Voltage Follower/Buffer Op Amps

Discussion in 'General Electronics Chat' started by theavi, Jan 6, 2010.

  1. theavi

    Thread Starter New Member

    Dec 19, 2009
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    What good are they? Not in the respect of why were they ever created but more along the lines of "how are they used". I tried searching for example circuits in Google but I'm not coming up with much outside of how they work, etc. After reading the AAC section on negative feedback and several off site articles on Voltage Followers/Buffers I'm under the impression they can be used to boost the current of an input signal to drive a load or something to that effect. Typically in cases such as these, seeing a few circuits diagrams that utilize the component helps clear up what they are good for. Anybody got a few examples or maybe a go to website where finding example circuits is a heck of alot easier? Thanks!
     
  2. thyristor

    Active Member

    Dec 27, 2009
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    In the circuit diagram below, a voltage follower (IC1c) is used to buffer the two 10K resistors that form the zero ground reference. ie: the output of the buffer amp is the mid-rail voltage thus producing effectively a +V and -V supply with zero being the buffer amp output.

    One could produce the mid-rail voltage by simply using the two resistors alone of course. The issue then would be that they would have to be much lower values so that they could supply any current drawn. The buffer amp is already there in the LM324 package (it contains 4 such op-amps) so there is no additional cost in using it in this circuit, and its output is a low impedance source, unaffected by the current drawn in this circuit.

    [​IMG]
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Sheesh, thyristor! Is that the worst example you could find?:D
    The load on the voltage follower is a few nanoamps. It is free, as you said, but since it is unnecessary, it is a poor example. Sorry to get on your case.:(
    I went through a bunch of my old schematics and found this one, which has 3 voltage followers that are put to good use, buffering some voltage dividers. RHI and RLO are the ends of a resistor divider string inside the 3914.
     
  4. theavi

    Thread Starter New Member

    Dec 19, 2009
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    Very nice. I appreciate both the examples. I'll save the first one for later on as I continue to read up on Op Amps. The second looks pretty interesting, I may actually experiment with that circuit a little bit.

    It seems like voltage followers/buffers aren't used too often though. I gave it a shot looking for some more circuits that incorporated them and couldn't find any. Ah well. Thanks anyway!
     
    Last edited: Jan 6, 2010
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    They are actually very common.
     
  6. JimG

    Member

    Dec 7, 2009
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    I used buffers recently in two instances:

    • The output current from a commercial thermocouple-reading instrument was so low that it took forever to charge the capacitor in a differentiator circuit I built (I needed to measure the rate of temperature rise). So I buffered the input from the instrument using an op amp and boosted the current available to charge the cap.
    • I needed a very gradual, linearly rising voltage signal to use as a reference for calibration of an instrument. I used a typical astable 555 sawtooth circuit. But to get the very slow rise I needed, the timer resistor was > 3M, and the current into the cap was only around a tenth of a microamp. Without an output buffer, introducing even a high impedance Fluke DMM to monitor the capacitor voltage became part of the circuit and changed the behavior.
    Jim
     
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