URGENT: How to find an NMOS transistor with a low Vgs(th) in Multisim?

Discussion in 'Electronics Resources' started by Robotics Guy, Dec 4, 2011.

  1. Robotics Guy

    Thread Starter New Member

    Mar 11, 2011
    15
    1
    Hi,

    I'm at my wits end here -- I'm trying to finish a project and can't figure out how to find an NMOS with a low Vgs(th) in Multisim :( I've looked at the datasheets of several transistors in Multisim and found that they should work, but none of them turn on at low Vgs voltages, even though they are supposed to! Take the 2N6659 for example, the datasheet says it should turn on at a Vgs of 0.8V, but with the following circuit it doesn't turn on until 1.9V!

    [​IMG]

    Could anyone help me understand why this is happening and how to find one that will turn on below 1.2V?

    Thank you!
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    The lowest I could find is the BF904 (0.7V) and BSD214 (0.95V). I never tried to change the components simulation parameter. If you click on "add component" ,then select a MOSFET and click on "Details", it will show you a detailed report of the component's parameters. From what I understand +VTO determines the VGS voltage (turn ON).

    I don't know if there is another way than to go through all the components to find the suitable one. If this is only for simulation purposes use the one I suggested or go ver to the NI forum and ask them if there is another way to find the best MOSFET. You could probably create a new component too, with a modified VGS.

    The MOSFET you had chosen has a MINIMUM possible VGS of 0.8V (a real component could have anything from 0.8V to 2.0V), if you have a look at the datasheet this would be @1mA. At the minimum VGS you can not expect to draw the maximum Id. In the datasheets there is always a current Id specified for the minimum Vgs.

    Btw, my +VTO parameter for the 2N6659 says "1.72558"
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'm late to this party - but you need to understand that Vgs(th) is specified when Id is at a very low current. Basically, it is most useful to determine when the MOSFET is considered OFF, not ON. If you want to find out when it's considered ON, look at the Rds(on) characteristics. You will see at least one voltage and Rds(on) specification.

    You can also take a look for Vgs vs Id plots further down in the datasheet. These tend to be normalized "typical" values.
     
  4. BobaMosfet

    Active Member

    Jul 1, 2009
    109
    11
    You're misreading the datasheet. Aim for typical (which is 1.4) for the 2N6659. That is the 'expected' value. Anything else between .8 and 2.0 is "within the range of acceptability"- no single value is guaranteed.

    BM
     
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