FET transconductance vs Vgs

[gecky]

Hi All,

i need to extract the g-values of a FET for mixer analysis. Basically, the transconductance is expanded as a power series with:

Ids-Ids0=g0*(Vgs-Vgs0)+g1*(Vgs-Vgs0)^2)+g2*(Vgs-Vgs0)^3)+g3*(Vgs-Vgs0)^3.......

Ids0=Ids at FET biasing point.
Vgs0=FET gate bias volt.

(assume common source FET mixer)

I managed to extract the Ids vs Vgs curve from ADS.

Question is how can I fit the polynomial to the I-V curve?? Eg, using MatLab....I tried the curvefit function in matlab but it doesn't work.

Thanks!

Added after 1 minutes:

Left this out:
I want to find g0,g1,g2....

 

[flatulent]

You need to know the current at four Vgs values.

Then you write the equation four times with the Vgs values the same in each equation and the drain current for that value.

Then you solve for the four unknown g values.

Matlab is good for solving simultaneous equations.