Layout question about W/L of transistors

[tieuquaybk]

If in our schematics most of our transistors has size of w/l (for example 10/2) then for good matching we use two transistors with w/l = 10/2 in parallel instead of 1 trans 10/1. How should we do if in we have a transistor with w/l = 10/100 for good matching? maybe use 50 10/2 transistors is not a solution.

 

[memed09]

layout question

as far as i know 2 parallel trans. wit w/l=10/2 means 1 trans. wit w/l=20/2...
am i wrong or?

 

[yankuangtu]

Re: layout question

this case is seldom appear in practice. what is your application?

 

[tieuquaybk]

Re: layout question

yeah memed09! two parallel 10/2 trans <=> 20/2 or 10/1 transistor.

@yankuangtu: that is start-up circuit in Baker's 2nd edition.

 

[tieuquaybk]

layout question

no one helps?

 

[SP24]

Re: layout question

If the w/l =10/2 then for matching we make it 2(w=5/l=2) and not 2(w=10/l=1).

 

[Saravanan.G]

Re: layout question

It depends on size of transistor with which you want to match.
normally divide the larger width into smaller fingers of same length and arrange in such a way that centroid of all matching transistor is same.
refer Book "Art of analog layout"

 

[analayout]

Re: layout question

Hii,

You cant reduce the length

You can reduce the width & you can increase the no. of fingures

Regards,

Analayout.

 

[varma_cs012]

Re: layout question

Buddies,

Let me clear you the concept.

Need for fingering---> If the width of the transistor is more, The poly resistance will be more.
then input for the transistor may not be going with same singal strength to the end of the transistor.

So we will do fingering to decrease the poly resistance.

How do we do it ?

For example you have a transistor in you schematic with 20/2 (W/L).
What you will do now ? Make it 5/2 M=4 (i.e M is number of fingers.)
If you do fingering you will never decrease the length of the transistor. You are just cutting the width and connecting them in parallel. Questions are expected. Thank You.[/b]

 

[raj_shekar]

Re: layout question

connect 10 transistors in series having W/L of 10/10 with all the gates connected together. But it depends on where u r going to use it and how much over drive u can tolerate.

 

[vaibhavbanga]

Re: layout question

i hav seen the answer but actually i want to know that how can i decide the no of fingers .why only i divide a 20/2 in 4 transistors 5/2 why not 5 of 4/2 or 10 of 2/2 ?????????? plz send me reply thanks.......

 

[varma_cs012]

Re: layout question

See for every condition we have some trade off. what will happen if you do more number of fingers, The source and draing cap will become more. If we increase the drain cap un-necessiarly it will make much load on output net.

So we should think about area constraint and source drain cap and fingering too.
In general we prefer to put less than 10um of the width for each transistor.

For example you have 20/2 transistor and better you can make it as 5/2 m=4 or 4/2 m=5.
If you put 10/2 you are not going to reduce more gate resistance, and if you some 1/2 m=20 or 2/2 m=10, then you are going to increase the source drain cap.

So you can decide 5/2 m=4 or 4/2 m=5, according to your layout floor plan. Guess understood. Correct me if I am wrong anywhere. Thank You.