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RLC Circuit with a Gyrator as an Inductor

  1. Dec 19, 2016 #1

    Im trying to build a resonant circuit with a ~low frequency resonance (<1kHz). Im using a Gyrator (https://en.wikipedia.org/wiki/Gyrator#Application:_a_simulated_inductor) as the inductor.


    Im using:
    RL = 100ohm
    R = 27kohm
    C = 1uF

    I put another 1uF capacitor in front of Zin, to form a series RLC circuit with C, RL, and L=RL*R*C

    This should give a natural frequency of 96.8Hz and a damped natural frequency of approx the same:


    But, when I measure the frequency response of the thing using a spectrum analyzer, I get something that is slightly off in frequency:


    The frequency is off by over 12Hz. The damping estimates (based on the half power method) are also off by a factor of ~3.

    Are there any errors that you can think of that would lead to this? Am I missing something in my usage of the gyrator as an inductor?

    Ive tried different resistors and swapped out the capacitors, but they all have the same results. Regardless of what resistors I use, I always seem to get measurements that are off from the transfer function predictions.
    Last edited: Dec 19, 2016
  2. jcsd
  3. Dec 20, 2016 #2


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    Gold Member

    Not quite sure about your graphs, but in spite of the equivalent circuit you have drawn, you seem to be simulating a parallel resonant circuit. Resonance is defined as the frequency where the phase shift is zero, not the max voltage, and for a parallel circuit the two are different.
  4. Dec 20, 2016 #3


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    Science Advisor

    Not all operational amplifiers can handle 100% feedback. I created an equalizer many years ago where I used simple gyrators. Instead of an operational amplifier with 100% feedback, I just used an emitter follower (which also have a gain of +1).
  5. Dec 21, 2016 #4


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    Staff: Mentor

    You have measured the actual component values, so you accurately know its theoretical response?

    Keep signal amplitude low to avoid overloading the op-amp, and monitor Vout. Your circuit has a relatively low input impedance, so it's loading whatever is driving it.

    I'd be happier if RL were to be about 20 times larger and C were 20 times smaller.

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