Creating a varriable frequency wien bridg oscillator

Discussion in 'Analog & Mixed-Signal Design' started by Arnab Kumar Mondal, Jul 5, 2016.

  1. Arnab Kumar Mondal

    Thread Starter New Member

    May 19, 2016
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    I had been trying to build a variable wien bridge oscillator . The schematic is shown below - I wanted to change the frequency by using a pot . But it starts to distort after 12 Khz. Also when i am reducing both the capacitors to 1 nF I find the waveform to be triangular more than sinusoidal. Also if I want to drive 0.5 w 8 ohm ospeaker should i use an op amp or an lm386 ? Also how do i change the frequency with one control pin?

    IMG_20160705_120430749_800x569.jpg
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Adjusting that particular resistor to change the frequency is not a good idea.

    Your problem is addressed and solved (I think) on pages 10 and 11 of the Ron Mancini / Richard Palmer T.I. Application note at the URL below.

    www.ti.com/lit/an/sloa060/sloa060.pdf
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    To adjust the frequency you should use a dual gang pot, so varying both the 10k resistor values simultaneously.
    An opamp is not a good choice for driving a speaker. The LM386 is a much better choice.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The LM741 is old, virtually indestructible, well known in the industry, and old. Old means slow. The triangle shape is the opamp going into what is called slew-rate limiting. Basically, its output stage cant change voltage fast enough to keep up with the faster signals.

    If you really want to change frequency by adjusting a resistor, you should use a dual-gang pot to change both Wein network resistances at the same time. Tracking between the two pot sections will not be perfect, but it will be way better than keeping one resistor fixed.

    ak
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Arnab Kumar Mondal

    Get a modern op amp - it doesn't even need to be modern, something first sold in the 1980s is good enough, like the TL082 or TL072. The NE5532 or NE5534 or nearly anything except the 741. You were lucky to get 12kHz with that. The TL072 will get to 100kHz if your caps and pots are well matched.
     
  6. Arnab Kumar Mondal

    Thread Starter New Member

    May 19, 2016
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    I had built the oscillator and now I want to drive it using a speaker . So i Had used an emitter follower . BUt somehow the speaker is giving a very reduced voice output. How do i increase the power more WIN_20160706_18_00_43_Pro.jpg
     
  7. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    That kind of circuit might be ok for earphones but a bit feeble for a speaker. You really need something to give a bit more umph to a speaker. The LM386 is a good choice and simple to use, but follow the example in the datasheet closely.
     
  8. Arnab Kumar Mondal

    Thread Starter New Member

    May 19, 2016
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    Should I use the transistor or should I connect it directly from the op amp output
     
  9. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The output capacitor should be 330uF or bigger (much bigger if you have one).

    You should put a 100 ohm pot in series with the 1k pot - in case you accidentally turn the pot to zero resistance. That zero resistance could destroy your transistor.

    You could replace your transistor bias resistors to a linear pot 1k. Then adjust the pot as needed to get the right bias (and loudest output). This ability to adjust input bias (on the transistor) is important on this circuit because the bias point will change as you adjust the 1k output (emitter) pot.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I would also add another op amp amplifier ( non-inverting) to act as a buffer. Right now, the 1k resistor output is loading your wein bridge oscillator. The additional amp would allow you to adjust amplitude and prevent the transistor circuit from loading the oscillator (and possibly change its frequency).
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Almost direct connect. If you use an LM386, I don't think you need the transistor buffer stage:
    Opamp output > input coupling capacitor > volume control > LM386 > output coupling capacitor > speaker.
    The LM386 datasheet has an application circuit with a gain of 20 and minimum parts. Follow that.

    With a 10 K volume control, the input coupling cap should be 2.2 uF or larger.
    With an 8 ohm speaker, the output coupling cap should be 2200 uF.
    These capacitor values are based on a corner frequency of 10 Hz. This should give flat frequency response down to 100 Hz. If a 2200 uF cap is too large or hard to get, you can reduce it to 220 uF and the output amplitude will decrease by about 3 dB at 100 Hz.

    ak
     
    Last edited: Jul 6, 2016
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