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Homework Help: Help with voltage threshold calculations

  1. Jun 8, 2016 #1
    • Thread moved from the technical forums, so no HH Template is shown.
    Please can someone tell me if this calculation is correct? Please see attached question.
    I have used the +/-3V as the saturation voltage and multiplied it by R2/R2+R3 to give me +/-1.5V.

    Attached Files:

  2. jcsd
  3. Jun 8, 2016 #2
    Is this a Homework ?
    For the circuit in FIGURE 3(b) your answer is wrong.


    Notice that you have additional +3V voltage source (V4) in your circuit. And this voltage source will have big influence on threshold voltage.
  4. Jun 8, 2016 #3
    Sorry yes, I think I may have posted it in the wrong place. First time using this site. I will have a rethink and post it back in the correct section.
    Thank you very much for your help.
  5. Jun 8, 2016 #4
    So now, can you show us the "right" answer? And my tip is: Find the voltage at non inverting input for two cases: one for Vout = +3V and Vout = -3V. And do not forget about V4.
  6. Jun 8, 2016 #5
    Struggling to find the formula I should be using. I think this is a Schmitt Trigger and all calculations I have seen include a third feedback resistor which isn't here.
  7. Jun 8, 2016 #6
    OK. But are you able to find the Vx voltage for this circuit?
  8. Jun 8, 2016 #7
    Would it be:
    Vx = R1/R1+R2 x Vout = 0.5 x 3 = 1.5V?
  9. Jun 8, 2016 #8
    No, wrong again? Why do you keep ignoring the V4 source?
  10. Jun 8, 2016 #9
    OK using basic ohms law:
    I = 3/1000 = 3mA
    So voltage dropped across the 1K resistor would be 9mV making Vx =2.991 V?
  11. Jun 8, 2016 #10
    OK, let us start again from the beginning.
    The circuit look like this:

    And notice that now V4 = 0V and R1 and R2 are connected in series.
    The voltage at Va is equal to Vout = 3V, the Vb voltage is 0V because V4 = 0V.
    So the voltage across (R1+R2) is Va - Vb = 3V - 0V = 3V and the current is I = 3V/(R1+R2) = 3V/2kΩ = 1.5mA therefore Vx = Vout - I*R1 = 3V - 1.5V = 1.5V
    But in your circuit V4 is not 0V it is 3V so the situation looks like this (for Vout = +3V)

    And when Vout = -3V

    Can you solve it for Vx ?

    And I highly recommend you to read this by starting at page 3

    Attached Files:

    • 1.png
      File size:
      1.7 KB
  12. Jun 8, 2016 #11
    OK thank you very much.
    So here goes:
    Vx when Vout is -3V:
    Vb =V4 = 3V
    Voltage across R1+R2 = Va-Vb = -3-3 = -6V
    Current I = -6/2000 = -3mA
    Vx = Vout - (I x R1) = -3 - -3 = 0V

    Does that mean the upper threshold voltage for this op amp is +3V and the lower threshold voltage is 0V?
  13. Jun 8, 2016 #12
    Yes exactly, this additional source (V4) will "shift" the threshold voltage up or down depend on V4 sign.
  14. Jun 9, 2016 #13
    Brilliant, thank you very much for all your help!
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