transistor output resistance

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tyanata

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typical ro output resistance transistor

In old Spice lybrarise we could first order calculation of output resistance of the transistor in saturation mode using parameter LAMBDA, but in new spice libraries there is not LAMBDA. Ro=1/(LAMBDA*Idssat)
Can any tell haw to make manualy first order calculations of output resitance of the saturated transistor. :?:
 

output resistance rout transistor

Even you know the arly voltage, or, if the model is correct, ask the simulator to print gds
 

Yes this I know this, but I asked for some first order formule
 

The first order equation is

gds = Id/Vea with Id the drain current and Vea the Early voltage.

Anyway, gds is the partial derivative of the drain current respect to the drain voltage at a fixed gate voltage. Up to you to chose the drain current equation that you want. You would take, for example, Id from the EKV model.
 

Easiest and most accurate way I think is to run a simulation with one transistor with different channel length. so you will know what kind of length you need to use.
 

gds is really what you want, tynata. remember that any transconductance (g term) is an inverse resistance, so 1/gds => Rout

here's how it fits together.
Rout = 1/LAMBDA*Ids where LAMBDA = 1/Va

so you could write Rout = Va/Id

and assuming gds=Id/Va just invert to show

Rout = 1/gds

partial derivative that humungous mentioned is true, but you can just remember this as being small signal output resistance. set up a bias point, and then any small deviation around this point sees an output resistance of 1/gds. Sounds good?
 

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