Inverting Op Amp Trouble

Discussion in 'Analog & Mixed-Signal Design' started by Jem2200, May 15, 2018.

  1. Jem2200

    Thread Starter New Member

    May 15, 2018
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    So I have been trying to build an analog computer for a circuits project, but it hasn’t been going smoothly. Right now I’ve just been trying to operate a single inverting op amp and I can’t get it to work. I’ve followed my text book and internet sources very closely to construct the amp but it still doesn’t work. The + input is always at 0V, but the - input is always equal to my input voltage and the output voltage. In other words my output is always just my input regardless of the resistors I use. There’s always a gain of 1 and no inversion. I’ve tried many different op amps so I can’t believe they’re all blown. In my picture, the right red power rail is powered with 12V, and the left red power rail is powered to -12V. My input would then be connected to row 11 and the ground. If you’ve got any suggestions it’d be greatly appreciated. Thank you.
     
  2. Sensacell

    Moderator

    Jun 19, 2012
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    Your negative supply voltage is connected backwards, at least in the sim.

    Reverse the polarity.

    The positive one is ok.
     
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Does it work correctly in LTspice?
    Which opamp have you got on the breadboard?
    But it is given a negative voltage so all is well there.
     
  4. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Thank you for your replies. It does work in ltspice and I am using lm741 op amps.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Are you sure you have the plus and minus inputs on the correct op amp pins? Sounds like they are reversed.
     
  6. Jem2200

    Thread Starter New Member

    May 15, 2018
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    I’m pretty certain they’re correct. In the pic the resistors are on pin two which according to the lm741 pinout diagram is correct. I thought I tried switching the two just to see anyways but maybe I didn’t. I’ll try that when I get back from class.
     
  7. AlbertHall

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    Check that the negative wire of the +12V supply and the positive wire of the -12V supply are connected together and to pin 3 of the opamp.
     
  8. Jem2200

    Thread Starter New Member

    May 15, 2018
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    The negative wire of +12V and the positive of the -12V are connected together and to pin 3.
     
  9. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Please clarify the four power input wires. Which voltage on which color? Common ground connections?

    ak
     
  10. Jem2200

    Thread Starter New Member

    May 15, 2018
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    The red rail on the right is +12V, the red rail on the left is -12V and the two blue rails are connected to make the common ground.
     
  11. Jem2200

    Thread Starter New Member

    May 15, 2018
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    I just tried switching the inputs with no luck.
     
  12. danadak

    Active Member

    Mar 10, 2018
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    Should look like this -

    [​IMG]

    Regards, Dana.
     
  13. Jem2200

    Thread Starter New Member

    May 15, 2018
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    I’m sorry I wasn’t clear in my post. The ltspice diagram works the way I want it to. The output is the same as the input but with opposite sign. My trouble is that I can’t get the real life version on my bread board to work.
     
  14. ebp

    Active Member

    Feb 8, 2018
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    I recommend probing each pin of the op amp using a meter or oscilloscope.
    Connect the meter negative or scope ground to the junction of the two power supplies.
    Probe each pin right on the pin - NOT via the breadboard.
     
  15. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Thank you for your reply. I tried probing each pin of the op amp with a multimeter bypassing the breadboard. The Vcc and Vee inputs are correct. The + input is 0V which is correct, but the - input and the output still are both the same as the input.
     
  16. ebp

    Active Member

    Feb 8, 2018
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    I think the only possible conclusion is that the amplifier itself is defective.

    More things to try:.
    • Instead of connecting the non-inverting input directly to "ground", connect it through a resistor - 10k would be OK. Then check the voltage at the input again. It should be within a few millivolts of zero.
    • Connect the output of the amp to ground through a resistor - again 10k would be OK. Check the output voltage. My suspicion is that the amp is not working properly and the reason you see the input voltage at the output is because no current is flowing through the resistors so you are effectively just measuring the input voltage through 20k ohms. Your meter probably makes a load of 10M, so there is a very small current. Connecting the output to ground through a resistor just makes a current path. With three 10k resistors in series the output voltage will be 1/3 of the input voltage if my guess is correct.
     
  17. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Thank you for your response. I’ll try that. I does really seem like the op amps themselves are broken but I’ve tried 13 op amps and they all acts the same way, so I have trouble believing they are the problem. 10 were from the same batch I got online and 3 are from my classroom so it’s not like they all come from the same bad batch. If that were the case the ones from the classroom would behave differently.
     
  18. crutschow

    Expert

    Mar 14, 2008
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    Is the (+) input really connected to ground?
    Probing the pin would indicate 0V even if it were not connected
    With power off, use an ohmmeter to check for zero resistance to ground.
     
  19. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    In the protoboard, one of the 10K resistors has a terminal unconnected. Input?
     
  20. Jem2200

    Thread Starter New Member

    May 15, 2018
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    Ebp, I tried your solutions with no luck. And crutachow, yes there is no resistance to ground. The + input is to ground.
     
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