Minimum lenght in analog design?

[eda4you]

Maybe someone can help me answering the following question!

For instance assuming a differential nmos input stage. Making the Area W*L for each transistor larger as well as the channel lenght, assures a better matchig and (?) a lower input noise. But on the other hand a larger area will be consumed and (?) the cut off frequency will decrease. Are my consideration right?

To sum it up, I don't know how to choose the L for my input stage in a 0.25µ process. Looking at other designs a lenght is choosen about two times the minimum channel lenght. Why???

 

[guamak_menanak]

For your information, definning W & L are very important to ensure your design meet your spec such as to define your current, gain and etc.

To define a better W & L is very important to ensure low noise because it relate to the noise calculations and simulations! For example, for flicker noise...

Idn = Kf*Id/(Cox*L*L*f0) for NLEV=0

= Kf*Id/(Cox*L*W*f0) for NLEV=1

*NLEV is set when you need to do the simulations

So, it's better to have a wide W & L values but the drawback is it will limit your freq response as wide W&L will have a bigger parasitic cap! Therefore, an acceptable value of W&L are needed to achieve your spec!

The minimum value of L for 0.25um tech is 0.25um itself, but you can have larger value to have smaller noise with acceptable freq response!

 

[electronrancher]

start with your desired gain. we know that gain of a diff pair is gm * rout1||rout3 where rout1 is output reisistance of the diff transistor and rout3 is output reisistance of the mirror transistor.

since your W/L ratio can be the same for many different L don't worry about W now - you will seem to solve for it later.

if you want 50V/V gain (Av) you can either have a 50uA/V mosfet, W/L of 1 and Rout of 1,000,000.
or...
50uA/V mosfet, W/L of 100 and Rout of 100,000
or..
100uA/V mosfet, W/L of 50 & Rout of 100k


now you already know a big mosfet is quiet and slower, and a small mosfet is fast and noisy but remember that larger has much better matching which i think is much more important. it is all these tradeoffs that make your final W/L

in fact, most of the designers i know only use minimum L (0.25u) for their digital! they feel that the very small Rout of a 0.25u device gives such bad current accuracy it's no use!