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Op amp

  1. Oct 31, 2005 #1
    are the input impedances for both inverting and non-inverting amplifiers infinity?
     
  2. jcsd
  3. Nov 1, 2005 #2

    berkeman

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    Staff: Mentor

    Nope. Just look at the equivalent circuit diagram in the datasheet for whatever opamp you are using. It will either have bipolar or FET input transistors. The datasheet will list the input impedances. The Zin will be much higher for a MOSFET input opamp, but the Zin will not be infinite. Also keep in mind that the input capacitance will limit Zin at higher frequencies.
     
  4. Nov 1, 2005 #3
    but i was using a 411 amplifier and not transistors
     
  5. Nov 1, 2005 #4

    berkeman

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    What's the full part number of the opamp? The transistors that I'm referring to are inside the opamp -- that's how an opamp is made. Take a look at the datasheet for your opamp, or the one for a simple LM741, for example. In addition to listing the input characteristics of the opamp, they will usually show you a simplified equivalent circuit. That will show you approximately how they have built the opamp out of transistors, resistors and capacitors on the monolithic opamp IC.

    http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=14&M=LM741CM
     
  6. Nov 1, 2005 #5
    Oh, I see what you're saying. I used an LF411. I guess my question was more of a theoretical one. Would it be infinity theoretically for both types?
     
  7. Nov 1, 2005 #6

    berkeman

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    Well, the LF411 has JFET input transistors, and its DC input resistance is listed as 10^12 Ohms typical. That's pretty high, but not infinite. The datasheet doesn't list input capacitance, unfortunately, but guess it's around 10pF and that gives about -j160kOhms of impedance, definitely not infinite.
     
  8. Nov 1, 2005 #7
    oh thank you so much berkeman! i guess it's not infinite but too large to measure here. thank you!
     
  9. Nov 1, 2005 #8
    It would be best to refer to a text on basic op-amp theory for the explanation, but the input resistances of the inverting and non-inverting configurations differ vastly.
    The effect of feedback on the inverting configuration is to reduce the input resistance; perhaps a few ohms, while the non-inverting configuration increases it beyond the intrinsic input resistance of the op-amp itself.
    An splendid text is Thomas Frederiksens's "Intuitive Operational Amplifiers"
    ISBN 0-07-0211966-4
     
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