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Homework Help: Capacitors (filter signals)

  1. Jul 27, 2009 #1
    Could someone explain to me the meaning of the following concepts:

    1. Filtering out AC: is used to reduce the ripple of a DC power supply.
    2. Filtering out DC: Only time-varying signal will pass through a capacitor. The circuit on either side of the capacitor can be at different DC voltages.
    3. And if one could help explain low and high pass filters, that would help a lot- since I have no prior knowledge in circuits.


    Last edited: Jul 27, 2009
  2. jcsd
  3. Jul 27, 2009 #2
    Hey Jeff,

    The first concept comes into play in devices known as rectifiers. These are used to convert AC to DC and they employ diodes. Diodes are "directional current valves" and only allow the current to flow one way. As you will recall the AC that comes out of the wall is a sine wave and has a series of positive and negative "humps". The rectifier will output a voltage waveform consisting of the positive "humps" of the AC signal. You can see pictures of the output at http://en.wikipedia.org/wiki/Rectifier.

    You see those parts where it goes back down to zero? Well this is where the capacitor comes in. The capacitor will charge on the upwards swing and then begin discharging. The voltage across a capacitor is related to how much charge is on it so with a right sized capacitor and a small enough time you can basically hold the voltage closer to the desired output.

    As for the second concept do you remember Z = 1/(jwc)? DC is at 0 frequency and this makes the impedance almost infinite. This lets us use capacitors as DC blocks. I've commonly used them in this way when I'm designing amplifiers to isolate the stages from one another since you can pass an AC signal (music) through all of the stages while setting the DC conditions in each stage to whatever I want.

    Now for the filters. An ideal low pass filter will pass frequencies below the cut off frequency while completely blocking every part of the signal that is above that frequency. Vice versa for the high pass filter. As an example look at this filter http://en.wikipedia.org/wiki/File:1st_Order_Lowpass_Filter_RC.svg. For DC (0 Hz) the capacitor has infinite impedance so it basically behaves like an open circuit and my input voltage and output voltage are the same. Now say I progressively increase the frequency of my input voltage. The impedance of the capacitor will start to go down until it eventually looks like I have a short circuit. Basically what I have just done is allow lower frequencies through and attenuate higher frequencies.

    I suggest you practice thinking one through by swapping the positions of the capacitor and resistor. You should be able to think it through in a similar way and see that creates a high pass filter. Hope that helps!
  4. Jul 28, 2009 #3
    What are DC blocks? How are DC blocks filtering out the AC? Are there pictures of this also?

    And for the second concept, why wouldn't the current just skip the capacitor that adjoins the top wire with the bottom; similarly for the opposite case, where current would skip the resistor that adjoins the top and bottom wire?
  5. Jul 29, 2009 #4


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    Think of a capacitor as being a short circuit at high frequencies, and an open circuit at DC (this is ONLY an idealization).

    When the capacitor goes between a wire and ground, you can see (with these idealizations) that high frequency signals will short out to ground, DC will be unaffected. So it acts to filter out noise on the line (hence calling it a filter capacitor, it's also often referred to as a bypass, or decoupling capacitor). For more:

    When the capacitor goes in series, you can see that AC frequencies will pass, but DC will be blocked (because it looks like an open circuit). Hence the reason it's called a DC block (it's also referred to as a coupling capacitor because it couples the--usually AC--signal from one part of the circuit to another). For more:
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