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Opamp oscillator with amplitude stabilization

  1. Nov 16, 2015 #1

    etf

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    It is known R4=R5=1.2k. My task is to calculate amplitude of oscillations at v0:

    limmm.png

    Opamps and diodes are ideal.

    I did analysis of circuit without amplitude stabilization and I got oscillation frequency, but I cant figure out how diodes stabilize amplitude in this circuit.
    Any sugestion?
    Thanks in advance.

    Edit: I simulated this circuit in Multisim:

    sim.png

    It looks like amplitude is 8.954*sqrt(2)V.
    One strange thing: how can voltage on diode be 3.409V ??
    I was thought that when diode is direct biased, voltage drop on it is about 0.7V.
     
    Last edited: Nov 16, 2015
  2. jcsd
  3. Nov 16, 2015 #2

    NascentOxygen

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    Staff: Mentor

    In your simulation, V4 and V5 should have opposite polarities.

    One diode conducts during part of vo's positive half-cycle, and the other diode conducts during part of vo's negative half-cycle. When conducting they supply additional heavy negative feedback (an additional path for signal vo to feed back to the OP-AMP's inverting input).

    A diode has 0.7V across it when conducting, but can have any voltage at all when reverse-biased.
     
  4. Nov 17, 2015 #3

    LvW

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    Because the gain is only slightly larger than the critical value (3.03>3) we can assume that the diodes will start to conduct (and to reduce the gain) for values smaller than 0.7 volts (perhaps 0.4...0.5 V).
    Without oscillation and for small oscillation amplituts both diodes are biased in reverse direction.
    Only amplitudes above a certain value will slightly open the diodes - thereby reducing the gain (increasing feedback).
     
    Last edited: Nov 17, 2015
  5. Nov 19, 2015 #4

    rude man

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    Homework Helper
    Gold Member

    I think the diodes are shown in backwards. Your reading of U2 is impossible with the circuit as shown.
    If the diodes are reversed then the negative clipping feedback thresholds make sense.
    In your simulation did you run Vcc and Vee to the op amp, and what were the voltages if so?

    Since the problem stated to assume ideal diodes, don't worry about 0.5V vs. 0.7V or whatever. Assume forward V = 0V.
     
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