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Operational amplifiers

  1. Mar 7, 2016 #1
    Ah, the op amp... so easy to learn. Yet i have a question:
    Why the op amp does not input any current? Is it made by some sort of CMOS technology?
     
    Last edited by a moderator: Mar 7, 2016
  2. jcsd
  3. Mar 7, 2016 #2

    BvU

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    The ease of learning in my past (long ago) was in remembering that the input voltage has to be zero because the amplification should be considered nearly infinite...
    And that was "all I needed to remember". (so I did :smile:)
     
  4. Mar 7, 2016 #3

    jim hardy

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    Find an opamp datasheet
    look for parameter "Input Bias Current"
    you'll find it small but nonzero.
    The circuit designer must be aware of its magnitude when he chooses resistor values for the circuit with which he intends to surround his opamp. He must not use resistor values large enough that "input bias current" would cause more than a negligible voltage drop .

    http://www.st.com/web/en/resource/technical/document/datasheet/CD00000490.pdf
    upload_2016-3-7_5-6-35.png
     
  5. Mar 7, 2016 #4
    Op amps existed before solid state electronics back in the paleolithic age of vacuum tubes.
     
  6. Mar 7, 2016 #5
    Thanks all for the feedback :-D
     
  7. Mar 7, 2016 #6

    tech99

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    Vacuum tubes are used in satellites in the form of travelling wave tubes.
    Regarding the input resistance, the building block circuit used in an op-amp is the emitter follower, so it gives a high input resistance due to negative feedback. Emitter follower/source follower/cathode follower have the same action.
     
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