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Why is there an offset on my oscillator?

Discussion in 'General Electronics Discussion' started by george2525, Nov 11, 2016.

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  1. george2525

    george2525

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    Jan 30, 2015
    Hello,

    Im hoping the experienced of you can help here as im still learning

    I have made an oscillator that goes from about 0.1 to 23 Hz

    it works pretty well but theres an imbalance of voltage swing on the output. like there's a negative DC offset happening but I cant work out why.

    Im using TL084's because thats all I had. They are not rail to rail but I assumed they would at least be the same magnitude in either direction?

    Anyway heres the circuit and the real output. Any pointers would be great. Thankyou!

    LFO.PNG
    lfo sq.jpg


    lfo tri.jpg

    lfo sine.jpg
     
  2. Alec_t

    Alec_t

    2,998
    813
    Jul 7, 2015
    The input signal of a TL084 can't be as close to the negative rail as it can to the positive rail, so there will be asymmetry in the output waveforms if the opamp inputs are driven close to the rails.
     
  3. george2525

    george2525

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    Jan 30, 2015
    Ok so should I be looking for rail to rail input op amps?
     
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    Try increasing R3, or reducing R1, to reduce the amplitude of the waveforms.
     
  5. george2525

    george2525

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    Jan 30, 2015
    Thanks but that only affects the triangle and sine. the square will still be the same
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    You can offset/reduce the opamp output levels by adding resistors a,b,c like this :-
    OutputSwingReduction.JPG
     
  7. george2525

    george2525

    170
    0
    Jan 30, 2015
    Thankyou

    I just tried it and it works well

    im not sure what Ra is for though. if I use it the output is good if I take the output from the op-amp (not from the node as you did)

    Or if i remove Ra it seems good taking the output from where you said.

    what is the purpose of Ra? to reduce magnitude only right? for some reason the offset remains when I use it

    Thanks for the help. very useful idea
     
  8. davenn

    davenn Moderator

    13,894
    1,969
    Sep 5, 2009
    good :)

    I assume you have made some drawing errors on your diagram around U2A VCC = +V not -V
     
  9. Alec_t

    Alec_t

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    813
    Jul 7, 2015
    Glad it works for you. The ratio of Rb:Rc affects the offset. Ra (in conjunction with the output impedance of the opamp) primarily enables the amplitude to be tweaked.
     
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