Circuit simulation with NMOS behaves weird

AI Thread Summary
The circuit simulation with an NMOS transistor shows unexpected behavior, particularly regarding its saturation mode, which occurs at 1.8kΩ instead of the anticipated 1kΩ. The threshold voltage of the n-channel MOSFET is 2V, yet the simulation results indicate a significant deviation from expected voltage values at various resistance levels. For instance, at 0.5kΩ, the simulation yields 0.606V, while the expected value is 1.154V. Additionally, calculations based on the simulation voltage suggest an output of 6.30V instead of the expected 12V. The discussion highlights the importance of verifying the specific transistor model's characteristics, as it may open at 1V and enter saturation around 7V, contrary to initial assumptions.
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I have got this circuit.
http://img225.imageshack.us/img225/4369/circuithf1.jpg
Threshold voltage of the n-channel MOSFET is 2Volts. RVAR varies from 0.5k\Omega to 2k\Omega.

When I simulate the circuit, I get:
http://img132.imageshack.us/img132/2779/simulationuw4.jpg

It should go into saturation mode when RVAR is about 1kOhms, but it doesn't. I nstead, it does it around 1.8kOhms.

Here are some values for comparison:
1/k\Omega | Expected| Simulation
0.500| 1.1540V | 0.606V
1.000| 2.1100V | 1.154V
2.000| 3.5800V | 2.105V

Here is another weird thing, say you use the simulation value of 0.606V to calculate the voltage above R1.
\frac{0.606}{0.5} \times (4.7+0.5) = 6.30V
Even though, it should be 12V.

If anyone could shed some light on this?
 
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The thing is that the specific transistor model that you use in your circuit starts to open around 1V (as you correctly pointed out) but unlike what you claim it is supposed to enter the saturation region around 7V (not 2V). Check it here: http://www.irf.com/product-info/datasheets/data/jantx2n6764.pdf
See Fig.
 
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