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Op-Amp Help

Discussion in 'Electronics Homework Help' started by shaneyj, Jun 10, 2017.

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  1. shaneyj

    shaneyj

    24
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    Jun 10, 2017
    Hello All,
    I am working on an assignment for an analog electronics course. The assignment is to take a pre-designed op amp circuit (several of them; non-inverting, inverting, adder, and differential) calculate the gain, then measure the gain of actual circuit and compare the measured results to the calculated results.

    The simulation software I am using is Multisim.

    The circuits are fairly simple, and they all call for the same op-amp- a LM741J.

    I have completed the first 3 parts of the assignment (non-inverting, inverting, and adder), and am seeing a trend that leads me to believe that I am doing something wrong, but the assignment is so simple it seems hard to mistake(not that I'm not capable, or anything).

    In scenarios where the gain is high, my measurements are capping out under where the output voltage should be.

    Here is an example...

    Non-inverting op amp circuit with the previously mentioned op-amp.
    Vin/Freq = 4Vrms/1kHz
    Rf= 5kohm
    Rg=1kohm
    I calculate the gain at 6... using the formula
    Gain=1+ (Rf/Rg)

    Simple so far, right?!

    Well my measured gain is 3.15. This is an anomaly. For this type of circuit, I have a total of 5 different circuits with varying component values and the other measured values equal their respective calculated values. (This one in particular does have the highest gain, the others being 2 or less).

    In my novice state, I think, "maybe this op-amp is limited and its hit its limit..."

    So I move on thinking nothing of it.

    But the same trend continues with inverting type (again 5 different circuits with varying component values) and it gets worse with the adder circuit. (By worse I mean I can't get higher than about 12Vrms when I calculate anywhere from 15-27Vrms.

    Can anyone shed some light and help to educate me, please?

    I hope you followed the scenario ok, and this is not disjointed.

    Feel free to ask for clarification.

    Thanks for your time!!
     
  2. davenn

    davenn Moderator

    13,765
    1,920
    Sep 5, 2009
    can you please post the circuits/exercises you are working with
     
  3. shaneyj

    shaneyj

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    Jun 10, 2017
    This is the non-inverting circuit that I calculated a gain of 6... as you can see the gain is just over 3
    upload_2017-6-9_23-6-42.png

    This is an inverting circuit and I calculated the gain at 5 and it is measuring at just over 3
    upload_2017-6-9_23-8-48.png

    Adder circuit with Vout calculated at 27.5V
    upload_2017-6-9_23-9-54.png

    Adder circuit, Vout calculated is 20V
    upload_2017-6-9_23-10-41.png

    Adder circuit, Vout calculated is 22.5V
    upload_2017-6-9_23-11-16.png

    Adder circuit, Vout calculated is 15V
    upload_2017-6-9_23-12-0.png
     
  4. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Have a look at the Data sheet. See if you can find a parameter called output voltage swing. That should help you.
    Thanks
    Adam
     
  5. duke37

    duke37

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    Jan 9, 2011
    Try 4mV in rather than 4V and see what gain you get.
     
    Arouse1973 likes this.
  6. shaneyj

    shaneyj

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    Jun 10, 2017
    Thank you, Adam. That cleared up the confusion!
     
    Arouse1973 likes this.
  7. shaneyj

    shaneyj

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    Jun 10, 2017
    Thank you Duke, I will do that...
    FYI, those values were the specified inputs for this assignment.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    It also may be worth looking at the input waveform compared to the output waveform.
     
  9. shaneyj

    shaneyj

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    Jun 10, 2017
    What is the comparison of the two waveforms going to tell me?
     
  10. shaneyj

    shaneyj

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    Jun 10, 2017
    Reduced the input voltage, as you suggested. All measured values match their calculated values.
    Much appreciated input! Yours, I mean... not the op amp's :)
     
    Arouse1973 likes this.
  11. shaneyj

    shaneyj

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    Jun 10, 2017
    Ch 1 (yellow) is the input.
    Ch 2 (blue) is the output.

    I don't think this o-scope has a small enough resolution to pick up the incoming wave.
    But what does this comparison show me?
    Assuming the incoming input is a sine wave, this is what I would expect to see.
    Is this for verification purposes? To prove that the circuit is functioning as desired or expected? Am I missing anything?

    upload_2017-6-10_6-42-6.png
     

    Attached Files:

  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
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    Jan 21, 2010
    That was with the 4mV input.

    Try again with the 4V input signal (or whatever was the larger value).

    Also, you should be able to increase the gain on the yellow trace so it appears larger.
     
  13. shaneyj

    shaneyj

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    Jun 10, 2017
    1. Interesting... The incoming signal is distorted at the zero crossing. What is the cause of this?

    Also, in order to calculate the Vout, I used the formula: Vout= -(Rf/R1)*V1+(Rf/R2)*V2+(Rf/R3)*V3...
    For this particular circuit with these component values I calculated a Vout of 27.7 volts- which is over the voltage output swing, as so graciously pointed out by @Arouse1973
    (The values used for V in the calculation were all rms values.)

    2. If you notice the output waveform, it has a pk-pk of 27.7V... Is this just a coincidence?

    3. Why is the output waveform not a smooth sine?

    And thank you for the dialogue... this is thought provoking and great education!

    upload_2017-6-10_8-26-52.png
     
  14. Audioguru

    Audioguru

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    Sep 24, 2016
    You are not supposed to look at the (-) input of the opamp because the signal is almost completely cancelled by the negative feedback when the opamp is not clipping. But your opamp is obviously clipping like crazy with such a high input level.You are supposed to look at the input of the entire circuit instead.
     
  15. Audioguru

    Audioguru

    3,087
    692
    Sep 24, 2016
    You calculated that the output should be 27.5V RMS. But the scope shows peak-to-peak (Pk-Pk) that is 2.282 times less. It is a coincidence that the numbers are almost the same.

    Don't you know about clipping? When the output tries to be 27.5V RMS x 2.828= 77.77V peak-to-peak but the output swing cannot go higher than 27.7V peak-to-peak then the peaks of the waveform are clipped off so it is not a smooth sinewave anymore. When an audio amplifier is turned up too high then its clipping produces severe distortion.
     
  16. shaneyj

    shaneyj

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    Jun 10, 2017
    What do you mean by "the entire circuit"? Channel 1 of the o-scope is connected to the - side of the op amp input, after the voltage source resistors, before it enters the op amp.
    Connecting to the positive side, which is grounded, produces no waveform on the scope.
     
  17. shaneyj

    shaneyj

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    Jun 10, 2017
    I learned about clipping in a week's unit lesson about 3 semesters ago... thank you for the reminder, and the clarification!
     
  18. duke37

    duke37

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    Jan 9, 2011
    In #13 the circuit is hidden but it looks as if it is a virtual earth amplifier. If the output clips, the amplifier cannot work properly. The only part of the waveform which is correct is that at the crossover positions.
     
  19. shaneyj

    shaneyj

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    Jun 10, 2017
    The professor calls it an "op amp adder" circuit...
    upload_2017-6-10_10-1-27.png
     
  20. duke37

    duke37

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    Jan 9, 2011
    As audioguru says in #14 the signal is almost complerely cancelled at the - input. Therefore this can be considered as a virtual earth.
    Current running into this position from whatever source has to be provided by the output of the amp. Thus the output current times the feedback resistor is the output voltage.
     
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