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What's wrong with my circuit simulation?

  1. Dec 12, 2012 #1
    What's wrong with my circuit simulation? No gain is observed at the amplifier output. The intent is for the output gain of the 741 operational amplifier to be equal to R2/R1.

    2kok1x.jpg
     
  2. jcsd
  3. Dec 12, 2012 #2
    You can't just leave pin 3 hanging. You have a wire wrong there, and when you look at what pin3 should do in your test circuit, you should see which one it is.
     
  4. Dec 12, 2012 #3

    AlephZero

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    Science Advisor
    Homework Helper

    As well as the wrong inputs, the output (pin 6) is corrected direct to ground, so what would you expect its voltage to be except 0V?

    The oscilloscope is connected straight across the AC voltage generator, so that is what it will display.

    A more subtle problem is that your 15V power supply isn't related to your "ground" voltage. The simplest way to fix that is use two 7.5V batteries between ground and pin 4, and ground and pin 7. if you don't do that, the op amp will probably "lock up" with the output stuck at one of the supply voltages, even when you fix the other problems.
     
  5. Dec 12, 2012 #4
    What am i doing wrong? Still no observed gain.

    1t4tp2.jpg

    1jtbg9.jpg
     
  6. Dec 12, 2012 #5
    You still have your output connected directly to ground... not sure what you expect that to do except act as an expensive but inefficient heater.
     
  7. Dec 12, 2012 #6
    5vnpra.jpg

    Ok, a gain is observed now. Output amplitude is 8.032V but a gain equal to R2/R1 is intent, what am I doing wrong?
     
  8. Dec 12, 2012 #7
    That looks better, you still have the scope lead hooked up wrong. You want to measure output voltage vs. ground (I imagine), not output voltage vs. oscillating input voltage.
     
  9. Dec 12, 2012 #8

    berkeman

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    Staff: Mentor

    Your output and input are both connected to Channel A input. Break the output connection and reconnect it to the Channel B input. Then you can compare Channels A & B to see the gain...
     
  10. Dec 13, 2012 #9
    juy793.jpg

    Like this? Output is square wave and amplitude is 6.618V, gain is still not equal to R2/R1.
     
  11. Dec 13, 2012 #10

    vk6kro

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    Science Advisor

    You are just overdriving the circuit.

    1 volt RMS sinewave is 2.8 volts peak to peak.

    So to have an undistorted gain of 10, the output would be 28 volts peak to peak.

    But the supply is only 15 volts total, so the opamp can't possibly produce 28 volts out.

    Try reducing the input to 0.1 volts RMS. This is 0.28 volts peak to peak, so a gain of 10 would mean 2.8 volts peak to peak.
     
  12. Dec 13, 2012 #11
    Thanks everyone, the circuit finally works.
     
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