Circuit simulation with NMOS behaves weird

In summary, the conversation discusses a circuit with a n-channel MOSFET and varying RVAR values. The simulation results show that the circuit does not enter saturation mode when expected, but instead at a higher RVAR value. The conversation also mentions some comparison values and a discrepancy in voltage calculations. The issue is explained by the specific transistor model used in the circuit.
  • #1
Sesse
5
0
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[itex]\Omega[/itex] to 2k[itex]\Omega[/itex].

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[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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  • #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.
 
  • #3


It seems like there may be a few issues with your circuit simulation. First, it is important to make sure that all the components in the circuit are properly modeled and have accurate parameters. This includes the MOSFET, resistors, and power supply. It is also important to check the simulation settings and make sure they are appropriate for the circuit being simulated.

In this specific case, it seems like the threshold voltage of the MOSFET may be causing some unexpected results. The threshold voltage is the minimum voltage required to turn on the MOSFET and allow current to flow through it. In your circuit, the threshold voltage is 2V, which means that the MOSFET will not fully turn on until the gate voltage reaches 2V. This could explain why the circuit is not going into saturation mode at the expected resistance value of 1kOhm. It may be helpful to try adjusting the threshold voltage to see if that affects the simulation results.

Another potential issue could be the use of the simulation value of 0.606V to calculate the voltage above R1. This value may not be accurate due to the threshold voltage issue mentioned above. It may be helpful to use a different method, such as using the simulation results for the voltage at the drain of the MOSFET and subtracting the voltage drop across R1.

Overall, it is important to carefully check all the components and parameters in your circuit simulation to ensure accurate results. It may also be helpful to consult with a circuit simulation expert or do some research on troubleshooting common issues in circuit simulations.
 

FAQ: Circuit simulation with NMOS behaves weird

What is NMOS and how does it behave in a circuit simulation?

NMOS (n-type metal-oxide-semiconductor) is a type of transistor that is commonly used in circuit simulations. It consists of a metal oxide gate, a source, and a drain, and it operates by controlling the flow of electrons from the source to the drain. In a circuit simulation, NMOS can exhibit unexpected behavior due to various factors such as temperature, voltage levels, and device parameters.

Why does an NMOS circuit simulation sometimes produce strange results?

There are several reasons why NMOS behavior in a circuit simulation may seem unusual. One possible cause is the presence of parasitic elements such as capacitance and resistance, which can affect the performance of the NMOS transistor. Another reason could be improper settings or incorrect assumptions made in the simulation software.

How can I troubleshoot issues with an NMOS circuit simulation?

If you encounter strange results in an NMOS circuit simulation, the first step is to check your circuit design and make sure it is accurate. Next, you can try adjusting the simulation settings, such as the time step or convergence criteria, to see if it improves the results. It may also be helpful to compare your simulation results with theoretical calculations or physical measurements to identify any discrepancies.

What are some common challenges in simulating NMOS circuits?

One of the main challenges in simulating NMOS circuits is accurately modeling the transistor behavior. This includes taking into account the non-ideal characteristics of the device, such as channel length modulation and threshold voltage variation. Additionally, parasitic elements and other circuit elements can also affect the simulation results and must be carefully considered.

How can I improve the accuracy of an NMOS circuit simulation?

To improve the accuracy of an NMOS circuit simulation, it is important to carefully select and configure the simulation software and settings. You should also ensure that your circuit design is accurate and includes all relevant components. Additionally, it may be helpful to perform multiple simulations with different settings and compare the results to identify and correct any discrepancies.

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