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Rectifier Help

Discussion in 'General Electronics Discussion' started by anthonyadams, Feb 22, 2014.

  1. anthonyadams

    anthonyadams

    5
    1
    Feb 22, 2014
    Hello,

    I'm building a "precision rectifier" using an op amp and some diodes. The schematic is attached. On a breadboard, this circuit behaves as expected, producing a clean, rectified sine wave. When I transfer the circuit to a protoboard, I get the signal shown in the attached scope capture. One side of the sine looks clean and the other is noisy. I've done this twice now, with the same results! Any ideas?

    One possible culprit... I'm driving the input with an iPhone signal generator app, because I don't have a proper function generator. Maybe it's not up to the task? It seemed to work fine when the circuit was on the breadboard. I've rung the circuit out multiple times with a multimeter and I'm still not sure what's going on here.
     

    Attached Files:

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Hi Anthony and welcome to Electronics Point :)

    Can you upload a photo of each side of your circuit board?

    How is the op-amp IC powered?
    Have you used decoupling capacitors?
    How is the ground rail obtained?
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
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    Jan 21, 2010
    To add to Kris's response:

    Can you upload photos of your breadboard and protoboard versions. If nothing else, we can eliminate wiring errors.

    I think I'd discount that as a possibility.
     
  4. anthonyadams

    anthonyadams

    5
    1
    Feb 22, 2014
    Thanks for the reply, guys.

    Attached are photos of my protoboard. I don't have the breadboard version built up anymore, since I transferred the parts to the protoboard. This is the second one I've built, with the same results.

    The op amp is powered through two 9V batteries.

    I haven't added any capacitors. The circuit is built exactly like the schematic in my original post (putting aside any wiring errors of course!)

    I'm using the battery ground. The batteries are wired like this:

    http://forum.allaboutcircuits.com/picture.php?albumid=159&pictureid=1967
     

    Attached Files:

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    I would start by shortening all of those long insulated wires. Personally I would try to use copper wires (component leads bent along under the board, and offcuts) for as many connections as possible. That might involve changing the component positioning slightly so that the whole board resembles a single-sided PCB layout, using wires instead of copper tracks.

    Also you should add some decoupling capacitors: a 0.1 µF 50V disc ceramic or multi-layer ceramic (MLCC) from pin 4 to 0V, and ditto from pin 11 to 0V. Keep them both as close as possible to the IC. I like to put decouplers underneath the board.

    I agree with Steve that the iPhone is not likely to be a problem. You may want to put a 33 ohm resistor across the signal, because the iPhone is designed to drive headphones. You can also check the signal at that point with a scope, to confirm that it's clean.

    Finally, you should tie all the inputs of the unused op-amps to something; it doesn't really matter what. You can connect them to +9V, -9V, or 0V, in an mixture you like. Or you can tie the non-inverting inputs ("+") to 0V and tie the inverting inputs to the respective outputs.
     
  6. duke37

    duke37

    5,211
    718
    Jan 9, 2011
    For an op amp to work properly, you need feedback from output to - input.
    The diode will isolate the feedback on half the cycle allowing the op amp to go mad.

    It seems you are going to rectify the output. Why not make a standard precision rectifier?

    Edit I think I am wrong, it does happen ! Perhaps a diode is in the wrong way round.
     
    Last edited: Feb 23, 2014
  7. anthonyadams

    anthonyadams

    5
    1
    Feb 22, 2014
    Thanks again guys. I'll add some decoupling caps and shorten up the wires a bit. My second attempt actually looks better on the scope than my first, so I hope I'm getting closer to a stable circuit!
     
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