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Suppose we have chosen this setup.... schematic circuit on the right.
( More on that basic setup here: http://everycircuit.com/circuit/6063244598050816 )
Let's say we want to have a bought Nmos instead of that bottom right resistor....as now seen in the left schematic similar circuit.
If we know that this Nmos has an Rds of say 0.05 ohm and a Vth of say 3v
but no other details, then how do we procede, and how do we simulate it in EC?
Here's how:
1: Set up the real Nmos on a board like the left schematic circuit.
2: Turn up the Vgs very gently until it switches on at Vth of 3v.(thus finding the Vth here if unknown).
3: With a volt meter across the Nmos, watch the Vds drain volts drop until it is at exactly 1mV just like the circuit on the right.
4: Make a note of the Vgs gate volts at that point ... it means that the Rds is set at 0.05 Ohm ie:
FormulaA: Rds=Vds/Ids where Rds=0.001/0.02
Note that the 20mA drain current Ids is the same as on the right. ie 0.02 as used just above here.
Let's suppose the gate volts Vgs reads as 8v on this example when Vds just reaches 1mV ie: 0.001
FormulaB: Rds = 1/[2Kn(Vgs-Vth)]
Manipulate that formula to obtain the fixed Kn of that Nmos:
2Kn = 1/ [Rds(Vgs-Vth)]
The (Vgs-Vth) here is (8-3) ie 5v
That 5v is called the overdrive, Vov
and the Rds is 0.05 Ohm as set by that 1mV drain.
(We are assuming that a volt meter measuring Vgs reads as 8v when Vds just reached 1mV on that particular bought Nmos ... any other mosfet may give a different valid Vgs reading to use instead of 8v).
We get 2Kn = 1/[0.05 x 5] for this particular Nmos.
So 2Kn = 4 .... ie: this Nmos Kn=2
FormulaC: β = 2Kn
so β = 4
We can now put a default Nmos in EC with an identical setup as here and adjust any of KP, W, L until that combination as KP(W/L) is 4 ie: that combination is β
FormulaD: β = KP(W/L)
We are then simulating a real Nmos in EC with our chosen setup:
Kn = 2
β = 4
Rds = 0.05 Ohm
That β does not alter, so we can use that Nmos or simulate it in any new circuit, knowing what its β is.
Here is that FormulaB from above, now in a more useful description:
Rds = 1/[β(Vgs-Vth)]
Knowing the fixed β and the fixed Vth, which do not alter, we can now use that formula to set any new Rds if we should wish to do so....
Simply by altering the gate volts Vgs
(Then use Rds=Vds/Ids to check the value of that Rds).
Also...
If you wish to keep the same Rds eg 0.05 Ohm, with the fixed Vth (3), and the fixed β (4) ........
Then you can calculate how much gate voltage Vgs you will need for that circuit to work:
Vgs = [1/(β x Rds)] + Vth
eg: from our values above ...
Vgs = 1/(4 x 0.05) + 3
so Vgs = 8v .... confirming the above.
(We could later make these calculations first, then set about wiring the board knowing what values to use/expect).
Now check these (real pretended) Nmos settings with your x-ray vision (ie: view the Nmos settings and try using them in the above formulae) and see if you get the Kn=2 and β=4 which we achieved here without seeing those settings ..... (they do!)
Extra:
FormulaE: Ids = KP(W/L)(Vgs - Vth)Vds
ie: Ids = β x Vov x Vds
All of the above is for Nmos full Ohmic switching (not for saturation state).
Simple EC Nmos settings with zero Rds:
W ... full clockwise
L .... full anti-clockwise
KP .. full clockwise
VTO.. ie Vth ... set it to 3v
Lambda..... ignore
Vgs ... ensure that it well crosses that Vth for on/off
eg: 5v to 10v
Source arrow on Nmos .... connect directly to earth.
Use a 10k resistor from gate to earth.
One of the main reasons for finding the unchanging fixed Kn value here in Ohmic like this, is that we can then use it during a (non-Ohmic) Saturation setup where it plays a more important rôle.... eg: to obtain an estimate of the real drain current (id), as opposed to the theoretical drain current (Id)... the latter of which does not take into account the slope of the saturation with differing Vgs. ... ref: lambda.
For Ohmic and Saturation practice, see here: http://everycircuit.com/circuit/6347349325774848
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