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Homework Help: Diodes, exponential model.

  1. Jan 20, 2014 #1

    1. The problem statement, all variables and given/known data

    The circuit in the diagram utilizes three identical diodes
    having I_S = 10−16 A. Find the value of the current I required
    to obtain an output voltage V_o = 2.4 V.

    If a current of 1 mA is drawn away from the output terminal by a load, what is
    the change in output voltage?

    2. Relevant equations

    What does the value of I_S tell me? should I multiply that by the # of diodes, 3(10^(-16)) A, plug that into the 'exponential equation' I have in my notes and solve for 'i'?

    Really not sure whats going on here. Going back to read the text again.

    3. The attempt at a solution

    What I tried is using i=I_S(e^(V_D/V_T)-1)
    using 25 mV for V_T, and 0.8 V for V_D (2.4/3). Got that, 7.9 mA for i, the current required. Yay.

    The next part I don't know what they mean. What would a 1 mA current drawn away do to the output voltage? How do you express that?

    In general I'm finding myself doing plug and chug and kind of flying blind with someone else's equations. A big picture description of what I'm trying to figure out, for starters.
    Last edited: Jan 20, 2014
  2. jcsd
  3. Jan 21, 2014 #2


    User Avatar

    Staff: Mentor

    A load connected at the terminal Vo that "steals" 1 mA will reduce the current through the diodes accordingly, hence changing the diode potential drops. That's just KCL applied at the junction of the current source and diode chain.


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