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Design >> Analog Design >> regarding W/L ratio of transistor
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Message started by srinivasan0901 on Nov 29^th, 2018, 2:12am

Title: regarding W/L ratio of transistor
Post by srinivasan0901 on Nov 29^th, 2018, 2:12am

In analog IC design , is it correct to make W/L ratio as 1. or L must always be less than W?

Title: Re: regarding W/L ratio of transistor
Post by DanielLam on Nov 29^th, 2018, 12:25pm

In general, the W/L ratio tends to be greater than 1. I do not know of any cases where you may want it equal or less. In my experience, you can't even do that. The minimum width is usually a little longer than the minimum length.

There are cases when you do not want minimum L maybe due to mismatch or for flicker noise reduction. But in those cases, the W/L ratio is still greater than 1.

**Edited to make statements correct.

Title: Re: regarding W/L ratio of transistor
Post by srinivasan0901 on Nov 29^th, 2018, 2:24pm

But in my class , i have designed diff amplifier, where width is several times more than length,which makes w/L >1. my doubt was for current measuring based circuits, usually L is more and almost similar to W or can be more than w. Is that way of proceeding design is correct or not?

Title: Re: regarding W/L ratio of transistor
Post by DanielLam on Nov 29^th, 2018, 3:20pm

Whoops, you're right. I was thinking one thing and wrote the other. I corrected my statements above.

Title: Re: regarding W/L ratio of transistor
Post by polyam on Dec 1^st, 2018, 7:51am

It depends on the design and the circuit you are working on, the length can be chosen more than W. I've seen a lot of circuits where the L was more than the width of the transistor especially when the device operates in weak-inversion.

Title: Re: regarding W/L ratio of transistor
Post by Tako on Mar 28^th, 2019, 6:10am

Sorry for late reply.

In general CMOS analog IC design (comparators, opamps single-to-hundreds MHz bandwidth, bandgaps etc.), generally W/L > 1 or even W/L >> 1. For example, a single transistor may be 10/2 um in 0.18 um technology.

Minimum and maximum dimensions depends on the technology, but generally Wmin is slightly larger than Lmin, e.g. 0.22/0.18 um.

You may choose W/L < 1 or even W/L << 1, especially in transistor which works as resistor (current source reference, RC filter), e.g.: 0.5 / 10 um.