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To find drain voltage for ntype JFET.

  1. Feb 7, 2008 #1
    Hi all,
    Ckt diagram for ntype JFET
    The values are as follows
    V2 = -10 V , V1 = +10V
    R1 = 1M ohm
    R2 = 10 k ohm

    IDss (drain current with shorted gate condition [tex]^{}_{}Vgs[/tex]= 10mA
    Vp = 4v {Vp= pinch off voltage}
    To find drain voltage Vd =?????



    ///////// SOLVED //////

    For gate source loop
    Vgg = V2 = -10 V
    Vdd = V1 = +10V

    From kirchoffs law

    (-Vgg ) – (Ig)* (Rg) – Vgs = 0 since (Ig ~= 0)

    Vgs = (-Vgg) ------------------------------------------------------- ***********

    By Shockley’s equation

    Id = Idss( 1-(Vgs /Vp)^2

    Id = 22.5 mA

    For drain source loop

    Vdd – (Id)*(Rd) – Vds = 0

    Vds = Vdd – (Id)*(Rd)
    Getting an absurd answer for Vds (= -215V)

    aim was to obtain Vds then Vds = Vd – Vs; as Vs = 0, Vds = Vd

    Can any one help me out by pointing my fault in analysis. Or is there any other method to solve for Vd .

    Attached Files:

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    Last edited: Feb 7, 2008
  2. jcsd
  3. Feb 21, 2008 #2
    Actually your answer if correct in one sense.

    1. First notice that 22.5mA is greater than the Idss. So you have a problem.
    2. Vgs= -10V. So what does this tell you about the state of the JFET. ON/OFF? The negative value of VDS is also a hint here.

    Directly solving by equations by brute force won't help.
  4. Feb 24, 2008 #3

    brute force method really worked ;) and thanks u gave me a wonderful idea

    as Id > Idss my assumption that its biased in active region is false... it must be in ohmic region(biased) same for Vgs although i could have inferred from vgs itself :(

    the ohmic resistance is Rds =_____
    replacing JFET by Rds and applying Voltage divider formulae to find Vd______________:cool::rofl::tongue2: got it!!!!!!!

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