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For a better sine wave

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by MD MUBDIUL HASAN, Dec 19, 2017.

  1. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    Hello sir,

    This post might be a silly one, but my stupid brain does not find it well.

    I am looking for a signal generator circuit to fed different frequencies in other circuit.
    I have found one, take a look
    sine_wave_genator_circuit.png
    But, the person who posted this did wrong on inverting and non inverting sign.

    Anyway, I simulate this circuit, but noticed that the peak of sinewave is not much sharp rather than it flat.
    Take a look this circuit, I found measurable change while screwing the POT R3.

    sine_wave_simulation.PNG

    Kindly discuss this design again.
    1. Feedback path.
    2. Inverting and non inverting inputs with filter.
    3. Contribution of the potentiometers here.
    4. Reason of using follower op amp.
    5. Further improvement.
     
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Get rid of the '741s
     
  3. AnalogKid

    AnalogKid Well-Known Member

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    The 741's are fine for this circuit. They are an old design, and have a bit more noise and distortion than newer parts, but have nothing to do with the problem you are seeing.

    The problem is that this circuit requires that the oscillator stage gain, set by the two pots marked R6 and R7, must be *exactly* the right value. That value changes constantly as the components warm up, and age over time. If you search for 'Wein Bridge oscillator schematic', you will see many examples. ALL of them have some kind of automatic gain adjustment in the negative feedback loop. Often this is a small light bulb, but sometimes it is a voltage controlled resistor such as a FET.

    ak
     
  4. dave miyares

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    The main problem with your Wein Bridge oscillator is that there is no Automatic Gain Control. Minimum gain required for oscillation is 3.000+Δ. If gain is just slightly more, then the amplitude builds up, and the sine wave becomes a clipped sine...

    Drs Hewlett and Packard solved that problem with their entry product, call a HP200 sine wave oscillator in the 1940s. They used the non-linear resistance properties of an incandescent lamp to stabilize the amplitude of their Wein-Bridge Oscillator.

    wb.png
     
    Last edited: Dec 20, 2017
  6. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    Crude way of adding amplitude stabilization. Note the improved distortion, and the reduced amplitude. Question for the student: Why is the % distortion less in V(+) than it is in V(output)?

    wb1.png
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Don't you know what "clipping" is? It is when an amplifier output tries to go higher and/or lower than possible and the top and/or bottom of the sinewave is clipped off. Also, please learn about the correct timebase setting for your 'scope.
     

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  8. dave miyares

    Dave New Member

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  9. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    Dear Audioguru,
    Thank you to pick the point nicely. "clipping" is here for sure, but wanted to know how the clipped off region is flat here.
    May be simulation tool cant show more sharp than oscilloscope can.
     
  10. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    This circuit can produce maximum 2kHz frequency wave, I need to change those frequency to see how my internal circuit works.
    Could you provide any other design?
     
  11. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    My application is + & - 9V maximum.
     
  12. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    Very nice informative post reply.
    Yes, this reason is acceptable why the sine wave is clipped off.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your circuit cannot go higher than 2kHz because the timing capacitor values are too high then the frequency adjustment pot values must be too low.
    Most opamps cannot drive a load resistance less than 2k ohms but your pots are even lower at 1k ohms.
    Also, your 741 opamp is 49 years old and was never designed for high frequencies. Most modern opamps easily go to 100kHz.
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I notice that your circuit is wrong. The frequency is adjusted with R1 and R3 that should be a dual pot but your R1 is not a pot.
    I do not know why it has R7 as a pot.
    Will you add automatic gain control?
     
  15. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    Dear Sir Audioguru,

    I have no word to say how much you are helpful every-time.
    Thank you so much.

    Its an important point I wanted to share with people, your idea means even I would add a 2k ohms pot, the circuit will not go to 2kHz.

    If its wrong, kindly show a corrected design. R7 seems to be changing the negative feedback gain contribution.
    Lets go through with this idea, if this design belongs to wien bridge

    Wien_Bridge_Oscillator.png

    Yes, if necessary.

    People also suggest me to try with TL07x and TL08x series,
    Do you want me to consider these idea here, http://www.eevblog.com/forum/beginn...ve-without-distorsion/msg1150871/#msg1150871?
    I know you have a great contribution here also.


    N.B: I need this circuit for hearing tone control as we have discussed.
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    No, do not use 2k pots, 2k is the minimum, use 20k or 200k for the pots in series with 2k resistors.
    Your capacitor values are much too high so the frequencies will be too low, and even lower when the pot resistances are higher. So use 10nF or 1nf capacitors (calculate the capacitor value with your pot value for the frequency range you want.

    No, Replace R7 with a piece of wire. R6 adjusts the amount of negative feedback, It is Rf and the light bulb.

    A TL07x opamp is much better than a lousy old 741 but it is also old and does not work if its inputs get to close to its negative supply voltage. Use newer and better opamps like OPA134 or OPAx134.
     
  17. BobW

    BobW Active Member

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    I'll just toss this circuit in, because it uses only one transistor, and operates off of a single 5 volt supply. This will annoy those who believe that an op amp is the answer to all of life's problems.

    PhaseShiftOscillator.png

    It's easily recognizable as a classic phase shift oscillator. It operates in the audio range with the component values as shown (about 250 Hz to 1000 Hz).

    The inverse parallel diodes in series with the 0.0068µF capacitor look very much like a limiter, but are in fact a non linear harmonic filter that do a very nice job of minimizing sine wave distortion.
     
    Last edited: Jan 1, 2018
  18. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    BobW,
    Maybe you do not understand that your oscillator uses highpass RC filters that pass distortion. It should be made with lowpass filters that reduce distortion.
    It is normal for a transistor to produce horrible distortion at high output levels but an opamp has extremely low distortion that is almost unmeasurable.
     
  20. tomizett

    tomizett Active Member

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  21. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    I do believe also with Sir Audioguru that,

    My application is not for 250 Hz to 1000 Hz.
     
  22. MD MUBDIUL HASAN

    MD MUBDIUL HASAN Member

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    Yes I am interested, but for sure this issue will take a lot of physics and mathematics.
    Just I need to put them in circuits.
     

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