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[SOLVED] [Moved] Minimum size inverter

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Newbie level 5
May 31, 2012
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How to design a minimum size inverter?I am not getting any clear idea on how should i decide the minimum width for the nmos device on the basis of which i will design my pmos to nmos ratio for eual rise and fall time.And how to decide the maximum frequency of operation for this inverter.
I newly started my career as an analog designer and undergoing training for the same.Kindly give any suggestions on how to start with my new career.:wink:

Absolute minimum size inverter is defined by the process you are using. So basically the minimum size inverter is the minimum size they can manufacture. However that is not to say that would be a good inverter. It would definitely have asymmetrical rise and fall times because the fact that NMOS of same size is stronger. And there are a billion issues with yield and connecting so small devices together like antenna effect and a lot more.

If you want symmetric rise and fall, an easy way to do it is connect the input and the output of the inverter together and try to get VDD/2 at that node. That is the point where your PMOS and NMOS actually has the same current driving capability. But again there will be difference in normal operation because what I just said just guarantees the operation during saturation.

Maximum frequency operation heavily depends on what is acceptable for your application. Would the circuit that is driven with this inverter be able to recognize signals that doesn't even look like square wave? If the answer is yes you can even operate at 10s of Gs with 0.35 um process :D. But if the answer is no, how much timing slack you have, how good settling you need, what is the noise margin are good questions to ask.

Good luck on your carreer, but one advice I can give is that, forums are really good way to enhance your knowledge, but asking very general questions generally end up getting you ignored. So try to be precise, read a lot about the general stuff and ask the specific questions.

Read about logical effort. There is also a book defining the concept. By understanding this concept, you can easily get the idea on sizing the inverter.


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