# MOSFET Bias Circuit and Equations

Hello
I'm starting to design my amplifier with Mesfet.I calculated my stable amplifier with scattering parameters,however I don't know how do dc bias because I don't have any dc equations for Mesfet and and I can't extract dc information from datasheet (because datasheets only define IDSS (without threshold voltage such as FETs equation) and Vp (without define K coefficient such as Mosfet's equation).

You can always measure your MOSFET and then you will know Vp and K coefficient.

https://www.physicsforums.com/threa...vds-for-mosfet-amplifier.660440/#post-4206467

Or simple take two measurements Id1 for Vgs1 and Id2 for Vgs2 .
And use this equation to solve for Vgs(th) = Vp = Vt

$$\Large Vt =\frac{V_{GS1} \sqrt{ \frac{I_{D2}}{I_{D1}}} - V_{GS2} } { (\sqrt{ \frac{I_{D2}}{I_{D1}}} - 1) }$$

And K = Id1/(Vgs1 - Vgs(th))^ 2

But this equations are not very useful in real world because MOSFET show great process spreader. Also real world MOSFET characteristic is far from theoretical quadratic equation.

• baby_1
Thanks dear Jony130
as you know In Mesfets' datasheet we have only IDSS and VP parameters.does we have any equation for Mesfets? because it doesn't clear when we should use Mosfet equation or JFET equations for MESFET.
and does them accurate for Mesfet?