Aspect ratio of transistor

Hi,
I am new to analog circuit design. I wanted to know if it is always better to have the aspect ratio (W/L) of the transistor greater than 1. For processes such as the 130nm technology node, the values of µCox are so large that some of my design are starting to have aspect ratio less than one. Should I try to avoid this? thanks for all the help.

Rajdeep

No... But a transistor is approximated to ideal when it is large and has a square form

No you will use W/L in all shapes and sizes. W/L>1 is good for gm, W/L< is generally good for low gm (current sources).

W/L ratio isn't so important as Gm or Gm effectiveness (Gm/Id). Only by going though trade-offs between gain, speed, accuracy you can get resonable W/L ratio. General rule of thumb which you can use in many cases you can find in this article 1212king.pdf from

Also there exist good methodology of device sizing called "Gm/Id methodology". You can find a lot of information and related articles on this forum.

Regarding your question: size W/L as desired and it doesn't matter is it more or less than 1.

thanks for all the help guys!!

Added after 12 minutes:

hello again, just one more question...
so far whenever i was designing any analog circuit, say a simple differential pair, the method i was following was to just design the size of the transistors based on the current flow that i want in each of the transistors and the Vgs and Vds voltage drops, and overdrive voltage. i wasn't considering any other parameters (such as gain, speed, output impedance, slew rate etc). Is this the right approach to designing analog circuits?
I know this must sound very basic...but i am new to analog design, so please help me out...thank you so much

You'd better read some books because your questions is very general and it could be a lot of answers here and all answers will be right but confuse you because of complexity of a subject. Everything depends on the particular goals. You could download any datasheet of an operational amplifier and to see how many parameters it has. I'm sure you don't know what they mean. The first task to understand them... hm.. you see? I've started to confuse you because of your questions. Parameters such as Vgs, Vds,Vov or Vds(sat), gm, size, and so on, is one level of abstraction. They relate to the transistors, but gain, speed, impedance, slew rate, concerned with the circuit design that is higher level, such as filters, amplifiers and etc. These parameters can be determined by a group of the transistors or even blocks. In this forum a huge amount of speculations about "I'm beginner. What book can you advice?" and you will find these topics without any problems. To me, the best book is A.Sedra "Microelectronic Circuits". Good luck