Colpitts oscillator ampop

Discussion in 'General Electronics Chat' started by beirao, Dec 11, 2012.

  1. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Hi,

    I´m designing the colpitts oscillator in attachment. At first stight, the oscillator is stable, but the frequency oscillates over time; the oscillation is notorious in my aplication. Can you help me?

    Thanks!

    (sorry for my bad english)
     
  2. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Hi,

    My previous circuit is wrong. The C10 don´t connect the pin 2 of UA741 connect but the R11 and L1; but the circuit continues instable.
     
  3. bertus

    Administrator

    Apr 5, 2008
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  4. tshuck

    Well-Known Member

    Oct 18, 2012
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    Also, your second stage is an open loop amplifier, you will saturate to +5V and -5V very quickly....
     
  5. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Thanks for your answer. Yes, I use the schematic of this website.

    My inductor is the 5mH. In this case, my goal is vary the frequency by vary the inductor. I replace the capacitor of 10uF by 1uF but is instable too...
     
  6. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Yes, I want read a frequency in a uC so I transform the output of oscillator in rectangular wave...
     
  7. tshuck

    Well-Known Member

    Oct 18, 2012
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    C10 is connecting to the inverting input of stage 1 when it should be connecting to the node R11 and L1 are sharing...
     
  8. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Yes, I correct my first post in my second post.. ;)
     
  9. tshuck

    Well-Known Member

    Oct 18, 2012
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    Ah, yes, you did write that:p.... You should update your schematic when you change it
     
  10. beirao

    Thread Starter New Member

    Feb 22, 2012
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    I can´t modify the schematic in my first post. So, is there
     
  11. tshuck

    Well-Known Member

    Oct 18, 2012
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    It may have to do with your feedback resistor values....

    Try making R11 1kΩ...
     
  12. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Hum.. I will test. But I think that I tested and the instable is the same...
     
  13. beirao

    Thread Starter New Member

    Feb 22, 2012
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  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Tell us why you want to make a Colpitts oscillator.
    Does it have to use an op amp?
     
  15. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Hi,

    I need a solution for vary a frequency by inductance variation. I use a colpitts oscillator because thought that was a stable oscillator. Any sugestion? :)
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This Colpitts oscillator works well in simulation.
     
  17. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Thanks for your answer!

    But your circuit is different of my... what is the better?

    In this momment I have a oscillator to stable in a breadboard but in PCB is instable.. I think that is layout of my PCB is wrong in terms of GND, for example..
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The circuit you posted won't work, partly because the op amp output is connected directly to one of the capacitors in the LC resonant circuit. My circuit works, at least in simulation.
     
  19. beirao

    Thread Starter New Member

    Feb 22, 2012
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    Thanks for your attention. In this momment my circuit too works :) How I saida, the problem is in PCB and is problem os stability; but I will test your circuit too!

    Thanks!
     
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