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Determine Kn (constant) of MOSFET easyway

  1. Jan 10, 2012 #1
    long time when i had tried more on how to extracting Kn from mosfet datasheet
    finally i found it;
    i datasheet look at gfs parameter with its details
    lets take IRF510
    ----------------------------------------------------------
    gfs------------ 1.3 ----------- @3.4 A ----------- simens
    ----------------------------------------------------------
    gfs is another name of Gm

    thus
    Kn= (gfs)^2 / (4*Id) where Id specified in datasheet under test condations of gfs

    Kn= (1.3)^2 / (4 * 3.4) = 124 mA/V2

    please if =there are something wrong in my calculation then do not stop reply and give us your opinions
     
    Last edited by a moderator: Jan 10, 2012
  2. jcsd
  3. Jan 10, 2012 #2
    Well in saturation,

    I = kn/2 * W/L * (Vgs - Vth)^2

    Or = Kn/2 * (Vgs - Vth)^2 if you include W/L in kn.

    Then Gm = Kn * (Vgs - Vth)

    Thus, Kn = (Gm^2)/(2I)

    putting values, = 1.3^2 / 2*3.4 = 248.529 mA/V^2
     
  4. Jan 10, 2012 #3
    there is something strange in my lt-spice
    when i build my circuit mosfet with voltage divider + NO Rs (@Source Lead)
    i did not get the corresponding Id current WHY?

    my parameter:
    R1=R2=100K
    Rd=400
    Id=10 mA
    where i used IRF510 with Kn= gfs^2 / 4Id = (1.3)^2 / (4 * 3.4 ) = 124 mA/v2
    i want to see at least V0= 1 or more than this

    but unfortunately i see very small voltage on Vds about 200 mV

    please some one help me
     
    Last edited by a moderator: Jan 10, 2012
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