Circuit Using OP-AMP

Discussion in 'General Electronics Chat' started by rdevendrakumar1989, Aug 27, 2012.

  1. rdevendrakumar1989

    Thread Starter New Member

    Aug 9, 2012
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    How to choose resistor values in Op-amp circuits.. Am Using OP 07cp.. What are the considerations..
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    I have no words for questions like this.

    Place yourself in our shoes, we have no idea even what the general context of the question is, how could we possibly render a useful answer?
     
  3. takao21203

    Distinguished Member

    Apr 28, 2012
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    A special category "bad posts" seems to be useful. There are posts like this one on daily basis.

    While I don't like the idea to silence people (means to lock thread or to delete it), also the wrongdoing should be pointed out.
     
  4. atferrari

    AAC Fanatic!

    Jan 6, 2004
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  5. Austin Clark

    Member

    Dec 28, 2011
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    The question is fairly broad, but not un-answerable or even necessarily a bad post. Maybe he really does need to have a LOT answered for him. Not all the questions here have to be specific. In fact, I'd prefer people asking questions that aren't specific to a particular circuit, because they learn to figure out problems themselves when they learn the theory. Probably the best way to handle this is to give a few links to good information, and maybe a few hints, pointers, rules of thumb, and things of that nature. Maybe suggest looking at voltage dividers, feedback loops, and voltage drop?
    Then again, if they don't care at all about theory, and want a specific answer without giving enough relevant info, you could end up wasting your time... HOWEVER, people in the future viewing the post might gain from it, so there's still value.
    I dunno, that's just my thought :)
     
  6. atferrari

    AAC Fanatic!

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    Who is "we" here?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Here are three considerations:

    1. The op amp input bias current will cause a voltage offset (drop) across any input resistors so you must pick resistor values where this voltage offset does not cause excessive errors in your circuit.

    2. Higher value resistors will cause high frequency peaking and response problems with the stray circuit capacitance. You want to pick resistor values low enough to keep these anomalies below the circuit frequencies of interest.

    3. Resistor thermal voltage noise goes up with the resistance value. High value resistors can cause excess noise in a low noise circuit, so you must be aware of that if your circuit needs to have low noise.

    Typically to minimize these problems you use resistors with the lowest value consistent with circuit operational requirements, op amp output current capabilities, and power considerations.
     
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  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Also - In the case of inverting amplifiers, the input resistor becomes a load on the source, and the resistance of the source adds to the input resistor when calculating the gain.
     
  9. ramancini8

    Member

    Jul 18, 2012
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    Crutschow and Ron H have done a pretty good job answering this question. I want to add something to their comments; op amp gains are set by resistor ratios, so you can pick a wide range of resistors to start. Suggest you start with a 10K input resistor if your source can drive 10K.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Good point, Ron. I guess Carl and I assumed the OP knew that. That is probably not a good assumption.:)
     
  11. Sensacell

    Well-Known Member

    Jun 19, 2012
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    I see your point, the responses illustrate a more broad and open minded approach to looking at these types of questions. I am always looking for specific answers to specific questions, this colors my view.

    I have learned something from this.
     
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