Op amp gain question

Discussion in 'General Electronics Chat' started by computerlen, Nov 26, 2011.

  1. computerlen

    Thread Starter New Member

    Nov 26, 2011
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    I have always wondered about the difference, if any, the values of resistors would have on the design of an op amp circuit. What I mean is, let's say that you were to design an op amp circuit for a gain of approximately ten times. You could use a 10k resistor as the input resistor to the op amp and a 100k resistor as its feedback resistor. Is there an impedance difference, noise difference or any other difference if you were to use, for example, a 100k ohm input resistor and a 1 meg resistor as the feedback resistor? Thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    896
    You are talking about an inverting opamp circuit. A non-inverting circuit can have as much gain as you want but it uses 'normal' resistor values.

    A very old 741 opamp was designed 43 years ago. Its input resistance is as low as 300k ohms so it cannot use a high value input resistor in an inverting amplifier circuit.

    Many modern opamps have Fet inputs that have an input resistance that is almost infinite so they can use any high value resistors until stray capacitance and leakage current cause trouble.

    High value resistors have "thermal noise".
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    The frequency response may suffer if very high values of resistor are used, as at higher frequencies the reactances of stray capacitances may fall to values comparable to or lower than the resistors.

    For this reason, lower values of resistance may be required in higher frequency applications.
     
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  4. CraigHB

    Member

    Aug 12, 2011
    127
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    I also find the same kind of thing with pull-downs and pull-ups. Used to be they were often specified as 4.7KΩ, but 47kΩ is a better value with the high impedance inputs you see on modern chips since it results in less power loss. I've even seen pull-downs specified as high as 1Meg. The only thing is if you go too high, then parasitic characteristics come into play and noise or poor response can become an issue.
     
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