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LMC6492: Wien bridge oscillator doesn't work.

KYUNGSHIN JEONG
Prodigy 10 points
Part Number: LMC6492
Other Parts Discussed in Thread: LM2904, OPA333

HI.

Wien bridge oscillator circuit using LMC6492 is as follows.

However, there are cases where it does not actually work, and there are cases where it does.(Even when LMC6492 was replaced with LM2904 in the non-working sample, it did not work.)

Q1) Simulating with Ti TINA does not work. The sample doesn't work either. I would like to know the cause of it not working.

Q2) There is also a working sample. I want to know why samples are working and not working.

Q3) An output of approximately 20kHz is required. What parts do I need to modify to work?

Q4) An output of 40kHz is required. Can I use the LMC6492 as is? If it is not appropriate, please recommend OPAMP.

All/any help is very appreciated.

  • 0
    TI__Mastermind 21427 points

    Kyungshin,

    I was able to get the circuit to oscillate, by changing the amplifier.  I simulated the circuit that you designed exactly except that I used OPA333 (a different 5V amplifier).  Here is a document covering Sine wave oscillators.  The frequency of the circuit isn't what you were looking for and also the signal slammed rail to rail rather than provided a sine wave output.

    I will look into oscillators some more and see if I can find you a better solution.  Truthfully, I don't work on many oscillator applications, so it may take some time.  Are you open to a two amp solution?  My recollection on the Wein-Bridge Oscillator is that it can have startup issues.  Also, I don't think it is easy to scale the amplitude of the output.  I have an idea for an oscillator that would follow a relaxation oscillator with a band pass filter.  I can also look at the Wein-Bridge more.

    wein-bridge-osc.TSC

    Best regards,

    Art

  • 0 in reply to Art Kay
    TI__Mastermind 21427 points

    Kyungshin,

    1. Below is an oscillator that may work for you.  It is more complex than the wien-bridge, but in discussions with a few of my colleagues these circuits can be problematic from a start up perspective.  The circuit uses a relaxation oscillator to generate a triangle wave, and a band pass filter to convert the triangle to a sinusoidal wave. 
    2. The triangle oscillator is documented in http://www.ti.com/lit/ug/slau508/slau508.pdf.  It is designed for a 20kHz triangle.
    3. The band pass filter is tuned to 20kHz.  The idea is that the band pass filter will filter out all the harmonics in the triangle wave and just leave the fundamental 20kHz.  This circuit can be designed using https://webench.ti.com/filter-design-tool/
    4. Let me know if this works.  

    relaxation oscillator and band pass filter.TSC

    Best regards, Art Kay