How to know unCox and upCox when calculate gm?

Hi all:

I am designing a OPA and I set the input gm = 10uA/v = upCox*W/L*(vgs-vth)=(2*upCox*W/L*Id)^(1/2),
but I found the upCox doesn't a constant value,
it changes with W/L or vgs or vds.

How to know the upCox value?


Thanks for your reply.

mpig

Hi mplg09
You are rite. UnCox (UpCox) depend on W/L or vgs or vds. In submicron (45nm or below) the quadratic formula of Ids doesnt work well. I think you should you gm/Id methology (from UC Stanford lecture). This method you dont care about process parameter

Hi tompham:

I studied some doc of gm/id method before,
if you have used gm/id method, I have two problems of this method:

when simulation gm/id, does the vgs and vds need to a constant value?
and the value need to the same with the circuit application?

If you have any suggestion, please let me know.
mpig

One problem is that u0 becomes meaningless when you
get into velocity saturation and such things (i.e. when
the long channel assumption becomes invalid).

u0 is a model fitting parameter used in the classical
device model equations. You might extract a reasonable
value by measuring a device of long dimensions, and
you might fit the geometric effects by measuring and
refitting short-L and short-W devices based on the large
device fit.

I think you want to simulate your circuit at corners of
gain-stage and output-stage Vds(es); many topologies
see large gain variation across the common mode output
range (lose plenty, when you creep into linear region
near the endpoints of output swing, and there will be
a "sweet spot" you'd prefer to operate near, by higher
level circuit biasing perhaps).