Nodal Analysis with a FET

Discussion in 'The Projects Forum' started by Engin_Nerd, May 1, 2011.

  1. Engin_Nerd

    Thread Starter New Member

    Oct 22, 2010
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    I'm trying to do nodal analysis of a DC circuit that uses a mosfet and want to compare my results to what I would get using PSPICE....how do I do this?

    If i was to remove the FET from the schematic what would I use to replace it for the circuit to behave the same (for the analysis). BTW the fet i'm using is a 2n7002.

    Thanks in advance.
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But where is the diagram?
     
  3. Engin_Nerd

    Thread Starter New Member

    Oct 22, 2010
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    Heres the diagram in a jpg, hope thats ok.

    Also, this might be a seperate topic, but I'm not sure how I would calculate the rise time and the fall time at the load??
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Is this a homework problem?

    hgmjr
     
  5. Engin_Nerd

    Thread Starter New Member

    Oct 22, 2010
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    project i want to do to learn about fets. did not really cover them in my electronics class.
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    For your MOSFET circuit when the Micro 5V signal is in high state (5V).
    You can treat MOSFET as a short circuit.
    As long as the drain current does not exceed 75mA you can treat your MOSFET as switch with Ron = 3.5Ω.

    And when the Micro 5V signal is in low state then you can remove the MOSFET form your circuit.
     
    Last edited: May 3, 2011
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  7. Engin_Nerd

    Thread Starter New Member

    Oct 22, 2010
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    Thanks for the

    When you treat it like a switch, do you need to consider the 5v from the micro? that was confusing me...

    Also when I want to know the rise time and fall time do I use this equation:
    V(t) = Vo*(1-(e^(t/T))

    When I used this equation i got a rise time in nano-seconds, but pspice gave me a rise time in micro-seconds..i'm a bit lost about how to do this.
     
  8. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Yes, you can ignore the 5v from the micro.
    Yes
    Hmm, the rise time is defined as
    tr 2.2RC
    So for your circuit
    tr 2.2* 770Ω*26nF ≈ 45μs for load
    And Tr 2.2 *740Ω*13nF ≈ 22μs for ADC input.
     
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