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Op Amps

  1. Sep 10, 2009 #1
    What's inside an op amp? Is it just an npn transistor used for amplification? So can we explain it in the terms of base and collector current?
    Please elaborate simply (It's a high school project)
  2. jcsd
  3. Sep 10, 2009 #2
    Op amps contain many transistors, some are used for amplification, others for current sources.

    http://members.arstechnica.com/x/so1os/500px-OpAmpTransistorLevel_Colored_Labeled.svg.png [Broken]
    Last edited by a moderator: May 4, 2017
  4. Sep 11, 2009 #3
    Ok, so if I know what npn transistor, its structure, its working and its formulas like current gain=[tex]\beta[/tex]=Vo/Vin are and how it can be used as an amplifier and as a switch, will I be able to understand op amps? In other words, does it have the same basics? Is it a combination of many npn transistor amplifiers (as depicted by the image you posted)?
  5. Sep 11, 2009 #4


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    Instead of trying to understand op-amps as the sum of their parts, youy're probably beetter off learning op-amp theory as a fundamental unit. Think of an op-amp as a black box; you don't need to know what's inside in order to understand it, you need only know how it behaves given specific input.

  6. Sep 11, 2009 #5
    ok, that means its a black box which amplifies, that's it, right?

    @waht how did you drop the white background in the image you posted?
  7. Sep 11, 2009 #6


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    It may not hae been initially clear, but there's a link in my prior post which answers your op-amp question. An op-amp is an amplifier which has the following characteristics:

    EDIT: I've bolded an error I just noticed in the previous link. Everything else looks okay, but I'll provide an alternate link with more accurate information:

    Last edited: Sep 11, 2009
  8. Sep 11, 2009 #7
    Oh K.....I checked out the link you gave and I have a question regarding it::


    Can you tell why the second equation has Rf+Rin while the first one does not?
  9. Sep 11, 2009 #8
    That's because the image has .png extension which is basically a multi layered image, and it does not have a background encoded so it could save memory.

    There are two assumptions when analyzing op-amp circuits:

    1. The inputs have infinite input resistance.
    2. Output has infinite gain.

    and when you apply this you are going to get alot of weird equations.

  10. Sep 11, 2009 #9
    Can you explain the second step of the derivation where i is calculated for both resistors?

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    Last edited: Sep 11, 2009
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