Finding Current and Voltage on Ideal OP-AMP! Calculated vs Simulated. HELP!

Discussion in 'General Electronics Chat' started by mrvaledon, Oct 16, 2016.

  1. mrvaledon

    Thread Starter New Member

    Mar 1, 2016
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    Hey Guys! I calculate the i1 and i2 currents and Vout. They gave me a different value from the simulated one. I simulate them on Falstad Circuit and they said that the Vout is -13.75 and i2 is 2mA.

    See attachment.

    Thanx.

    Juan.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    Is this your homework for school or personal study issue?
     
  3. mrvaledon

    Thread Starter New Member

    Mar 1, 2016
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    A practice excercise for test.
     
  4. DGElder

    Member

    Apr 3, 2016
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    Your solution is wrong, the simulation is a possible solution. How did you simulate this without an input signal?

    Ambiguously, you show 0.5mA at the inverting input without indicating a direction. Is that how the problem was presented?
    What in that schematic and problem statement are your assumptions and what are the givens in this problem?
     
  5. mrvaledon

    Thread Starter New Member

    Mar 1, 2016
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    They give me i1 and i2 directions. i2 go to node Va and i1 is exiting Va.
     
  6. DGElder

    Member

    Apr 3, 2016
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    If that were true, how could the output be negative? Please show the complete problem without your additions. Also a screen capture of your simulation circuit if you want to know why it got -13.75V
     
  7. mrvaledon

    Thread Starter New Member

    Mar 1, 2016
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    Here it is. See attachment
     
  8. DGElder

    Member

    Apr 3, 2016
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    Now that we know the actual problem: the direction of the 0.5mA current means that Va has to be negative and this can only happen if Vo is negative. Which means that current and the current from the ground through the 2.5K resistor combine through the 5K resistor. So I1 and I2 are both negative. The voltage drop across the 5K adds to Va for Vo = -13.75V.

    Your voltage and current directions are reversed at the Va node. Current flows into the positive terminal of a passive device.
     
    Last edited: Oct 17, 2016
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