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MOSFET Question

  1. Feb 26, 2015 #1
    1. The problem statement, all variables and given/known data
    2wdx2eh.jpg

    Questions ask me to find the value of VG and R2 as well as Vs
    2. Relevant equations
    R = V/I
    Id = Kn(Vgs - Vth)^2

    3. The attempt at a solution

    Before i find VG and R2 , I would like to find Vs first.
    Vs = R*I where R= 1k ohm and I = 1mA ? hence , Vs = 1V
    Is it correct for Vs?
     
  2. jcsd
  3. Feb 26, 2015 #2
    Vs would be a sensible place to start. Your calculation is correct. I'm not sure why you are having doubts.

    You do seem to have assumed that IS = ID. If you are not confident about that assumption, ask yourself where the current ID can go. It is, after all, a flow of electric charge, so what goes into the MOSFET must come out. And where can it go? Let's think about the gate current first. Remember that the O in MOSFET stands for oxide: the insulating layer of silicon dioxide. How much current flows through an insulating layer? So where does that leave for ID to go?

    Apparently we are supposed to answer questions this way, not just doing your homework for you. But I presume your next step will be to work out Vgs by plugging the numbers straight into the second equation, won't it? :wink: And that should tell you the current through R1.

    And, using your conclusion about the gate current, you should be able to work out the current through R2 without needing a calculator... and thus the value of R1.

    Alternatively, using your conclusion about the gate current, you could treat R1 and R2 as a potential divider - you may have derived a formula for this in your lessons. It's the same idea but just means you don't have to work out the currents explicitly. For instance if the required voltage at the junction of R1 and R2 were 4.5V (it's not) then R2 would have to be 9 times R1.

    Is that helpful or did you have a different difficulty?
     
  4. Feb 26, 2015 #3
    Hi

    Thanks for your reply. I am confused when I saw the question stating that there is no body effect. What does it do and how will it affect the circuit? If there is body effect and there isn't
     
  5. Feb 26, 2015 #4
    The body effect is about how the MOSFET itself works and makes the body of the MOSFET act like a second gate, squeezing or opening the channel according to the voltage on it. As the body is internally connected and there is no external connection to it and you've said there is no body effect and you've quoted the device characteristic to be Id = Kn(Vgs - Vth)^2 it seems reasonable to forget about it. It would have the effect of altering Vth and generally mucking the characteristic up. It would also require Vbody to be included in the characteristic! But I do wonder - if you have studied the operation of the MOSFET and the question sees fit to talk about body effect, where did the characteristic come from? It's only an approximation - a very good one with some devices, a pretty bad one with others.
     
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