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Homework Help: RC Impulse Response

  1. Jan 28, 2016 #1
    Hello. I would need some clarifications about the impulse response of a RC circuit.

    1. The problem statement, all variables and given/known data

    Find the impulse response relative to the vc(t) voltage.


    NOTE: delta(t) is the exact dirac's delta of infinite amplitude, not an approximated peak.

    2. Relevant equations

    δ(t) = d θ(t) / dt

    3. The attempt at a solution

    First of all I have a doubt of theoretical nature. If we know that capacitor's voltage is a continuous function (vc(0-) = vc(0+), and know that vc = 0 for t < 0, how can voltage change? Shouldn't it remain 0 for the impulse duration, considering the circuit completely ideal?

    Anyway, is it correct to consider the impulse source as a step one, and then derivate the function?

    Specifically talking, I found the Thevenin equivalent between a and b, and it comes out:

    Rth = 1,6344 Ω
    Vth = -0.3313 V

    Then I calculate the step response as:

    vcθ(t) = -0.3323 θ(t) (1 - e-t/1.6344)

    From here is it correct to just derivate this function to get the impulse response?

    vcδ(t) = -0.3323/1.6344 e-t/1.6344

    Thank you very much.
  2. jcsd
  3. Jan 28, 2016 #2


    User Avatar

    Staff: Mentor

    That works. Is it possible that you've got the sign wrong? My interpretation of the voltage labeling (arrows) on the schematic would make the node b end of the capacitor the positive reference for the capacitor voltage, and the negative reference for Vx. In other words, Vc = Vb - Va. But the numerical constants look right to me.

    You could also solve the problem using Laplace Transforms. The transform of the dirac impulse is simply "1".

    Oh, your thread title is misleading: There's no "L" in the circuit, so "RL Impulse Response" doesn't match the problem given.
  4. Jan 28, 2016 #3
    Yes I probably mistakened the sign.

    Unluckily the university course does not consider laplace transform as a way to solve circuits.

    I apologise. Basically I had written RL, then I noticed the error and wrote RC, but the log in expired and when I refreshed the page there was RL again, but I couldn't change the title once the thread was opened.

    Thank you very much.
  5. Jan 28, 2016 #4


    User Avatar

    Staff: Mentor

    No worries. It happens.
    Wow. Laplace transforms must be one of the most powerful methods for handling differential equations. They're used all the time in circuit analysis. That's a bit of a handicap for you.
    No problem, I can fix it for you.
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