Help In Function Generator

Discussion in 'General Electronics Chat' started by bilalsaeed, Sep 18, 2008.

  1. bilalsaeed

    Thread Starter Member

    Mar 30, 2008
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    HI.
    I m Making a function generator through opamp. I made an oscillator using rc feed back ( Wien Bridge) but the problem that i am getting is either it is clipping the output signal or giving too much noise... what can i do to over come both of these errors !
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Can you post up the schematic? You may be using a design that outputs a square wave. For a sine output, it is conventional to use a non-linear resistor in the feedback, like a lamp filament.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    To beenthere...

    Beatcha!
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Great minds...
     
  6. bilalsaeed

    Thread Starter Member

    Mar 30, 2008
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    I don't have the soft copy of the circuit...
    for the square wave i used a comparator with reference voltage 'o' the all problem is with the sine wave ok i'll try using a non linear resistor.
     
  7. bilalsaeed

    Thread Starter Member

    Mar 30, 2008
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    According to the book 'Floyd' feed back path must have a resistor of half value then the resistor used for inverting leg to make the gain equals 3 for eg if i am using 10k resitor for a feed back path i must have a 20 k resistor connected in series with the fb path and going into inverting leg of 741 op amp
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    A light bulb has an odd characteristic, it's resistance increases as the voltage across it (in this case AC) increases. This means that if the AC value of the waveform exceeds a value the resistance increases also, thereby reducing the gain of the amp, and compensating automatically. It is an absolute necessity of this type of circuit, even a little more gain than is needed will cause clipping, which is what you're seeing.

    It's why HP got his patent, it was a big deal at the time.
     
  9. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I'm sure Floyd etc says that the gain has to be exactly 3. So your resistors have to be fantastically accurate.

    If you make one of the two feedback resistors variable, e.g. by substituting a 47k pot for the 20k resistor you will be able to trim the value to achieve a sine wave oscillation.

    However you will quickly discover that the necessary setting drifts with time so some form of stabilisation is necessary.

    The correct amount of feedback is obtained from the output by potential divider. This can be varied by using a positive (light bulb filament) or negative (thermistor) or other non linear (FET, Diode) resistor in the appropriate leg of the divider.
     
  10. bilalsaeed

    Thread Starter Member

    Mar 30, 2008
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    I tried The Light Bulb Filament Its working !!! Thank u All !
     
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