k parameter in mosfet

[liquias]

Hi,
I have a kind of a thick question...

I know MOSFETs are governed by their current equations for each region.
My problem is that for every current equation I need the "k'" parameter, and I can't recall encountering it while looking at discreet MOSFET's datasheet.

Where can I find this parameter for discreet MOSFET transistors (like IRFP140, for example) ?

How is one to go about designing an amplifier without the ability to calculate the drain current in MOSFETs?

Can I calculate K' for discreet MOSFETs out of other parameters that do appear in the datasheets?

Is there a different attitude to designing amplifier circuits using MOSFETs ?

 

[jony]

We use MOSFET in the output stage, so we don't need to know K parameter.
Because we use Vbe multipliers to set the bias current.

But if you use this equation:

Id = K * ( Vgs - Vt )^2 then

K = Id / ( Vgs - Vt )^2

So from the datasheet
http://www.micropik.com/PDF/irfp140.pdf
Vgs(th) = Vt = 3V
And from figure 5
Vgs = 5V; Id =10A

K = 10A / ( 5V - 3V )^2 = 2.5

 

[liquias]

First, thank you.
OK. I agree you can extract k' graphically out of the datasheets. However think I have even encountered datasheets that did not include Vds to Id curves.

What then ?
What if the graph is not accurate enough ?

I have tried to google around but I find only referances for Vbe multipliers for BJTs. I am sure I am not familiar with Vbe multipliers (beta ? in MOSFETs ?)
What are Vbe multipliers and where can I read more about them ?
How do I go about designing any amplification stage using MOSFETs ? (Even final stages)...

 

[jony]

I have even encountered datasheets that did not include Vds to Id curves.
What then ?

But you can always do a simple measurement test.

What if the graph is not accurate enough ?

Don't worry about that.
This equation is not very accurate, and MOSFET show greater process spreader then BJT.
Look for example on Vhs(th) = Vt = 2V...4V.
So only real measurement can give as exact value.

I have tried to google around but I find only referances for Vbe multipliers for BJTs. I am sure I am not familiar with Vbe multipliers (beta ? in MOSFETs ?)
What are Vbe multipliers and where can I read more about them ?
How do I go about designing any amplification stage using MOSFETs ? (Even final stages)...

It is the same circuit as for BJT
Using HEXFETs in High Fidelity Audio (Q5 in Figure 4 set the bias current for the output stage)

 

[sawreyrw]

You should really be talking about Kp, not K, for the MOSFET model. Anyway, Kp is the transconductance of the device; sometomes refered to as gm. You can determine this from the data sheet, if the curve of Id vs Vgs is given. Otherwise, numerical values should be given in the data table.