I am having a heck of a time applying a linear fit to a small region of data (the goal is to extract vt, ala the method shown in Baker 3rd Ed. p 293).

I have a plot of gm vs Vgs, and I have tried making transforms similar to the ones shown in the fit_line demo, but it just does seem to understand limiting the fit to a specific range of values. Here is my code in my transform to try to get a linear fit to the gm data from vgs values of 2.3 to 4.3 V:

gm = DERIVATIVE2(vg.m, id.m, 1, -1)

OVERRIDE_LIMITS = "Y"

X_LOW=2.3

X_HIGH=4.3

fit = fit_line(vg.m , gm, 1, -1)

RETURN fit

I have tried defining X_LOW and X_HIGH in the Setup Variables tab as well, also with no luck. I have also tried the linfit function.

Any suggestions?

Thanks,

Paul

*edited to fix typos*

Edited by: Paul Shepherd on Apr 9, 2014 7:42 AM

I have a plot of gm vs Vgs, and I have tried making transforms similar to the ones shown in the fit_line demo, but it just does seem to understand limiting the fit to a specific range of values. Here is my code in my transform to try to get a linear fit to the gm data from vgs values of 2.3 to 4.3 V:

gm = DERIVATIVE2(vg.m, id.m, 1, -1)

OVERRIDE_LIMITS = "Y"

X_LOW=2.3

X_HIGH=4.3

fit = fit_line(vg.m , gm, 1, -1)

RETURN fit

I have tried defining X_LOW and X_HIGH in the Setup Variables tab as well, also with no luck. I have also tried the linfit function.

Any suggestions?

Thanks,

Paul

*edited to fix typos*

Edited by: Paul Shepherd on Apr 9, 2014 7:42 AM

If you are still looking for a transform to compute the threshold voltage (vt), I have a PEL transform that uses the linfit function to return it as the Y-intercept of the data. I will be happy to send you a copy.

Best Regards,

Mike