OP Amp LC ocillator

Discussion in 'General Electronics Chat' started by Dj Marik, Jan 5, 2014.

  1. Dj Marik

    Thread Starter New Member

    Jan 5, 2014
    3
    0
    Greetings.

    I am new here, and so i hope i am in the right place. I have a question to witch i need an answer, can anyone give me some details or direct me to a source of detailed info about this oscillator?:
    http://forum.allaboutcircuits.com/album.php?albumid=145&pictureid=1023

    As much as i tried, i could not seem to be able to find any info on it, just a pic with a schematic without any component values. I need to understand it better, to know how much positive feedback it needs to work well, does the OP amp need symmetrical supply and if so, what is the best choice... and so on.

    Thank you for your time and all the best.
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. LvW

    Active Member

    Jun 13, 2013
    674
    100
    The condition of oscillation requires the LOOP GAIN to be unity (real, with phase shift of zero deg).
    For an opamp as active unit this is identical to the requirement that there is one single frequency for which the positive feedback dominates slightly over the negative feedback.
    This can be realized, for example, using the shown LC-parallel circuit.
    However, as a drawback of this simple circuitry, there is a no "clean" sinusoidal output signal but a signal with a clipped amplitude. The degree of clipping depends on the resistor ratio - but a "smooth" clipping is possible using, for example, two antiparallel diodes across the feedback resistor R2 in the negative path.
     
  4. Dj Marik

    Thread Starter New Member

    Jan 5, 2014
    3
    0
    Again, thank you for your time, it is greatly appreciated.

    I do know that for a stable oscillation to take place, unity gain is required, less than means oscillation will dampen out, more than means saturation. And also positive feedback is required to sustain the oscillation, in the schematic from the link i posted, the LC tank circuit provides a resonant frequency witch the Op amplifies, R3 provides the positive feedback, and R2/R1 the unity gain. Now i am not sure how much positive feedback would be best, and so, what value should R3 have, how it is calculated. Another thing is the supply, does it need to have a negative rail, and if so, is it ok to be the same as the positive rail? I am asking cus i have tested it in multisim and found that not any value on the negative rail starts the oscillation, or maybe i am going wrong with it, i am not sure, and that is why i ask you for help.

    PS: Please excuse my bad english, i am not a native english speaker.
     
  5. LvW

    Active Member

    Jun 13, 2013
    674
    100
    Hi Dj Marik,
    In case you are using (or assuming) IDEAL properties for L and C (no losses), you know that the impedance at f=fo is infinite. Thus, you have full positive feedback - independent on R3. Thus, its value has no influence on the oscillation condition - it determines the bandwidth of the resonant circuit only.
    Therefore, the negative feedback loop must provide only slightly less feedback than unity. Example: R1/(R1+R2)=0.95.
    However, there will be a problem because R2 should be R2=0 for unity negative feedback. This cannot be realized with diodes or other non-linear elements.
    For this reason, it is helpful to place a resistor Rp in parallel to the LC circuit, thereby reducing the positive feedback at f=fo to Rp/(Rp+R3).
    In this case, it is no problem to give the negative feedback factor the same (that means: a slighly larger) value.

    EDIT: Regarding supply voltages: Of course, the simplest design is based on dual supply, however, single supply is possible if needed (with additional bias circuitry). However, the start of oscillations (in Multisim) does not depend on the value of the supply voltages. It´s best to give a starting aid by either switching on the power supplies at t=0 or to give the capacitor an initial condition (some millivolts). Ths should be sufficient to let the oscillation start.
     
    Last edited: Jan 5, 2014
  6. Dj Marik

    Thread Starter New Member

    Jan 5, 2014
    3
    0
    First of all i want to explain why the question with this oscillator. I want to built myself an multi testing digital device. I will have an C-meter, L-meter, ESR-meter, Amp-meter ( probably both AC and DC ), Miliohm-meter ( or rather any resistance under 1 or maximum 10 Ohm ), and that would be it, i plan an using PIC16F877 and an 2X24 LCD display... haven't decide yet on all the exact parameters for each measurement but those are details to be established later.

    Now, i need this oscillator for Lx and Cx, i will most probably use a separate one for each L and C, that way i can better configure them for the exact ranges wanted without resorting to relays. Studying the problem i have tested it further in multisim and i think i got what i needed using split supply, but this is where things get clouded. I do have a basic understanding of C language and i did wrote some working programs, but i am far from those experianced programmers that eat code lines for breakfast. I do know that i will need to configure the controller to freq counter, ( i think sinus or square wave would be all the same ) and that it involves timers, and those are at the moment sort of an issue for me, the fact that i couldn't find someone to get me started in the right direction might be one of the reasons, some help here would be awsome...

    Another thing is, that with split supply i have AC at the OP output, i do know that the microcontroller i plan on using has protection diodes to ground but are they enough to protect it from the negative voltage? Or would it be better to use an rectifier after de OP amp?

    What would you advice?
     
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