DC to AC conversion via Bubba oscilator

Discussion in 'General Electronics Chat' started by Stefan Zlatev, Jun 5, 2015.

  1. Stefan Zlatev

    Thread Starter New Member

    Jun 5, 2015
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    Greetings everyone! I built a "Bubba oscilator" circuit, which is supposed to convert DC signal into AC. (Resource: http://www.hscott.net/bubba.pdf ) The uploaded picture is the exact schematic that I built with only 1 modification. We did not have the TLV2474 ( http://www.ti.com/product/tlv2474 ) so I used 2xTLV272 instead ( http://www.ti.com/product/tlv272 ). When I first tested it the osciloscope showed a perfect sine wave output, but after my initial excitement passed I noticed that the sinewave actually has no negative values. It moves between approximately 200mV and 700mV. My question is: Can the different op amp be the reason for that? Or is there something entirely different I am missing?
    PS: I will upload a screenshot of the osciloscope with the exact values about 30 minutes later.
    Thank you in advance!
     
  2. Bordodynov

    Active Member

    May 20, 2015
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    To have your picture drawn by 0.5 V.
    Need to understand this is not like a 0.5V.
    You have to understand how 0.5VCC =0.5*5V=2.5V.
    This is done most easily resistive divider
    (two resistors to 10 kohm).
     
  3. OBW0549

    Well-Known Member

    Mar 2, 2015
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    Why would you expect the sine wave outputs of your circuit to have any negative values? Your circuit operates from only a positive power supply, so the outputs are constrained to between zero volts and +5 volts.
     
  4. Stefan Zlatev

    Thread Starter New Member

    Jun 5, 2015
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    So what will happen if I add a second supply of -5 volts on another op amp?
     
  5. OBW0549

    Well-Known Member

    Mar 2, 2015
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    If you want outputs from this circuit that go both (+) and (-), you need to power all four opamps from positive and negative supplies, such as +/-5V. Also, if you want the outputs to be symmetrical about zero volts (or as close to symmetrical as possible), connect the non-inverting input of the upper left-hand opamp to ground rather than to +0.5V.
     
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  6. Stefan Zlatev

    Thread Starter New Member

    Jun 5, 2015
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    Thank you!!
     
  7. ramancini8

    Member

    Jul 18, 2012
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    I am the original designer of the Bubba Oscillator, and a few things got mixed up when the circuit was copied (without giving me credit). The first stage non-inverting gain is 5.16, so the bias voltage should be 0.484V to obtain a dc output voltage of 2.5V (center of the supply range). A bias voltage of 0.5V should suffice for most cases, but the output dc voltage will be 2.588V. The load seen by the output should be high impedance and carefully protected because it is also connected to a oscillator op amp input. An op amp input is ideal to couple into, and If you want an ac output with a swing around ground, capacitor coupling into something like 10M gives it to you without the need for a second supply.
     
  8. Stefan Zlatev

    Thread Starter New Member

    Jun 5, 2015
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    Would you please explain to me a part of the calculations.
    If w0(omega) =1/RC that is 1/(10^4 * 10^-8) = 10^4 or 10k
    Then how does that fit into the gain equation which is 1/(sqrt(1^2 + (w/w0)^2) ?
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    THAT is interesting. When did you come up with the idea, and why?

    ak
     
  10. crutschow

    Expert

    Mar 14, 2008
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    You can also couple the output from a single-supply circuit through a capacitor to a grounded resistor.
    That will give a signal exactly centered around ground with equal plus and minus peaks.
     
  11. ramancini8

    Member

    Jul 18, 2012
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    I had a dog named Bubba who constantly chased his tail and I was watching him tirelessly chasing his tail when I got the idea for the circuit, thus the name Bubba.

    For Stephan: the circuit oscillates where the feedback equals -180 degrees, and each section give -45 degrees phase shift, so four sections yield an oscillator. Beware, the bandwidth of the op amp much be high enough so the op amp phase shift is small at the oscillation frequency or frequency errors occur.

    The theoretical gain works out to be 4, but to insure operation with normal component tolerances it is boosted up to 5.16. Higher gain insure oscillation but it introduces distortion. Try reading "Op Amps for Everyone" where all this is explained in detail; the book is free on the Texas Instruments web site.
     
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    My beard is the same color as yours. For those around here of fewer years, know this:

    Ron Mancini is one of the few long-term analog design gurus. His technical prowess, insight, creativity, and communication skills have inspired and coached decades of circuit and system designers.

    ak
     
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  13. #12

    Expert

    Nov 30, 2010
    16,261
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    I know. I am one of the herd he taught, but I still can't grow a beard. :(
     
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