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Help in a passive low pass filter

Discussion in 'Electronics Homework Help' started by ra585ed, Dec 15, 2011.

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  1. ra585ed


    Dec 15, 2011
    Hi everyone..
    Good morning/evening..
    I hope you all fine..
    I need your help..

    I have this RC CIRCUIT
    Assume the following:

    I need to find V(out) (magnitude only), in the Phasor domain, in terms of the frequency ω and I need to show that the cutoff frequency of this circuit is 1/RC.

    Can any body help me with that..? I will be Grateful.
  2. Laplace


    Apr 4, 2010
    In order to solve this problem you need to use phasor algebra. Phasor algebra has the advantage that it reduces steady-state sinusoidal circuits to the form of pure resistance circuits. Once the problem has been converted to the frequency domain it is essentially a resistive problem. The only difference is that the real-number algebra used for resistive circuits becomes complex-number algebra for frequency-domain circuits. I will presume you are proficient in doing simple arithmetic (add, subtract, multiply, divide) with complex numbers.

    The circuit is a simple voltage divider in the frequency domain; therefore, structure the solution as a resistor voltage divider but then use impedance instead of resistance. For cutoff frequency I would assume that means the half-power point. Since power is proportional to the square of the voltage, (Vout/Vin)^2=1/2, what is the frequency where Vo/Vi=0.707?
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Note that the formulae in "assume the following" give the cutoff angular frequency omega . If you want the cutoff frequency in terms of cyclic frequency, you'll have to factor in 2*Pi.

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