Sine wave oscillator produces almost square wave.

Discussion in 'General Electronics Chat' started by tracecom, Jul 2, 2012.

  1. tracecom

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    Apr 16, 2010
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    I have not have much luck with breadboards recently. I just built the attached (Forrest Mims) circuit, which is supposed to be a sine wave oscillator. As you can see from the scope trace, it is oscillating, but is not a sine wave. Any help would be very welcome.
     
    Last edited: Jul 3, 2012
  2. tracecom

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    I found an assembly error :eek: and now I have a sine wave, but I may have more questions.
     
  3. t06afre

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    May 11, 2009
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    Even if I have learned you are not girl. I might have something for you. How about this free ebook. http://www.ti.com.cn/cn/lit/an/slod006b/slod006b.pdf
    It has a good section on oscillators. Also hmm the lm741. It was great then it was introduced back around 1970. But modern opamps are much better. The lm741 is outdated and has been so a long time
     
  4. tracecom

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    :) Thanks for the e-book. I will use it; coincidentally, I just ordered a used copy of "Understanding Ic Operational Amplifiers" by Roger Melen, so I will have two points of view.

    And I know that the LM741 is old, but I needed a sine wave generator, and already had the 741. The dual voltage power supply is a little annoying, however.
     
    Last edited: Jul 3, 2012
  5. Brownout

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    Jan 10, 2012
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    Save the circuit with the error for the next time you need a square-wave generator ;)
     
  6. tracecom

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    It looks like the sine wave is about 16 Vpp, but I need a much lower level. A couple of 10 MΩ .25 watt resistors in series across the output drops the level to 1.2 Vpp. Looks like I am ready to start soldering.
     
    Last edited: Jul 3, 2012
  7. MrChips

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    Before you start soldering, don't feel that whenever you see a dual supply circuit that you are forced to do the same. You can adapt the circuit for a single supply op-amp if you alter the bias voltage on the amplifier.

    Take the R1 connection off GROUND and connect it to a bias that is halfway between the +V and GND supply rails. Two 10kΩ resistors to make a voltage divider should do the trick.

    Here is a recent post on a square and triangular wave generator:

    http://forum.allaboutcircuits.com/showthread.php?t=71821
     
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  8. Wendy

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    Creating a Virtual Power Supply Ground

    Problem with a 741 is the extremely high voltages it requires. You could use two 9V batteries for the ±9V (which is the lower end for this chip), or the above to make something equivalent from bench power supply. If you look at the datasheet for a 741 most do not list a low voltage spec.

    Look for low voltage op amps. A LM324 will go down to 3V, but is functionally equivalent to a quad 741. Just ground all the inputs you are not using.

    You could replace R1 and R2 with a 10KΩ pot, and make it a variable gain for the op amp. The problem you are fighting is the gain is way too high.

    High Speed Op Amp Query

    Bill's Index

    The fact is you never need dual voltage power supplies, if you know the techniques to simulate them. The concept of a virtual ground is extremely valid.
     
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  9. t06afre

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    Nah I will slightly disagree here. There are some inherent problems with virtual grounds if you have current demands above a few LEDs. So instead of trying to save some shekel. Build your self a dual regulated power supply. Very easy with LM317 and LM337. In the long run you will be far better of with such a solution.
     
  10. Wendy

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    Actually not. Dual resistors may give you a problem, but if you use an active solution that is capable of several amps you will have no problems. Read the link. :D

    There are virtual grounds, and there are virtual grounds.
     
  11. tracecom

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    Thanks for all the help and input. I needed this oscillator to use as a signal source for an MP3 amplifier that I am working on, so I went ahead and prototyped it with the modifications as shown in the attached pdf. It produces a good clean sine wave of about 1kHz at about 1.2Vpp, which will be fine to stand in for the MP3 output.

    I intend to study op amps some more and will probably attempt to build this circuit again with a more modern part than the LM741 and use a single voltage supply, but for now, this will do. Thanks again.
     
  12. daviddeakin

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    Aug 6, 2009
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    shortbus, dataman19 and tracecom like this.
  13. tracecom

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    Thanks. I saved the files. I suppose it looks a little better than mine.
     
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