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Question About W/L(MOS)

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yonzzan

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When we design op-amp, the first thing you need to do is the decision of length of transistors in your design. How do you determine the reasonable length?

For example, TSMC 0.18 micron Technology.. should the length be greater than 0.18um??? or does that depend on each topollogy??

I am designing Fully folded cascode op-amp with or w/o gain boosting.

Thanks ahead.
 

It depends on requirements of your design, for example max frequency, cutoff frequency, speed of your amplifier, and so on. Smallest L is in the other side the most difficult case, because of submicron, short-channel effects. So if you can, use as long channels as possible. A.
 

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    yonzzan

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for analog design, in general, we don't use the smallest length, because of the consideration of match, gain, linearity, etc.
but because of the speed, we can't select too long length in signal path
so there is a tradeoff for selection of length
referring to J. Baker's book, we may measure the transistor's characteristic for different lengths and determine one
simulate the circuit and adjust the length if needed
good luck
 

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    yonzzan

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Get a factor of 10 from the your min. length as the start for all transistors in analog design. It is best for doing matching and since sometimes, you cannot do matching easily in layout, to be honest.

my experience using 0.35um (even for such old tech) from a top 3 foundry showing that if you draw 3.5um as the length, there will be sometimes a drop to 3.28um. If you draw 0.35um in length, there can be a drop to 0.285um in final length, so beware of this, ppl always say 4-5 times for analog design, I can say, it is not equal if you do precise amplifier or matching.
 

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    yonzzan

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I believe it pretty much depends on the technology that you wanna work on. In my opinoin, the smaller the size of L, the better. (Moore's Law)
 

Use 3~5 multiply of your process minimum lenth to
design the length. For example, the 018 um process is good for 0.5u ~1um for analog design. Of course, it also depends on your Design Circuit requrement such as output resistance , transconductance, Bandwidth , ....
 

it's better to use lengths and widths for the design of current sources and current mirrors but i think it's ok to use minimum feature size for the design of the differential pair ( or near values to the min feature size ) when we designed u used values larger than the min feature size but not very much larger
 

its simple when you need speed choose length to be minimum, when you need gain and stability choose length to be nearly twice or thrice the mini value
 

Take the L of the MOS gates 3~5 times

This is done to minimise the affect of 2nd order effects on design like channel shortening effects etc.

Regards
Sarfraz
 

actually, you have to fact in another parameter, the mismatch, which is inverse-propretional to the area of the device. So, basically, if you have your Gm in mind, you get you W/L, with mismatch, you have W*L. with these two equations, you have you W and L.
 

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