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Earlier part 1 is here http://everycircuit.com/circuit/5245744790634496
Earlier part 2 is here http://everycircuit.com/circuit/5338883035496448
In part 2 some terms were used which now need some adjustment:
Notice that there is no resistor between earth and the Source terminal on the Nmos (arrow shows the Source terminal). Such a resistor would be called Rs and the volts across it Vs
The voltage at the gate was called Vg.... but since the Mosfet Source connects directly to earth in the same way as Vg, we could equally have called that Vgs instead of Vg. (ie Vg=Vgs+Vs but Vs is zero here so Vgs and Vg are the same when no Rs is involved).
When dealing with an Nmos, we are generally referring to the Mosfet Source as our reference point unless otherwise stated. So now I will stick to terms like Vgs and Vds....even though they are the same as Vg and Vd respectively whilst there is no Rs. .... having no Rs makes things a lot easier for now.
When the Vds dropped to a certain level, the Nmos gate bar showed suddenly as fully out as it changed from Saturation to Ohmic.
The voltage where that happened was in fact when Vds dropped down to the same as
(Vgs-Vth), or below. That happened when you raised the Vgs from 8.65v to 8.66v
Not only did the Vgs rise .... that raising of Vgs also caused Vds to drop slightly,
which now put the Vds at or below the (Vgs-Vth). The Nmos thus changed from Saturation to Ohmic,
and the gate bar showed us that !
Look at the Vds and compare it against (Vgs-Vth)
The (Vgs - Vth) is also known as the "Overdrive Volts", or Vov
That is how to tell whether you are in Saturation or Ohmic mathematically if seeing the bar is not enough for you.
Vds> (Vgs-Vth) .... Saturation half bar seen
Vds< (Vgs-Vth) ....Ohmic full bar seen
Vds= (Vgs-Vth) ...just changing state, call it Ohmic half bar seen to change to full bar when Vds is dropping.
Vth is the fixed threshold volts in the EC settings (called VTO there). Mine is set at 1v if you look there in the schematic Nmos settings. That threshold is where the Nmos turns on/off as Vgs crosses that volt setting.
Notice that my default Vgs of 8.65v - 1v as the Vth gives 7.65v as (Vgs-Vth).
Now look at the Vds .... my default there is 7.66v
See then how Vds is slightly greater than (Vgs-Vth)
When Vds > (Vgs-Vth) then the Nmos is in Saturation, with the gate bar half out, as with my default schematic
When Vds Is equal or less than (Vgs-Vth)
Then the Nmos is in Ohmic with the gate bar fully out.
You can now decide mathematically what Vgs and what Vdd and what Rd you require in order to set the Vds just as you want, to obtain Saturation or Ohmic, for your amplification or switching needs.
ie ....for fixed drain current ready for superimposed signal variations, or for a fast full blast to switch.
For amplification, the EC defaults are a quick setup, but it is better and often necessary to apply the principles outlined above..... eg especially keep lambda=0 for now..( but see footnote in part 6), and set the Vth as reqd.
For switching, here is a quick setup for EC settings of this Enhancement Nmos to give full SPST switching simulation:
W ....... fully clockwise
L ........ fully anti-clockwise
KP ..... fully clockwise
VTO.... set to 3V for general N-Mosfet simulation
Lambda... best leave it as EC default with these switching settings.
Ohmic full switching may need the mosfet Rds altering by more advanced users.... see here:
Rds₍ₒₙ₎ and heatsink help .... http://everycircuit.com/circuit/5362558166106112
Next ... Part 4 is here http://everycircuit.com/circuit/4863487332581376
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