Oscillators using OP-Amp

Discussion in 'General Electronics Chat' started by gamefreak, Apr 3, 2010.

  1. gamefreak

    Thread Starter New Member

    Apr 3, 2010
    Hello everyone:):)!!!

    I seriously need help with my DIAC(Discrete Integrated Arithmetic Circuits) assignment......If anyone of u would bother to have a look,it would be really appreciable...The assignment is based on buffered RC phase shift oscillators
    the assignment is an attached jpeg file.....

    THANKS in ADVANCE!!!!!!!!!
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    I am perhaps helping to much. But anyway. It is a free ebook from Texas Instrument named "op amps for everyone" (Google is your friend:cool:) It has an excellent section about oscillators.
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  3. kkazem

    Active Member

    Jul 23, 2009
    I don't want to just flat-out give you the answers, but I'll give you hints. First, as far as the frequency, you can use the formula: Fo=1/(2*pi*R*C), and since all 3 phase shift stages have the same value R & C, well, think about it and it's a fairly obvious conclusion. For the R2/R1 thing, I would have figured a value of gain just above 1, like around 1.01 or so would work. Now, don't forget that the left-most stage with the R2 & R1 is a non-inverting stage and therefore has a DC gain of 1+R2/R1, but when I simulated the ckt, it needed an R2/R1 of above 2 (430/187)=2.3, which is a stage gain of 3.30. For the phase shift relation for the 3 stages of phase shift, again, think about it; it takes a shift of 0 degrees or 360 degrees or some multiple of 360 degrees. If you look carefully, you will notice that the 2nd stage is an inverting amp, giving a phase shift of 180 degrees, which leaves another 180 degrees total needed. Now lets see, 180 degrees and 3 stages...try 180/3=?? To make the amplitudes equal, of course, the gain in the first stage could be distributed equally over the 3 non-inverting stages with the same configuration as the left-most stage, but divide the gain by 3 and use that to set the ratio of the Rfb/Rin(-) for all 3 of those stages. One very old, but still common method to this day of stabilizing an oscillator using non-linear feedback is to use a good old incandescent flashlight lamp as part of a resistive divider to set the gain, as in the left-most stage. The R1 would b e replaced with the lamp bulb, and its resistance will increase as the level of the oscillation increases. This may need some data to be taken on the lamp to be used and also require a change in the R2 value. You want the initial gain not to be so high as to saturate the op-amp outputs at the supply level, but it must be high enough to allow oscillation in the first place. Then as it heats-up a little, the output will stabilize at some point below saturation, giving a nice, low-distortion sine-wave at the outputs.
    Good luck and I would highly recommend that you model this circuit in LTSPICE (it's free on the Linear Technology website, including the library files and all documentation--no limitations on the number of nodes--go to www.linear.com and look for the LTSPICEIV link) or PSPICE so that you can understand it better by making changes and seeing what happens to the outputs. It's better than breadboarding and you won't have to buy parts, solder, or go thru the agony of blowing up circuits with mistakes.
    Kamran Kazem
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  4. bertus


    Apr 5, 2008
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