(W/L) ratios from specifications of analog circuits

[ashuanurag]

Hi,

I need to know how (in industries) a designer reaches the (W/L) ratios of the various MOSFETs in an analog circuit from its design specifications.
For instance allan holberg gives a method for op amps. where can i get such a methodology for other circuits?

Thanks a ton,
Ashu

 

[sutapanaki]

I don't think you can find one methodology good for all kind of circuits. Different circuits with different specifications would require different approaches to transistor sizing. For example, if you design for high frequency you'd strive to minimize W*L and perhaps chose W/L such that Vdsat is not minimum. For low noise, accurate circuits you'd like to have bigger transistors. For gain you may chose W/L that minimizes Vdsat so that you get higher gm for a given current. And then most often you mix all those together.

 

[ashuanurag]

Thank you for the reply. But are there any good examples of this anywhere? I need to understand how this procedure is followed for a few analog circuits. Do you have links to any such design examples or, say, a good book for such specifications to (W/L) conversion??

Thanks a ton!

 

[girih192002]

@ ashuanurag,

I understand your question. but, there is no specific define routine procedure to calculate aspect ratio of transistor in your design. you need to juggle all the way, play with Vdsat equation. apply simple theories. you should go through Gray meyer book and UCB lectures.

 

[shashikumar.22]

hey girish,

where can i get these UCB lectures???

thanks in advance

 

[EmbdASIC]

https://webcast.berkeley.edu/courses.php

 

[ashuanurag]

ok. that was quite helpful. But can anyone suggest a 8-10 MOSFET circuit apart from an op-amp?? actually all the books seem to be obsessed with deriving the ratios for op=amps(2stage, cascode etc). I need a new circuit with its W/L ratios derived from the basic specifications. I really need to do this fast.

Thanks in advance

 

[JoannesPaulus]

A band-gap reference is a simple enough circuit that includes an op-amp. So you can see the effect of your transistor sizing...

 

[holddreams]

You can search for the project solutions of EECS240, which gives the design flow for different types of differential OTAs.

Also EE214 lecture give an example of differential OTA design using gm/id methodology.

ok. that was quite helpful. But can anyone suggest a 8-10 MOSFET circuit apart from an op-amp?? actually all the books seem to be obsessed with deriving the ratios for op=amps(2stage, cascode etc). I need a new circuit with its W/L ratios derived from the basic specifications. I really need to do this fast.

Thanks in advance

 

[ashuanurag]

again, OTAs are parts of an op-amp right? band gap references are different, i agree but are these all?? i mean, i need lots of these kinds of circuits i need to analyse..
specifications and their equations etc. where can i get all this stuff??

any kind of help with the equations and useful circuits will be greatly appreciated.

Thanks in advance
:|

 

[wpchan05]

again, OTAs are parts of an op-amp right? band gap references are different, i agree but are these all?? i mean, i need lots of these kinds of circuits i need to analyse..
specifications and their equations etc. where can i get all this stuff??

any kind of help with the equations and useful circuits will be greatly appreciated.

Thanks in advance
:|

I believe the W/L of a ring oscillator is a trade off between phase noise, power, and frequency of oscillation. right?

 

[ashuanurag]

ok. ring oscillators. that seems nice, but where are the equations for the W/L ratios derived??
Can you send me a link or name of some textbook i could use??

 

[wpchan05]

ok. ring oscillators. that seems nice, but where are the equations for the W/L ratios derived??
Can you send me a link or name of some textbook i could use??

I have not seen exact formula for the trade off, but a good discussion in this paper might help:

https://www.ee.ucla.edu/~brweb/papers/Journals/BRMar96.pdf

 

[sutapanaki]

Yes, OTAs are not the only circuits done in analog design, that's correct. But don't expect that you can find recipes with how to size transistors readily available for any circuits you may think of. While there is a lot of information how to size OTA, you should look at it not as something set in stone but rather as just an example of an approach to the problem. Bottom line, you have to know how the circuit (really any circuit) works, be able to come up with some simple expressions suitable for hand design, know the dependencies between voltages, currents and size of the transistors and this is how you find your W and L. Then refine by simulation.
The links given above are good for just this - identifying design metrics and how to link those to W and L.