# Length and Width of Transistor(NMOS)

#### sivaraj

Joined Oct 8, 2007
6
Hai I am new to design. While I am design a small circuit I got stuck. So pls clarify my droughts.

I have taken a small ckt. From Microelectronic Circuit by Sedra and Smith. In that I have to find the Width and Length (W/L). I have found the W/L its 10.

Answer is right in book W/L=10 (W=8um/L=0.8um). But I dont know how to fix the value for W/L. I mean how much I have to fix for W and for L?

I know that W=2L, They have given in the that W=8um/L=0.8um.
Why these much variations. Why we should not take 90um/9um or
20um/2um ?

How to fix the value?

#### Dave

Joined Nov 17, 2003
6,970
I'm confused by the terminology of "fix the value"; are you looking at fixing the ratio or the physical dimensions? Can I ask which part of Sedra and Smith is the question in so I can have a look and give you further feedback?

Dave

#### sivaraj

Joined Oct 8, 2007
6
ya its in the page 265 Sedra&smith question no. D4.11.

more over, for this question and previous question (Ex 4.3) i have worked. i have designed with W/L 40u/1.2u the output voltage is Vd=0.75, but while i am designing in cadence i am getting 0.2 extra. i dont know why?

I am designing an CMOS opamp? (current mirror, Diff amp, Active load) i dont know how to design der length and width. can u give mi some idea.