I have got this circuit.(adsbygoogle = window.adsbygoogle || []).push({});

http://img225.imageshack.us/img225/4369/circuithf1.jpg [Broken]

Threshold voltage of the n-channel MOSFET is 2Volts. RVAR varies from 0.5k[itex]\Omega[/itex] to 2k[itex]\Omega[/itex].

When I simulate the circuit, I get:

http://img132.imageshack.us/img132/2779/simulationuw4.jpg [Broken]

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[itex]\Omega[/itex] | 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.

[tex]\frac{0.606}{0.5} \times (4.7+0.5) = 6.30V[/tex]

Even though, it should be 12V.

If anyone could shed some light on this?

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# Circuit simulation with NMOS behaves weird

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