How to eliminate harmonics in operational amplifier circuit?

Discussion in 'General Electronics Chat' started by yuesu jin, Sep 20, 2019.

  1. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
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    Hi all,
    I made an amplifier circuit using LM675, the schematic diagram is
    upload_2019-9-20_22-13-0.png
    When the circuit is directly connected to oscilloscope. Signal is heavily distorted by harmonics. The input signal is 5Hz and 2Hz sin wave, Vcc=27V, Vee=0, but the output waveform is distorted like spike. More harmonics component, more distortion. What I want to know is that what is the cause of harmonic distortion in low frequency, and how to remove this distortion? Thanks a lot!!!!!
    upload_2019-9-20_22-13-8.png
    upload_2019-9-20_22-13-16.png
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    If you have no negative supply you need to bias it differently.
    But somehow you show the output going below ground.
    What is the input voltage?
     
  3. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
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    Hello, ronv!
    The input voltage is 200mV
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    What is Vee?
    What is the dc level of the 200 mv signal?
     
  5. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
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    Vee is the negative power input, you can find it on the schematic diagram. DC offset is 0v.
     
  6. ronv

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    Nov 12, 2008
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    If Vee is zero volts (or ground) as I think you said in your post and the input goes below ground (0 volts offset) the op amp can't work on the negative 1/2 cycle of the input.
    So you should make Vee minus 10 volts or so.
     
  7. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
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    I don't know why, but LM675 could accept only positive +Vcc with no -Vee, in the datasheet
    http://www.ti.com/lit/ds/symlink/lm675.pdf
    there are one typical application:
    upload_2019-9-20_23-11-1.png
     
  8. ronv

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  9. ronv

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    What do you want your output to be?
    Would it be OK if it were 4 volts pp at 10 volts?
    Or do you want it to be 4 volts pp around ground?
     
  10. Ylli

    Active Member

    Nov 13, 2015
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    Yes, you need that Vee voltage. In additions, looking at the output waveform, I wonder if your Vcc supply has enough current capability.
     
  11. Audioguru

    Expert

    Dec 20, 2007
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    The amplifier with a single polarity supply needs to be designed to work with a single polarity supply, like this:
     
  12. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
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    Ah, I understand it, Vee is indispensable. Thanks a lot, I will add another battery for -Vee

    But there is another question, why the output has negative part?

    Thanks a lot, I understand how it should be! But how can I invert a positive power to negative, is there any circuit can do that?
     
    Last edited by a moderator: Sep 22, 2019
    ronv likes this.
  13. crutschow

    Expert

    Mar 14, 2008
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    Yes, you could use an inverting switching regulator.
     
  14. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
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    Thanks a lot!!
     
  15. DickCappels

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    Aug 21, 2008
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    An ICL7660 is often a good single-chip solition.
     
  16. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    @DickCappels , the ICL7660 is only capable of supplying 40 mA.
    I think the OP is using a much higher current.

    Bertus
     
  17. ronv

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    Nov 12, 2008
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    I don't see any way for the voltage to go negative in your circuit.
    Is the picture an actual scope trace or some type of simulation.
    If it is scope. Is the scope DC coupled or AC?
     
  18. DickCappels

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    Aug 21, 2008
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    Yes, Thank you Bertus. I see the quiescent current can exceed the ICL7660's rated capabilities. Not your every-day opamp.

    Then there is Audioguru's alternate bias arrangement in post #11.
     
  19. yuesu jin

    Thread Starter New Member

    Aug 22, 2019
    20
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    Hi, all, thank for all your help, the problem has been solved under your instruction! Thanks a lottttt!!!!!
    upload_2019-9-23_22-36-5.png
     
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