How to choose the width and lenghts of the device!

Hi ,

I am want to know how to determine the W/L ratios for the particular transistor needed.I am implementing a IEEE paper in 350nm tech process log domain filter(3.3V) .

But i am using 180nm technology(min L=180nm and W=2um)..so i tried to maintain all the aspect ratios and scaled down to fit 180nm tech.as

in paper(in IEEE paper) my design(180nm)

2.8um/4.2um 1um/1.52um

what i did for L is 4.2um/0.5um=8.4 (Lmin=0.5um for 350um)
so for 180nm :::: 8.4*180nm= 1.52um
and then derived the W from the ratio.


Is is correct approximation approach i am using..

Else the correct method is something other than this ??

Please help me on how to decide the W/L of the transistors in the circuit .

Also let me know why the L is greater than the W ..??


Thanks

You can use it as a starting point but you should also scale down the current, to avoid that too large overdrives prevent you to work with 1.8V. If you need an high current for noise or speed, you have to change your W/L because the Vth doesn't scale of the same factor from 0.35 to 0.18. You can be also quite sure that you will have to refine your sizing after this first try. Are you sure that 2 um is your minimum WIDTH??? (W means width not weight )