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Homework Help: Electrical Circuit Question - Integrating Amplifier

  1. Apr 13, 2013 #1
    1. The problem statement, all variables and given/known data

    http://img801.imageshack.us/img801/4227/capturebsw.png [Broken]

    2. Relevant equations

    3. The attempt at a solution

    Alright I'm not really sure how to solve this problem. I have redrawn the circuit below without the operational amplifier. In ideal operation amplifiers there's no current going into the positive and negative inputs and the voltages are also the same.

    http://img254.imageshack.us/img254/963/capturenpcq.png [Broken]

    My initial thoughts to solving this problem was to use KCL. I'm however unsure how to do this in this situation. [itex]I_{1}[/itex] is both entering and leaving the upper node at 0 V and [itex]I_{f}[/itex] is entering the node. So does this mean that [itex]I_{f}[/itex] is zero?

    Thanks for any help you can provide.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Apr 13, 2013 #2


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    Staff: Mentor

    Since the op-amp wants to keep the node at the top of I1 (Let's call it Node A) at zero volts, it must do so by dropping Vout to compensate for the voltage that builds up on the capacitor.

    If node A always at zero potential, what's the current through R1 at all times?
    So where does I1 go? What then is an expression for Vc (the potential across the capacitor) with respect to time?
  4. Apr 13, 2013 #3
    So if I understand correctly you mean that Vout becomes zero?
  5. Apr 13, 2013 #4


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    Staff: Mentor

    No, Vout becomes whatever is necessary to nullify the effect of Vc, keeping Node A at zero.

    Vout + Vc = 0V.
  6. Apr 13, 2013 #5
    Thank you for your help. I was able to solve the problem.
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