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Homework Help: Diodes Current–voltage characteristic

  1. Jan 11, 2010 #1
    1. The problem statement, all variables and given/known data
    http://img39.imageshack.us/img39/7496/59187398.th.jpg [Broken]
    VDON=0,7
    First it is asked to calculate V0 for Vin=-1V and Vin=-5V.
    Secondly it is asked to draw V0(Vin) for -10<Vin<10

    2. Relevant equations

    08d7bd7060be987d4da37b7fc263a740.png

    3. The attempt at a solution

    Using Kirchhoff Volt Law I've established that for Vin=-1V D1 is ON and D2 is OFF and for Vin=-5V D1 is ON and D2 is ON an I believe I got it right.
    I have also determined that D1 conducts for a Vin< -0.7V and that D2 needs Vin<-1.4V. Right now I'm stuck at linking their current–voltage characteristic and determining the slope value, which depends somehow of the resistance values, so that I can build the V0(Vin) graph
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 11, 2010 #2

    Redbelly98

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    I agree.

    Yes.
    Not quite. Think of it this way:
    With D2 right at the "on/off" threshold, it has 0.7V and 0 current. That means the current through the 1kΩ is ____?
    And therefore the current through the 4kΩ is _____?
    So the voltage across the 4 kΩ is ______?
    And Vin would equal _____?

    You can replace each diode with either an open circuit, or a 0.7V source, depending on whether it is off or on, respectively.
     
    Last edited by a moderator: May 4, 2017
  4. Jan 11, 2010 #3
    With D2 right at the "on/off" threshold, it has 0.7V and 0 current. That means the current through the 1kΩ is 0.7mA
    And therefore the current through the 4kΩ is 0.7mA
    So the voltage across the 4 kΩ is 2.8V
    And Vin would equal 2.8V

    Am I right?

    I can do 1 graph for each diode. My problem is when I try to put them together in order to represent the whole circuit nothing that I do makes enough meaning for me to call it an answer to the problem
    If the diodes were in parallel I knew how to solve it or even if they were Zener's. But with them connected in series I'm not making any sense of this.

    From what I can tell I'm getting a positive diode clipper similar to this one http://www.circuitstoday.com/wp-content/uploads/2009/10/Output-Waveform-Positive-Clipper-and-Negative-Clipper-300x123.jpg" [Broken] (graph on the left) but with its max Vout at -0,7V? Is that it? But wouldn't I get the same with D1 only?
     
    Last edited by a moderator: May 4, 2017
  5. Jan 11, 2010 #4

    Redbelly98

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    So far so good. :smile:
    There are two problems with this last step.
    1. Vin is not equal to the voltage across the 4kΩ resistor. Rather, it is the sum of the voltages across the 4kΩ, D2, and D1.
    2. Keep in mind the polarity: which end of D2 has a higher potential at the on/off threshhold, and therefore in what direction is current flowing through the resistors?
     
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