# Homework Help: Determine Kn (constant) of MOSFET easyway

1. Jan 10, 2012

### vco.asm

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. Jan 10, 2012

### Kholdstare

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

3. Jan 10, 2012

### vco.asm

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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