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Wien bridge oscillator distorted output

Discussion in 'General Electronics Chat' started by Jony130, Mar 9, 2012.

  1. wagnerlip

    wagnerlip New Member

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    You don't even need to think much. LM358 is not the best choice, no matter what other people say.

    As the Wien bridge oscillator tends to increase the output voltage as much as possible, and considering that LM358 has a horrible rail voltage limits, it WILL CLIP or hump the sinewave.

    Try a "more advanced and socialized" op-amp, several will behave much better. LT-1413 and OP-07 are some of them.

    The two diodes are largelly used worldwide to provide estabilization of the Wien bridge, they do a pretty good job, they are not the culprit.

    I design electronics for more than 40 years. Recently I designed and built the following circuit (part of a 60Hz inverter), it successfully worked with several op-amps, I decided for the LMC660 due little better performance above the others, but with advantage for low voltage power supply.

    Pay special attention for the distortion caused by wrong simetry (duty cycle) what makes the sinewave "bends" back or forward, caused by wrong power supply simetry. Adjust some resistors to fix that. Also, of course, "ground" should be zero volts, so, the op-amp must be connected to positive and negative voltage, or use a virtual ground as follows:

    As you can see in the following drawing, the LMC660 is great to operate even in 5V single supply. To make it works I divided 5V in the middle to create a virtual ground for the oscillator. The virtual ground is done by R1 (2k7) + trimpot (3k3) and C1 (100nF). Very few op-amps can operate well in this low voltage conditions.

    View attachment 63988

    The bottom sine of the following Tektronix scope picture is the sinewave generated by this circuit. The waveforms are part of the 60Hz inverter using PWM without the use of a microcontroller, what would be much easier... the client requested "no digital". The top is the sine generated after the transformer. The middle is the PWM signal, generated by comparing the bottom sine with a high speed triangle waveform.

    View attachment 63989

    Post addition:
    The top sinewave is the generated by the oscillator.
    The middle PWM is the generated by comparing (LM339) the top sinewave with a triangle wave (>2kHz).
    The bottom sinewave is the PWM after filtering with RCRC, or, a filtered voltage at the secondary of a transformer, driven by two Power MosFET, driven by the PWM.
    Observe a 107 degrees delay from the original sinewave to the PWM filtered.
     
    Last edited: May 7, 2012
  2. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    A little late huh? :)

    The LM358 only clips when the output tries to go to high. Keeping it lower and it works fine.
     
  3. unclejed613

    unclejed613 Well-Known Member

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    it's better to use the NE5532. obviously the LM358 not only has crossover distortion, but a bad tendency to hang on the rails when it clips, and also seems to have a bizarre problem with common mode signals (with oscillators, an op amp's common mode range is very important)... the NE5532 is definitely a better choice for an oscillator. an op amp's feedback controlled characteristics, common mode rejection, distortion, output impedance etc... get worse at higher frequencies because the gain margin is reduced, and there is less effect of feedback to control the amplifier. this also has an effect on any crossover distortion.
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    We talked about the LM358 crossover distortion and how to get rid of it in the other thread. I guess you'd have to find that thread if you want to see what was said.
    Basically the LM358 output stage can be biased so that the crossover distortion is eliminated. One of the points i brought up was that the data sheet did not properly describe how to do this. Once that is fixed, all is well.
     
    Last edited: May 7, 2012
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    But ther lousy old opamp still produces a lot of noise (hiss) and chops frequencies above only 2kHz. Actually its very slow slew rate causes frequencies above 2kHz to become triangle-waves which is severe distortion.
     
  7. unclejed613

    unclejed613 Well-Known Member

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    with a GBW of 0.7Mhz, a very poor choice for audio. a TL072 (GBW of 3Mhz)would be a much better choice, or NE5532 (GBW 5Mhz) (which runs much higher bias currents, and has no problems with crossover notch), but if you are running from batteries, stick with the TL072....
     
    Last edited: May 8, 2012

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