gm ploted doesnot equal gm readed

[microwaver]

I sweep the DC simulation to plot the gm(transconductance) with vgs but the operating point indicate different results. why?

 

[dyjguilin]

what the data present for in the right of picture?

gm is decided by the DC bias point, Does the point you polt out in the left of picture has the same DC environment as the gm you gave in th right?

 

[LvW]

I suppose the right side of your picture shows the model description of the part and on the left side the result of a DC simulation is shown. Correct ?
In this case, you cannot expect that ONE parameter of the model (gm) is identical to a parameter which is measured/simulated for the whole part (Yo).

This would be the case only for an oversimplified model (voltage controlled current
source) without a real physical background.

And, in addition, as said in the first reply: The transconductance depends of course on the bias point.

 

[microwaver]

thankyou for attention!
the picture of the right is indeed the model description @ certain DC environment and the picture of the left is DC sweep of gm ,in my opinion, we can read the plot @ the same DC environment the same value as the model description ,but the reslut tell us not, i really can not figure out why?

 

[refugee]

Cause :

gm=Id/(Vgs-Vt)，so the operation point would affect the gm

 

[edge_tv]

The Formula gm=Id/V_over (V_over = Vgs-Vt) is not currect for short channel Transistors, and for long channel transistors it is only a good aprox for V_over>>0. I can see in your sumulation that your Baising is in beginning of the transist rigion from Satur. to subTt, try to bias higher like gm/id=7 then you should get closer bout it will be still 20% off, check out Gm/id design Methode, there is a paper from 1996 from a south american guy (forget the name sorry)

 

[LvW]

In this case, you cannot expect that ONE parameter of the model (gm) is identical to a parameter which is measured/simulated for the whole part (Yo).

As I have mentioned in my first reply (see quote above) , you cannot assume that the internal model parameter gm is identical to the measured external value Yo.