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Homework Help: Circuits Help (Filters)

  1. Sep 6, 2010 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    *in picture

    3. The attempt at a solution
    Not sure where to start. This is a review but I have not covered it yet or if we did, it was very brief?
  2. jcsd
  3. Sep 6, 2010 #2
    if vi were a dc voltage source, it would have the lowest frequency possible (0 Hz). How do inductors and capacitors behave when DC voltage is applied to them as you look at t -> infinity? What would the voltage across the parallel combination of an inductor and a capacitor have to be if they behave that way?

    As freq -> infinity, inductors and capacitors behave oppositely to how they behave for f = 0. Apply the same reasoning as above except with the new simplifications for when f -> infinity.

    Oh yeah. I guess your first step would be understanding what the different types of filters are:

    if you find that the circuit has:
    nonzero 0Hz and zero inf. Hz response, it is probably a lowpass
    zero 0Hz and nonzero inf. Hz response, it is probably highpass
    zero for both, probably bandpass
    nonzero for both, probably bandstop
    Last edited: Sep 6, 2010
  4. Sep 6, 2010 #3
    Well, it looks like a bandpass filter. It allows frequencies between a certain range, depending on the values of your circuit elements. It becomes more apparent when you find the transfer function.

    If you change those circuit elements into the s-domain, and then write a node equation at [;V_o;], you get:

    [;\frac{V_o - V_i}{R}+\frac{V_o}{Ls};+V_o*Cs = 0;]

    Doing some algebra, you can get the transfer function,

    [;\frac{V_o(s)}{V_i(s)} = \frac{s}{RC*s^2+R*s+\frac{R}{L}};]

    with a little more algebra, we can get it into a useful form:


    For analysis, if you change it into the frequency domain ([;s = j*\omega;]), set some values for our elements and vary the frequency, we can see what will happen. I find it rather easy in MATLAB. There are equations to see where your range will be, but I don't know them off the top of my head. Something about 3dB. This is where you can get into design.

    I guess your best bet is to do a frequency response and graph the frequencies from like 0Hz to 1MHz. You'll see your bandpass

    Also, we can see as we increase R, the output will decrease. So I think it's safe to say that increasing R will increase your damping factor.

    Hopefully this helped.
    Last edited: Sep 6, 2010
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