched evenly the current splits evenly, so Id = 0.16mA.
    - ro = 1/λId = 62.5kΩ
    - Id = 1/2μCoxW/L(Vgs-Vt)^2(1+Vds*λ)... we are given or know everything except Vgs and so we solve for that and get 0.879 V.
    - Vov = Vgs - Vt = 0.179V
    - The trouble starts when finding the transconductance, gm.
    gm = μCoxW/L*Vov = sqrt(2*μCoxW/L*Id) = 2*Id/Vov

    However, i'm getting three different values when i use those three equations -- any ideas? thanks in advance....
     
  2. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    any thoughts?
     
  3. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    hmmm, also, to find the other parameters:

    i think i can use the same Id = 1/2μCoxW/L(Vgs-Vt)^2(1+Vds*λ) equation for the pmos on the top.... i know everything except Vgs and Vds... BUT i do Vg = Vd and I know Vs = 2.5.... so I can turn that into an equation with only one unknown... but that gives me the following equation which I don't know how to solve:

    01.Vg^3 + 0.75Vg^2 - 2.736Vg + 2.43 = 0.04

    any thoughts?

    thanks guys
     
  4. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    no one? really?
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Well I try to look on your problem tomorrow.
     
  6. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    thanks but its due in 2 hours
     
  7. ihaveaquestion

    Thread Starter Active Member

    May 1, 2009
    314
    0
    for the record, that post i made earlier about getting a third order equation is how you do it....
     
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