gm-id design methodology
Hi All,
I took the ee240 class at Berkeley, and the truth is that the best way to learn is to go through the pain of doing the posted homeworks and look at the solutions... but in a nut shell here is what is fuss is all about.
1) B-sim level models are far from your first or second order approximation hand calculations. So, why do hand calculations?! have the simulator do a bunch of calculations for you, and then graph them, then you just look at the graph and see where you want to be.
2) Ok, what graph? you make these plots... gm/Id Vs Vgs, ro Vs Length (pick typical Widths) Id Vs Vg for a series of Lengths (pick a typical Width again) , all for a simple common source transistor configured in a circuit like one that I will post soon. (slide 30 of **broken link removed**)
3) to extract these parameters you can use Hspice parameter list, i.e:
.print dc +
myGmOverId = '(M1(LX7)/M1(L5)'
or something to that effect... I don't remember...
or you can run a bunch of .op and lookup the parameters (gm, ro, Id ...) and graph them in excel *perl would be handy*
4) now you have all your data... you can setup what L gives you gmro that you want... if that meets bandwidth you reduce your gm/id and find a new Vgs that maps to a whole new set of L and W, ... this post is not intended to cover gm/id (aka V*) methodology in one shot...
I hope at least this would motivate more designers to respond and ask questions... Once you learn how to design with this method, you can optimize better, faster, and save yourself pain... if you want to design in the 90nm and bellow, you won't be able to do so without the above method... of course then chances are you already know this method if your given the task of designing in 90nm... but for the rest of you stuck in .3 micron world and above... you can also use this and save yourself the pain of velocity saturation and a bunch of other crap that only physicist and process engineers care about, and focus on Gain Bandwidth and other nice circuity stuff... because the quantum effects are all in SPICE ... so use it as a lookup table...
Hope this helps/wasn't too long an confusing.