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Diodes problem

  1. Feb 8, 2014 #1
    1. The problem statement, all variables and given/known data

    Hi so I have a problem that I don't exactly know how to approach, here's the diagram:
    http://images.fr1ckfr4ck.fastmail.fm/probA.jpg [Broken]
    The diodes are supposed to have saturation current I_s = 10^(-15) A

    2. Relevant equations

    What is V?

    3. The attempt at a solution

    First I tried assuming that 2mA must be going through D_1, which means 8mA must be going through D_2, so I used the exponential model to solve for the voltage drop across each diode. But then what? Is that even what I should be trying to figure out here?
    What that got me was that the voltage drop across diode 1 would be ~708 mV and ~742 mV across diode 2. But even if that's right I don't know what it means for the voltage at V.

    If I assumed that both diodes should have a voltage drop of 0.7 volts (which I don't think is right) they would have a current of 1.45mA through them - which doesn't seem to work. So that seemed like a dead end also.

    Any pointers would help, thanks.
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Feb 8, 2014 #2

    gneill

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    Staff: Mentor

    If you have the potentials across the diodes thanks to the known currents flowing through them, then you can fix the potential with respect to ground at the top of D2. Then you're just one diode drop away from the potential at the bottom of D1...

    Can you show how you computed your values of ~708 mV and ~742 mV? What value did you use for nVT?
     
    Last edited by a moderator: May 6, 2017
  4. Feb 8, 2014 #3
    For nVT I'm using 25mV, so to solve for the voltage drops across D1 and D2 I used;

    D2:
    8mA=(10-15A)*e(V2/25mV)

    solving for V2 it came out to 742.8mV
    I used the same equation for D1, V1 except swapping 2mA for 8mA.
    V1 came out to 708.1mV

    It does seem obvious now I could go up from 0V (below D2) to 742.8mV, then drop 708.1mV across D1 to give me V=34.7mV

    I think that might actually be the right answer..
     
  5. Feb 8, 2014 #4

    gneill

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    Staff: Mentor

    Yup, looks good if nVT is taken to be 25 mV. I think that 26 mV is the usual assumed value for room temperature silicon, but the 'n' can vary depending upon the diode's construction. n between 1 and 2 is typical.
     
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