# Mesfet Dc Bias Circuit and Equations

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#### baby_1 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).

#### BigBoss If Idss and Vp values are given, you can make an interpolation between them.
Gm=Idss/abs(Vp) ( very rough approximation ) then you can simulate it with a right model.

• baby_1

### baby_1

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#### baby_1 Thank you so much BigBoss
could you tell me where it comes from( Gm=Idss/abs(Vp) )? and how can we find the source-Drain current with specific gate-source voltage or vise versa ?

#### BigBoss Thank you so much BigBoss
could you tell me where it comes from( Gm=Idss/abs(Vp) )? and how can we find the source-Drain current with specific gate-source voltage or vise versa ?

It's not coming from somewhere, I have given a rough expression for that.There isn't a handy consistant equation for HEMT devices therefore you should do a rough approximation ( you presume that the Ids is rising monothonically and linear by Vgs voltage ) and that approx. will give you a triangle with Idss and Vgs.So you can calculate approximately Ids @ Vgs by using 90 degreee triangle equation.The exact solution is found by doing DC simulations with complex models.

#### volker@muehlhaus 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).

There are no curve plots for Id vs. Vgs and Id vs. Vds in the datasheet?

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