simple sine wave oscillator.. LMC6482 datasheet..dB

Discussion in 'General Electronics Chat' started by Zeeus, Jul 1, 2019.

  1. Zeeus

    Thread Starter Member

    Apr 17, 2019
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    12
    Hi..Please assist..According to book reading, this is meant to oscillate (with Capacitors of 16nF and 10k then around 1khz)..Fortunately, not oscillating...

    Please why no oscillation?



    And how can we go from calling something 20 db per decade to 6db per octave? (yeah voltage gain of 2 is 6db)

    Thanks Thanks
     
    Last edited: Jul 1, 2019
  2. Ylli

    Active Member

    Nov 13, 2015
    747
    218
    Flip the Vee power source around, or make it -7.8 volts. Then edit the simulation command to "Start external DC supply voltages at 0V".
     
  3. Zeeus

    Thread Starter Member

    Apr 17, 2019
    376
    12
    thanks, didn't notice Vee...did both but no sine wave oscillation..Something has to start the oscillation?

    Also, used this op amp http://www.ti.com/lit/ds/symlink/lmc6482.pdf
    Can use 7.8 and -7.8 for this op amp? there's no ground pin on the IC

    Thanks
    nvm, ground pin not needed
     
    Last edited: Jul 1, 2019
  4. Ylli

    Active Member

    Nov 13, 2015
    747
    218
    Works for me with the AD8031 op-amps. It does take a long time for the output to build to maximum - it is still increasing after 100 seconds.
    6 dB per octave and 20 dB per decade are the same slope.

    I would expect the LMC6482 to work. Are you trying it in LTSpice or on a breadboard?
    If in LTSpice, be sure you have the .model properly defined.
    If doing it on a breadboard, try changing R4 to 10K.
     
  5. Zeeus

    Thread Starter Member

    Apr 17, 2019
    376
    12
    Please how is it the same? " 20db per decade"...how to calculate from this to "6db per octave"?
    made this change on breadboard but nothing.. how it's working for you?? not working on spice
     
  6. crutschow

    Expert

    Mar 14, 2008
    23,095
    6,850
    Try increasing R5 to about 200kΩ to give more loop gain, use uic to skip the initial operating point, and simulate for at least 400ms.

    upload_2019-7-1_22-55-13.png
     
  7. Zeeus

    Thread Starter Member

    Apr 17, 2019
    376
    12
    works, thanks (even without changing R5)...Will try on breadboard in morning...How to calculate the frequency?

    Thanks

    btw, please what does that mean? "skip the solution of the initial operating bias point"
     
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