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Calculate the Vout vs Vin characteristics for a diode circui

  1. Jan 29, 2017 #1
    1. The problem statement, all variables and given/known data
    Calculate the Vout vs Vin characteristics for input voltages of 5V to 10V peak.

    upload_2017-1-29_0-53-5.png

    2. Relevant equations


    3. The attempt at a solution
    Would Vout simply be nothing because the diodes are connected in anti-parallel?
    Putting a volt probe at Vin and Vout gives something like: (Green is Vin, red is Vout)
    Why is Vin not shown as a sine wave?
    upload_2017-1-29_1-2-12.png
     
  2. jcsd
  3. Jan 29, 2017 #2

    gneill

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    Staff: Mentor

    Hi mveep, Welcome to Physics Forums!

    What do you think might be effect of the 5 V sources in series with the diodes? Isolate one of the diodes branches just to see how it behaves:

    upload_2017-1-29_6-11-59.png

    What potential would V1 have to reach for D1 to conduct (be forward biased)?
     
  4. Jan 29, 2017 #3
    Something higher than 5V? If there's no potential difference (if V1 is smaller than 5), the diode is off, correct?
     
  5. Jan 29, 2017 #4

    gneill

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    Remember that the diode is not ideal here (it's a 1N4001) so it will have a forward bias voltage contribution, too. What's the typical forward voltage of a silicon diode?
     
  6. Jan 29, 2017 #5
    0.7V
     
  7. Jan 29, 2017 #6

    gneill

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    Right. So add that to the 5 V you stated in your previous answer. That gives 5.7 V. So you can expect the diode to turn on when the potential V1 reaches 5.7 V.

    Now let's extend our view of the circuit slightly to include the power source and resistor:

    upload_2017-1-29_13-53-40.png

    If the diode forward voltage remains essentially constant so long as the diode is conducting (a pretty good approximation for this situation), what do you expect to happen to V1 for different values of Vin?
     
  8. Jan 29, 2017 #7
    V1 would be 0 if Vin is 5V to 5.6V, but the same as Vin if Vin is higher than 5.7V
     
  9. Jan 29, 2017 #8

    gneill

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    Make that 5.7 V rather than 5.6 V. You said the forward voltage of the diode is 0.7 V.

    How? Which one of the two voltages in the diode branch can change while the diode is conducting? Will the 5 V source change value? Will the 0.7 V diode forward voltage change?
     
  10. Jan 29, 2017 #9
    Ok, so V1 = 0 for Vin less than 5.7V

    The 0.7 forward voltage will change
     
  11. Jan 29, 2017 #10

    gneill

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    No, if the diode is not conducting what's the current through the resistor?
    No. It cannot. The forward voltage of a diode is (almost) constant.
     
  12. Jan 29, 2017 #11
    If the diode isn't conducting, there would be no current through the resistor correct?
     
  13. Jan 29, 2017 #12

    gneill

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    Correct. So what is the potential drop across it?
     
  14. Jan 29, 2017 #13
    If there's no current, there's no potential drop right?
     
  15. Jan 29, 2017 #14

    gneill

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    Correct again :smile:

    So what does that make V1 (so long as the diode is not conducting)?
     
  16. Jan 29, 2017 #15
    So V1 would be Vin?
     
  17. Jan 29, 2017 #16

    gneill

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    Yes! So V1 "follows" Vin until the diode conducts. Then it is held at 5 V + VD no matter how high Vin goes. That is, this branch limits the maximum voltage of V1 to 5.7 V.

    Do a similar analysis for the other diode branch alone.
     
  18. Jan 29, 2017 #17
    The voltage at V2 would always be Vin because the diode is in reverse saturation?
    upload_2017-1-29_12-18-6.png
     
  19. Jan 29, 2017 #18

    gneill

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    I don't think that "reverse saturation" is a valid term. Perhaps you mean cuttoff (not conducting)? That would be true if Vin could only take on positive values. What happens as Vin goes negative?
     
  20. Jan 29, 2017 #19
    reverse bias and reverse saturation were the terms I learned in class

    When Vin goes negative, it will add with the +5 source?
     
  21. Jan 29, 2017 #20

    gneill

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    You can think of it that way if you're doing KVL around the loop. You should be able to find the conditions for Vin where the diode is cut off and when it is conducting, just as for the other branch.
     
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