if the function of mos fet in saturation region is used:
i=1/2*ucw/l*(vgs-vth)^2
which ignore the second order effect;
to analyse the character of gm''(the second derivative of gm) of a simple full differential amplifier,then is the result helpful for design? or it is unreasonable?
why?
thx~

hi,
   sorry i didn't understand man..could you please explain clearly?

Thanks,
Rajasekhar.

sorry, may be i did not say clearly before;
if i=1/2*ucw/l*(vgs-vth)^2  is used
then the gm of full differential amplifier is
gm=1/2*ucw/l*(4*Iss/(ucw/l)-2*deltav^2)/sqrt(4*Iss/(ucw/l)-deltav^2) where deltav refers to the input differential voltage;
which appears in razavi's book <design of analog cmos integrated circuit> formula 4.10
and gm'' can be calculated though the expression of ids do not include second order effect!
but, if the channel length effect is included, do you know how long the expression can be? it is beyond hand calculation!
so , i hope that the earlier result is useful, but i am not sure
thank u again for your help!

Hi kibby,
               As Berti suggested, Why don't you use Gm/Id methodology for ckt design rather than hand calculations (taking into acc only 1st order effects ) ??

Gm/Id methodology employs technology plots which you can simulate from spectre/spice on only 1 MOSFET....say you can store FOMs for different vgs,vds,vbs,lengths etc...Now you while building a ckt, you can directly use these curves which include all the secondary effects. It's pretty simple too.

From personal experience, i feel using Gm/Id is the best methodology & gives pretty accurate results when used wisely....

regards,
Mayank.