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How far can i increase my channel length in OTA design?

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EmbdASIC

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Length in Analog Design

Hello all,

I have a naive question, whose answer i myself have given to people on this forum :)

How far can i increase my channel length( if its worth it) in OTA design. Lets say the allowable drawn length is from 140um to 10um. I would usually keep it nearly 4xLminimum.
Can i push it upto 1um if it benefits me and i achieve the other parameters ?
 

Re: Length in Analog Design

it depends on what u are doing.
if it is a current mirror/ current source/sink then u want better impedance so bigger length (i use 3-4 times the minimum), for input devices u want more gm without wasting too much area so use minimum length which also gives you more ft so more speed.

and one more thing is the models are not very linear so going from minimum L to 3L might give u a great impedance but going from 3L to 9L will not give the same increase . so u might want to get ur performance curves.
 

Length in Analog Design

Your numbers don't make sense. Do you mean 140nm to 10um?

Keith
 

Re: Length in Analog Design

Keith, Yes i meant 140nm to 10um

Added after 27 minutes:

Thanks steadymind.
It seems you keep different length for different trasistors (for mirror and input devices), a practice i have not seen a lot ?
 

Re: Length in Analog Design

EmbdASIC said:
Thanks steadymind.
It seems you keep different length for different trasistors (for mirror and input devices), a practice i have not seen a lot ?
Click to expand...

I often have large W and fairly high length trying to get high gm with good matching for the input transistors. You also need to be aware of matching for the mirrors which keeps the sizes well above minimum.

Keith
 

Re: Length in Analog Design

Thanks Keith,
but by matching you mean matching gms or impedance ?
 

Re: Length in Analog Design

EmbdASIC said:
Thanks Keith,
but by matching you mean matching gms or impedance ?
Click to expand...

To minimise input offsets.

Keith
 

Reducing length of a channel in the transistor is done mainly for one reason: to improve the speed of your OTA(the Ft of a transistor increases for lower L values). This is however true for your input transistors. If you increase your length you have other advantages like higher ro, higher current etc. SO it depends on what transistor it is. If it is in the signal path, lower the length the better. And of course, lower the L value, lesser the area hence more density -the concept of digital VLSI world.
 

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