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Capacitors & Resistors in Amplifiers

Discussion in 'General Electronics Discussion' started by ska214, Mar 5, 2013.

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  1. ska214

    ska214

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    Feb 25, 2013
    What's the benefit of using a capacitor and a resistor in an amplifier circuit to create a gain? Why not use just resistors?
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi there
    welcome to the forums :)

    neither capacitors nor resistors create gain, they are both passive devices as a result they actually introduce losses into a circuit
    you need active devices ... transistors ... to produce gain

    Have a look in the Tutorials dropdown menu up above this thread and start learning about what resistors, capacitors and other components do in a circuit :)

    Dave
     
  3. ska214

    ska214

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    Feb 25, 2013
    Thanks for the quick response!

    Sorry, I was talking about when they're attached to an op amp. Instead of having the capacitor and the resistor in parallel, what if we replaced that with a resistor (of course, with its resistance modified so that we get the desired gain)?
     

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  4. BobK

    BobK

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    Jan 5, 2010
    They make it a filter. The one on the input of your circuit is odd. I would expect the capacitor and resistor to be in series, not parallel. The one in the feedback loop is a low-pass filter. As the impedance of the capacitor gets low (at high frequencies) it limits the gain to 1.

    Bob
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    OK

    well Bob told you what the paralleled use is for :) but...

    you also need to understand components ... hence why I told you to go read through the tutorials
    Resistors and capacitors are very different devices and behave totally differently in a circuit
    So you cannot replace a capacitor with a resistor or visa versa
    that would be like giving me an orange when I asked for an apple ;)

    Dave
     
  6. sirch

    sirch

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    Dec 6, 2012
    If you just want a simple amplifier to get some gain then you can just use resistors with an op amp. As others have said capacitors are added to create filters (e.g. to remove noise) and in series to inputs for decoupling DC.
     
  7. ska214

    ska214

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    Feb 25, 2013
    Thank you so much for your feedback! I'll go look at the tutorials right now.
     
  8. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Just curious, what kind of noise would filters remove from an OP Amp?

    Would it be customary to have a low pass filter remove mains hum and have a high pass filter remove RF interference?

    When I see resistors and capacitors involved in an audio amplification application it screams low pass filter and high pass filter to me.
     
  9. BobK

    BobK

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    Jan 5, 2010
    Filters are not necessarily to remove noise, they shape the response of an electronic circuit. For example, a specific response is needed to play back vinyl records. Resistors and capacitors are used for many purposes other than filters.

    Bob
     
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