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In part 3 we saw how to recognise and adjust EC Nmos setup to achieve Ohmic or Saturation region for our requirements.
Earlier part 3 is here http://everycircuit.com/circuit/6179495276183552
Now let's look at the Nmos settings:
In a N Mosfet there is a movement (ie a mobility) of electrons called .... μ
It is a capacitive event called .... C
The layer on which it occurs is an oxide layer (as in metal oxide...MOsfet) called .... ox
So don't be put off if you see μCox mentioned somewhere.
μCox has an easier name which is ... KP
KP is related to the Width and Length of a layer, ie to W, L
Each individual N Mosfet has its own specific value for that relationship ...it is the Kn
Here is how it is related:
Kn= 1/2 x KP x W/L ie:
Kn=0.5KP(W/L) ...... and KP(W/L) is called beta in a mosfet.
So ....that is an important formula which we can use in order to find the Kn for an individual Nmos.
Being individual, it will not be found on a datasheet.
Mosfets are generally better suited for full switching in the Ohmic state, but it is useful to explore their Saturation state... especially so that we understand the difference and know how to select to our requirements... (see part 3 for quick, easy EC Ohmic setup for full switching)... Here we will continue with the Saturation state:
If we know a few common values, then we can calculate our Nmos Kn for the Saturation region:
Id= Kn(Vgs - Vₜₕ)² ..... simplified temporarily here ..... advanced users: see part 6 end footnote.
So ... Kn=Id/(Vgs - Vₜₕ)²
That is another important formula for us to find our individual Nmos Kn
Notice that the (Vgs-Vth)... called the Overdrive or Vov, is familiar to you from part 3 earlier, and so is Id
By taking a few readings in a simple setup as shown in earlier schematics here, we can find Kn
Alternatively, if we are designing a circuit in EC, then we can put in a KP of our choice and then use it to determine Kn which is often used in these formulae.
( straight to Ohmic setup Simulation Guide here: http://everycircuit.com/circuit/6260747729633280 )
But what about W/L .....it is explained in the next part....
part 5 here: http://everycircuit.com/circuit/5707117962199040
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