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Homework Help: Alternating current : Solving for low-pass filter

  1. Feb 18, 2008 #1
    1. The problem statement, all variables and given/known data

    Q] The figure below shows a typical circuit for a low-pass filter. An AC input [itex]V_i~=~10~mV[/itex] is applied at the left end and the output [itex]V_o[/itex] is received at the right end. Find the output voltage as a function of [itex]\nu~(\textrm{frequency})[/itex]

    http://img252.imageshack.us/img252/3724/lowpassfilterrb2.jpg [Broken]

    2. Relevant equations

    well.. maybe:

    Z = \sqrt{R^2 + (X^2_C - X^2_L)}

    i = i_o sin(\omega t + \phi)

    \phi = tan^{-1}\left(\frac{X_C - X_L}{R}\right)

    3. The attempt at a solution

    Well.. i'm just thinking that the voltage across the capacitor will be the output voltage. I found out the current in the circuit using the equations and it is coming to be:

    i = i_o sin\left(\omega t + tan^{-1}\left(\frac{10^6}{2\pi \nu}\right)\right)

    This is the current in the first loop. I thought of using Kirchoff's law first.. but i have no idea how to do this in this case as the current will split at the resistor-capacitor junction.

    Any help is appreciated. Thanks.
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Feb 18, 2008 #2
    I think you can assume that the output will be connected to a very high resistance (such as an oscilloscope), so there's no current going into the output.

    You know how to express the impedance of R, C and the input voltage as complex numbers? then the circuit is just a voltage divider.
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