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Circuit simulation with NMOS behaves weird

  1. Oct 16, 2007 #1
    I have got this circuit.
    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:

    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?
    Last edited: Oct 16, 2007
  2. jcsd
  3. Oct 26, 2007 #2
    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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