Wien Bridge Oscillator Help!

Discussion in 'Homework Help' started by ARealHero, Dec 7, 2014.

  1. ARealHero

    Thread Starter New Member

    Dec 6, 2014
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    Hello all!

    I have to design a wien bridge oscillator using a two stage BJT amplifier. I have done all kinds of research and found that I need to have a gain of roughly 3v/v, and my frequency of oscillation is equal to 1/(2pi(RC). The problem I am running into is properly biasing and then setting the gain of my circuit. I have attached a screenshot of my schematic and simulation output.

    Thank you in advance! schematic.png simulation.png
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The R2 of the timing circuit is now part of the biasing of Q1.
    The bias of Q1 is made with R2 and R5 now.
    Is this intended?

    Bertus
     
  3. ARealHero

    Thread Starter New Member

    Dec 6, 2014
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    It definitely is not intended! I will make a few modifications to my design and will post my new findings.
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. ARealHero

    Thread Starter New Member

    Dec 6, 2014
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    Before I completely scrap this design (I just did some math trying to bias this circuit properly), do you have any suggestions about what could be going wrong here? I got this design from: http://www.daenotes.com/electronics/digital-electronics/rc-feedback-oscillators
    I went ahead and posted the simulation as well. I can tell its doing something, but I have no clue what that something is...

    fail2.png schematic2.png
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Now R4 of the timing circuit and R3 are parallel and influencing the bias of Q1.
    Put a capacitor between point B and the base of Q1 and see what happens.

    Bertus
     
  7. ARealHero

    Thread Starter New Member

    Dec 6, 2014
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    I added the capacitor and it did not change much other than smooth out the initial spike
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,ncy again

    Try raising the timing resistors to about 50 K and lower the capacitors according to match the frequency again.
    The timing circuit may be loading to much now.

    Bertus
     
    ARealHero likes this.
  9. ARealHero

    Thread Starter New Member

    Dec 6, 2014
    8
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    Hello!

    I return with good news and bad news. The good news is it oscillates! The bad is it's oscillating at a frequency that is 10 times faster than I calculated...
    As always I will post screenshots, but I was wondering if the reason it is oscillating so fast is because I am neglecting the input impedance of the BJTs or could it be that I messed up a resistor value and the gain of my circuit isn't what I think it is?


    PS thank you bertus for all your help tonight!

    oscillation!.png
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It shows that the gain is to high as the signal is clipping now.

    Bertus
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,801
    1,105
    Since you are simulating this, it would help others to help you if you use the 'Upload a file' button to upload your .asc file.
     
  12. ARealHero

    Thread Starter New Member

    Dec 6, 2014
    8
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    Here is the schematic file!
     
  13. ARealHero

    Thread Starter New Member

    Dec 6, 2014
    8
    1
    After drawing my hybrid pi model of the two BJTs I am wondering if RB11 and RB12 effect the input impedance. I know with a typical voltage source they don't because of super position but does that change in this case due to the nature of the feedback loop? Is it necessary to include the feedback resistor in my gain analysis?

    Also is it safe to assume that Vout of Q1 is equal to Vin of Q2? wienbridgehybridpi.jpg
     
    Last edited: Dec 7, 2014
  14. ARealHero

    Thread Starter New Member

    Dec 6, 2014
    8
    1
    To everyone out there who is struggling with this circuit or just interested in my progress, I got a good output and a changed the circuit a tiny bit. OutputWaveformAndFFT.png problem2finaldesign.png

    For those who are wondering the gain of the circuit is controlled via R3 and R8. A = 1 + R3/R8 so R3 = 2R8. As you can see from the FFT the center frequency is around 31.5KHz and I was shooting for 30.3 KHz.

    Thanks again to everyone who helped over the past couple of days!!
     
    atferrari likes this.
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