E-MOSFET Formula

Discussion in 'General Electronics Chat' started by NeRdHeRd, Feb 11, 2008.

  1. NeRdHeRd

    Thread Starter New Member

    Feb 5, 2008
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    I'm trying to determine what the Vds would be for a given Vgs. Is there a formula I can use for this calculation? I was thinking of using the following formula:

    Vds = Vdd - IdRd

    I just don't know if Id(on) is the same as Id.

    Could someone please clarify this for me. Thanks
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    A Mosfet has a range of Vgs for it to conduct a certain amount of current.
    Most are guaranteed to be fully turned on with a Vgs of 10V. They are barely turned on (called their threshold voltage and their current is only 250uA) with a Vgs from 2V to 4V.

    The transconductance is what determines the current with a certain Vgs but it is also a range and it is not linear.
     
  3. NeRdHeRd

    Thread Starter New Member

    Feb 5, 2008
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    So, if I have a Vgs of say +2.5v is there a way of determining what the vds would be?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Or perhaps in the spec sheet for the individual device. As in transistors, all FET's are not alike. Some are "logic-level", meaning that they are fully turned on at 5 Vgs. One might expect that such a device might have a characteristic quite different from a warhorse IRF510.

    By the way, Vds is not dependent on Vgs. An N-channel FET will start to enter the conducting region sometime after the gate is taken more than 2 volts positive with respect to the source, but the voltage on the drain is the result of the external circuit. It is a bit more meaningful to inquire as to the degree of conduction resulting from a certain Vgs.
     
  6. Audioguru

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    The datasheet shows some Vgs that make a certain drain currents for a typical Mosfet. Your Mosfet might be typical or be below or be above.
     
  7. NeRdHeRd

    Thread Starter New Member

    Feb 5, 2008
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    Alright to be more specific I have a E-MOSFET in a circuit with the following characteristics:

    Vgs(th) = 2v
    Id(on) = 5mA
    Vgs(on) = 5v
    Rds = 500 ohm
    VDD = 15v
    RD = 1k ohm

    With that being said, if I have a Vgs of +2.5v or +3.0v can I determine what Vds would be. Do I need to determine the Id(sat)? I've been searching everywhere for some formulas and I can seem to find one that applies to this.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Vgs(th) is the threshold voltage where the device just starts into conduction. If you have the data sheet for the specific device, you could run out the transfer curve and make a guess, but FET's just aren't like BJT's. All the data you have says for certain is that it starts to conduct with 2 volts on the gate, and it's fully on with 5 volts on the gate.

    I could be too ignorant to help, too.
     
  9. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    K = Id(on) / (Vgs(on) - Vth)^2

    Id = K (Vgs - Vth)^2

    Vds = Rds * Id

    Assuming that the MOSFET is in active region, where Vds > Vgs - Vth, which you should check after obtaining the Vds value.
     
  10. harrypotter

    New Member

    Sep 26, 2007
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    How do I find VGS(th) if given: ID(on), k, and VGS(on)?
     
  11. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    Use the first formula above. Vgs(th) is Vth.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The transconductance of a Mosfet also has a wide range of values. The transconductance is not linear at low current.

    Simply design the circuit with plenty of negative feedback to lessen the effects of the variations.
     
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