Electronics > Beginners
Help in deriving Zin small-signal expression
<< < (9/11) > >>
promach:

--- Quote ---In saturation region:
Cgs= (2/3)WLC'ox
Cox=WL*C'ox
--- End quote ---

I found the above in one other forum. Why would Cox be different when it is not in saturation region ?

Besides, U0 could be found directly inside the model itself as in https://github.com/imr/ngspice/blob/master/examples/xspice/table/modelcards/modelcard.nmos#L55

Wimberleytech:

--- Quote from: promach on May 29, 2018, 02:21:15 am ---
--- Quote ---In saturation region:
Cgs= (2/3)WLC'ox
Cox=WL*C'ox
--- End quote ---

I found the above in one other forum. Why would Cox be different when it is not in saturation region ?

--- End quote ---

Because in saturation, the charge in the channel (inversion layer) varies from the source to the drain.  This is because the electric field is decreasing in magnitude as you get closer to the drain.  For a non-saturated transistor, the charge in the inversion layer is closer to constant across the channel.  Of course, these are APPROXIMATIONS.  Simulators use a more thorough charge-storage model than this simple relationship.

--- Quote ---Besides, U0 could be found directly inside the model itself as in https://github.com/imr/ngspice/blob/master/examples/xspice/table/modelcards/modelcard.nmos#L55

--- End quote ---
Somebody had to put it there!
promach:
Do you guys have any idea how to compute value of Cox using https://github.com/imr/ngspice/blob/master/examples/xspice/table/modelcards/modelcard.nmos ?

Could anyone show ?
Wimberleytech:

--- Quote from: promach on May 30, 2018, 04:05:03 pm ---Do you guys have any idea how to compute value of Cox using https://github.com/imr/ngspice/blob/master/examples/xspice/table/modelcards/modelcard.nmos ?

Could anyone show ?

--- End quote ---

EPSROX*8.854e-12/TOXE
promach:
Cox = e0*er / Tox

Where is the missing parameter "area" ? I suppose capacitance equation is something like C = e*A/d

Besides, someone told me that calculating gm and ro by hand will not be very useful. What do you guys think ?

No matter what, using wxmaxima software, I have also came up with the extremely long expression of Vout/Iout which I am still not quite sure how I would use it effectively. Almost all parameters are both in nominator and denominator of the expression, this makes mosfet sizing decision very difficuly indeed.


\[ node\_1: V1/ro4 + gm4*V1 + (V1-V2)/ro6 + gm6*(Vin-V2) = 0\$ \]

\[ node\_2: (V2-V1)/ro6 + (V2-V3)/Rs - gm6*(Vin-V2) = 0\$ \]

\[ node\_3: (V3-V2)/Rs + (V3-Vout)/ro5 + gm5*V3 = 0\$ \]

\[output\_node: -Iout + gm3*V1 + Vout/ro3 - gm5*V3 + (Vout-V3)/ro5 = 0\$ \]

\[ sol: linsolve([node\_1, node\_2, node\_3, output\_node], [Vout, Iout, V1, V2, V3])\$ \]

\[ Rout: Vout/Iout,sol,factor; \]


\[
\frac{ (\mathit{ro3} (\mathit{Rs}\, \mathit{gm4}\, \mathit{gm5}\, \mathit{gm6}\, \mathit{ro4}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm4}\, \mathit{gm6}\, \mathit{ro4}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm4}\, \mathit{gm5}\, \mathit{ro4}\, \mathit{ro5}\, \mathit{ro6}+\mathit{Rs}\, \mathit{gm5}\, \mathit{gm6}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm6}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm5}\, \mathit{ro5}\, \mathit{ro6}+\mathit{Rs}\, \mathit{gm4}\, \mathit{gm6}\, \mathit{ro4}\, \mathit{ro6}+\mathit{gm4}\, \mathit{ro4}\, \mathit{ro6}+\mathit{Rs}\, \mathit{gm6}\, \mathit{ro6}+\mathit{ro6}+\mathit{Rs}\, \mathit{gm4}\, \mathit{gm5}\, \mathit{ro4}\, \mathit{ro5}+\mathit{gm5}\, \mathit{ro4}\, \mathit{ro5}+\mathit{gm4}\, \mathit{ro4}\, \mathit{ro5}+\mathit{Rs}\, \mathit{gm5}\, \mathit{ro5}+\mathit{ro5}+\mathit{Rs}\, \mathit{gm4}\, \mathit{ro4}+\mathit{ro4}+\mathit{Rs})) }

 { (\mathit{Rs}\, \mathit{gm4}\, \mathit{gm5}\, \mathit{gm6}\, \mathit{ro4}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm4}\, \mathit{gm6}\, \mathit{ro4}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm4}\, \mathit{gm5}\, \mathit{ro4}\, \mathit{ro5}\, \mathit{ro6}+\mathit{Rs}\, \mathit{gm5}\, \mathit{gm6}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm6}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm5}\, \mathit{ro5}\, \mathit{ro6}+\mathit{gm4}\, \mathit{gm6}\, \mathit{ro3}\, \mathit{ro4}\, \mathit{ro6}+\mathit{gm3}\, \mathit{gm6}\, \mathit{ro3}\, \mathit{ro4}\, \mathit{ro6}+\mathit{Rs}\, \mathit{gm4}\, \mathit{gm6}\, \mathit{ro4}\, \mathit{ro6}+\mathit{gm4}\, \mathit{ro4}\, \mathit{ro6}+\mathit{gm6}\, \mathit{ro3}\, \mathit{ro6}+\mathit{Rs}\, \mathit{gm6}\, \mathit{ro6}+\mathit{ro6}+\mathit{Rs}\, \mathit{gm4}\, \mathit{gm5}\, \mathit{ro4}\, \mathit{ro5}+\mathit{gm5}\, \mathit{ro4}\, \mathit{ro5}+\mathit{gm4}\, \mathit{ro4}\, \mathit{ro5}+\mathit{Rs}\, \mathit{gm5}\, \mathit{ro5}+\mathit{ro5}+\mathit{gm4}\, \mathit{ro3}\, \mathit{ro4}+\mathit{gm3}\, \mathit{ro3}\, \mathit{ro4}+\mathit{Rs}\, \mathit{gm4}\, \mathit{ro4}+\mathit{ro4}+\mathit{ro3}+\mathit{Rs}) }
\]
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod