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Why resistor between +input and ground necessary in OP-AMP?

  1. Aug 26, 2015 #1

    goodphy

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    Gold Member

    Hello.

    Please see the attached image first.

    1. The textbook seems say resistor to + input in this image is necessary to provide a return path to ground. I completely don't understand even what it mean. Could you help me to clarify this?

    2. Instead of understanding directly what the book said as above, I interpreted combination of capacitor and resistor on + input as high pass filter to block the DC. In high pass filter, what is the role that resistor take? Mathematical formula for high pass filter operation requires finite value of resistor to have desired cut-off frequency, but I would like to know physically the reason of presence of resistor.

    Thanks!
     

    Attached Files:

  2. jcsd
  3. Aug 26, 2015 #2

    berkeman

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    Staff: Mentor

    Opamp inputs are not self-biasing. They need to be biased with external connections.

    The input capacitor is used to remove the DC component from the input signal, and as you say, the RC combination does form a high-pass filter.

    Have you seen the equivalent circuits of the inside of typical opamps? You can look at datasheets for LM741, LM324, TL084, etc. to start to get a feel for why external biasing of the inputs is necessary. :smile:
     
  4. Aug 26, 2015 #3

    donpacino

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    to elaborate on what berkman said....

    in DC conditions capacitors can be represented as an open circuit. This means that without the resistor, the positive node will be floating, which is NOT good. so at low frequencies, the resistor provides a bias point!
     
  5. Aug 26, 2015 #4
    Ad1
    Every op amp has a differential amplifier at his input. And every transistor need a "base" current to flow to work as a amplifer.
    And this is why op amp need a path for his bias current.
    0.9.PNG


    Ad2
    If we talking about the ideal the high-pass filter the capacitor together with series resistor form a voltage divider.
    0.7.PNG

    So without this resistor (R1) the one plate of a capacitor is connect to input AC voltage, and the second plate of a capacitor is left floating.
    So we do not have voltage divider anymore, and sometimes we add this "extra" resistor (R1) simply becaues R_load has a very high resistance (almost ideal case) and we need this resistor to determine the filter cut-off frequency.
     
    Last edited: Aug 26, 2015
  6. Aug 26, 2015 #5

    goodphy

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    Hello.

    Thanks for making reply detailed.

    Thus OP-AMP requires tiny DC current as a basing on the inputs, but without input resistor in connection to ground, this current is blocked thus OP-AMP can't work. Is my summary right?
     
  7. Aug 26, 2015 #6

    berkeman

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    Yes, that is correct. There are a number of other tips for working with opamp circuits. Are you studying them in school? Or for hobby use? :smile:
     
  8. Aug 26, 2015 #7

    goodphy

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    Hello.

    Thanks for giving me kind comments and I would like to ask one question regards of your reply.

    I'd not understand why bottom side of the capacitor is floated without R1 since that side is still in connection to other such as the load. But I immediately got a flash that OP-AMP inputs has very very high impedance thus in ideal case capacitor is floated.

    Could you tell me my reasoning is correct?
     
  9. Aug 26, 2015 #8

    berkeman

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    As I said in my reply, each opamp input needs a DC bias connection. Did you follow my advice and look at the internal equivalent circuits shown in the datasheets for typical opamps? :smile:
     
  10. Aug 26, 2015 #9

    goodphy

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    Thanks.

    I'm actually not electrician but my physical experiment have forced me to study electronics thus I'm now in some sort of tougher time:
     
  11. Aug 26, 2015 #10

    berkeman

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    Staff: Mentor

    There are lots of resources on the Internet to help you learn about opamps and opamp circuits. What have you been reading so far?
     
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