Understanding Op Amps Ch8

Discussion in 'General Electronics Chat' started by quique123, Jul 5, 2016.

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  1. quique123

    Thread Starter Member

    May 15, 2015
    Ok just to provide some background. I got into Arduino about a year ago. I love to research what I do, so I've taken a couple of informal incomplete electronics courses covering Ohms Law, Kirchoff's Law, a little on Node and Mesh analysis. But Im a very visual learner so Im slow to understand abstract concepts and especially such confusing ones as electricity and electronics where for example current flows in a conventional sense and in a real sense.

    That said, I've been battered in the Arduino forums for not understanding opamps and even though I've read a few articles, seen tens of videos and read Ch8 up to Divided Feedback, Im still having a few comprehension issues. Although I must say that AACs Ch8 was the best so far at clearing up a few things. I think it didn't help that I got into opamps because I found a video of how to create an amplifier on youtube () using a pc mic and some headphones. In any event, I tried reproducing the project and failed miserably. I also didnt have an LM324 so I used a TL071 that I found in a charge controller. At first I thought there was something wrong with my wiring or that the tl071 wasnt properly suited as a replacement. As I asked around I was told that tl071 was not able to produce enough current to be able to listen to the mic sounds on headphones, that I needed more current and I should use a power amp.

    Anyway, after more studying of opamps, I ran into Ch8 and I've been trying to understand the "flow of current" in the Divided Feedback page. As the text states, in this circuit:
    If Vin at the non+inverting input is 6V, then the opamp with neg feedback will make the -inverting input = 6V as well. This gives 6V at the voltage divider. How does that make Vo = 12V? And why are the arrows pointing from left to right, I thought conventionally we wrote current as flowing from + to - terminals.

  2. #12


    Nov 30, 2010
    About your circuit. When +6 volts is inflicted on the non-inverting input, the output goes positive (in voltage). How far? Until the inverting input is 6 volts. How much is that? Given a pair of 1k resistors as a voltage divider, it must be 12 vols.

    As for current flow, there are 6 kinds of people:
    1) Those that don't think at all
    2) Those that think electrons flow
    3) Those that think current flows from the positive to the negative terminal
    4) Those that can think in either direction
    5) Those that argue about it
    6) Those that just shut up and get the job done.

    I am type 4 and 6.
    Sinus23, absf and OBW0549 like this.
  3. crutschow


    Mar 14, 2008
    That diagram shows electron flow. I believe all of ACC's tutorials use electron flow (I don't).
    You are correct that conventional current flow is in the opposite direction.
    As #12 noted, if you need +6V at the voltage divider then how much output voltage do you need at the output to generate 6V at that divider, given that the other end of the divider is a 0V.
  4. JohnInTX


    Jun 26, 2012
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