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simple op-amp woes

Discussion in 'Electronic Basics' started by Chet Hosey, Feb 25, 2004.

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  1. Chet Hosey

    Chet Hosey Guest

    I'm trying to construct a virtual earth mixer using an op-amp. In
    order to test my understanding of the way op-amps work, I threw
    together a quick test circuit on a breadboard. I'm having trouble
    getting the circuit response I expect, so hopefully somebody here can
    point out the source of my troubles.

    It's been a long while since I've dabbled in things electronic, so
    I'll try to describe my circuit and expectations as briefly yet
    completely as possible.

    I'm using a 120 VAC-to-12.6VAC stepdown transformer through a bridge
    rectifier. I'm using a 100uF electrolytic capacitor to smooth the
    ripples, and using it to feed a TL082 op-amp. I've measured the
    difference between positive and negative supply voltages at about
    18.5V. Using two 220K resistors to obtain ~9.25V, I set the
    non-inverting input to 9.25V and jumpered the non-inverting input to
    the positive supply.

    As the non-inverting input was at a lower voltage than the inverting
    input, I expected the output to swing down to 0V. However, the
    measured difference between the op-amp's output and ground was about
    18.1V. Thinking that I'd been mistaken in my expectations, I reversed
    the jumpers connecting the two inputs and again measured 18.1V between
    the op-amp's output and ground.

    Without a feedback resistor, shouldn't the output be near the negative
    supply voltage when the inverting input is at a lower voltage than the
    non-inverting input? I'm doing something wrong here, but I'm at a loss
    as to what's wrong.

    I've posted a schematic at
    http://www.nauticom.net/www/chosey/opamp.gif. My eventual goal is to
    make an audio mixer as a gift for a friend, so of course I would be
    most appreciative of any input anyone cares to offer.
     
  2. You have confused yourself. You have the noninverting input connected
    to two things. It may be simpler to refer to the inputs as + and -.
    Agreed, if the opamp were ideal.
    So reversing the inputs does not change the output voltage?
    Do you have the data sheet handy to make sure of the pinout?
    http://focus.ti.com/lit/ds/symlink/tl082a.pdf
    Take a look at page 8 of the data sheet for the specification of the
    common mode input voltage range. Many opamps act weird if either
    input voltage is outside this range. This design needs to have both
    inputs a few volts from the negative supply rail for everything to
    work as expected. It is not obvious to me that taking either output
    to the negative rail would lock the output negative, but to make sure
    this is not happening, change your input divider to 3 equal resistors
    in series, and try connecting the inputs to the two 1/3 supply points
    alternately, to see if your unit acts more reasonably.
     
  3. Jamie

    Jamie Guest

    when ever the voltage on the - input over comes the + voltage you get
    negitive effects, the results should be aprox 0.8 volts.
    just think of the + input giving you the expected positive effects.
    when voltage on the + input exceeds the voltage on the - input it
    the output goes high..
    do that the same for the - input and it simple does the oposite..
    unless you have something not correctly wired?
    are you using a single supply or a +/- common ?
     
  4. Skeptic

    Skeptic Guest

    Your description does not match your schematic. According to your
    schematic the non-inverting input (+) is connected to the negative
    supply or 0V. The inverting input (-) is connected to the junction of
    R1 & R2 or 9.25V. Since the non-inverting input is higher than the
    inverting input, the output should be high. This is what you observed.

    If you're looking for a mixer circuit, you might try this one.

    http://www.aaroncake.net/circuits/mixer2.htm
     
  5. Chet Hosey

    Chet Hosey Guest

    Aye, I have. One of those should have been a reference to the
    inverting input. I'll stick to + and -.
    No, it wasn't affecting the output voltage -- either orientation would
    result in the output running very near the positive supply voltage.
    I've verified the pinout both against the data sheet (thanks for the
    link, by the way) and against the packaging in which it came. I had
    much better luck with your suggestion of taking inputs which weren't
    so near the supply voltages -- with the + input at halfway between the
    positive and negative supply voltages I was getting the results I
    expected (ie, a virtual ground amplifier).

    This leads me to what was probably the cause of my misunderstanding --
    I've been treating the negative supply rail as "ground", and jumpered
    the + input straight to the adjacent negative supply pin (pins 3 and 4
    on the TL082).

    It was a big forehead slapper, and reminds me of the time back in 1993
    when I wondered why op-amp schematics always showed +VDC and -VDC
    instead of just calling it +VDC and ground. Unfortunately I'd moved on
    to less costly hobbies before learning the answer.

    I really appreciate your response. I'm no longer thinking that trying
    to build a homebrew audio mixer was an ill-conceived gift idea, and
    it's really nice to rediscover an old interest.
     
  6. Chet Hosey

    Chet Hosey Guest

    I think I discovered the source of my misunderstanding -- I'd jumpered
    the + input to the negative supply rail, when it seems that it should
    be kept halfway between the positive and negative supply voltages (0V,
    not -VDC).

    Thanks for the response!
     
  7. Chet Hosey wrote:
    (snip)
    You are welcome. Opamps are fun and easy to use compared to almost
    any other amplifier mechanism, once you get your mind wrapped around
    their limitations.

    Good luck on your project.
     
  8. Outrider 141

    Outrider 141 Guest

    How many inputs do you want to use, how many outputs (mono or stereo)?
    There are schematics on the web for multi input mixers, Craig Anderton
    has an 8 in 1 out mixer that I use quite often. Cheap and easy to
    build, and can be modified for high or low level inputs. It was from
    the Electronic Projects for Musicians book, and I think PC boards are
    available from PAIA.com - could be wrong on that one, tho.


    Use the usual techniques to reply via email.

    Molon Labe!
     
  9. Chet Hosey

    Chet Hosey Guest

    My description was a tad off, although the confusing part was that
    reversing the inputs had no effect on the output voltage.

    As I discovered last night after reading an earlier response to my
    post, attaching the + input to the negative supply rail was not what
    I'd actually wanted to do. I appreciate your response, and had I seen
    a schematic such as the one to which you'd posted a link when
    prototyping my design I'd have noticed that what I actually wanted to
    do was to split the difference between the positive and negative
    supply voltages and feed it to the + input.

    Thanks for your response! The circuit you pointed me towards is
    actually pretty close to what I'd had in mind, and will serve as a
    handy reference.

    -chet
     
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