How to calculate W/L value of a transistor using its datasheet?

Hello there, I gotta bias a basic circuit with NMOS transistor. But while doing it and finding Id, i need W/L value in the equation, so i am stuck. All i have is this datasheet : http://www.datasheetcatalog.org/datasheet/philips/BS108_3.pdf

I have searched a lot tho but still nothing. (I know resistance values, Vdd, and got this datasheet. Transistor is in saturation region btw.)

Thanks in advance, I will appreciate all the tips.

Besides the W/L you would also need the Uo and Cox constants which are technology dependent and you won't find them anywhere. Besides that, the equation you're talking about (Id = W/L U0 Cox (Vgs-Vth)^2) is for small-signal analysis only. For large signal (including bias) that equation is insuficient, it lacks the non-linearities and other stuff.

Anyway, you can use the Transfer Admitance value on the datasheet which gives you the Id/Vgs variation rate. This value was calculated for a specific circuit but it should be good enough for you.

Do not forget that this transistor is used mainly as a switch and not a linear circuit and therefore small-signal parameters are not provided by the datasheet (except the admitance) nor the transistors performance will be as accurate as you might expect.

Thanks a lot for the tips. Anyways, i assume you mean this by transfer admittance

SYMBOL---PARAMETER------------CONDITIONS-----------------MIN.TYP.
Yfs--------transfer admittance----ID = 300 mA; VDS = 25 V-----200 600

aren't the conditions too high? Don't know how these work tho, even if i have a circuit with ID around 5mA, can i still use it? If so, which value to use, typical or minimum? Thanks.

Thanks a lot for the tips. Anyways, i assume you mean this by transfer admittance

SYMBOL---PARAMETER------------CONDITIONS-----------------MIN.TYP.
Yfs--------transfer admittance----ID = 300 mA; VDS = 25 V-----200 600

Click to expand...

Yes thats right.

aren't the conditions too high? Don't know how these work tho, even if i have a circuit with ID around 5mA, can i still use it? If so, which value to use, typical or minimum? Thanks.

Click to expand...

VDS doesn't interfere with Yfs a lot, so I think you can ignore the difference between your circuit and the one used on the datasheet. However, for the ID thats another story. Transimpedance (Yfs) is the inverse of transconductance (gm) and gm = 2 ID / Ve where ID is the DC bias current and Ve is a constant which we don't have. What this means is that Yfs varies linearly with ID.

Yfs = 1 / 2 ID (I'm ignoring the Ve constant here)

So if Yfs = 400 mS for ID = 300 mA we'll have roughly Yfs = 6.6 mS for ID = 5 mA

But like I said, this is all small-signal stuff. For biasing, I bet the equations won't work.

Hmm, i have covered some distance but still stuck at a point. I have VGG value, VDS equation (including ID) which is VDS = 12 - 1220*ID, and VGS equation (including ID) which is VGS = 3.44 - 220*ID but still don't know what do with all those parameters. This is my circuit by the way : https://i.imgur.com/jWTNI.png

You have to indicate exactly what do you want to obtain from these equation. VDS? VGS? ID?

ID, then the rest can be acquired.

With a gate voltage of (330/(330+820)*12V=3.44V and a max. Vgs=1.8V you get an imprinted current Is=Id=7.5mA . With a min. Vgs=0.4V you'd get 13.8mA, but this isn't possible because of the limitation by R1; by estimating the saturation voltage Vds,sat=100mV you'd get Id=Is=9.75mA .