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Making resonance using simulated inductor (op amp)

  1. Feb 23, 2014 #1
    Making resonance using "simulated inductor" (op amp)

    I want to have a circuit that responds strongly (gives me a good measurable voltage) to a 10kHz sound input. To do this, I thought about hooking up a microphone and making a resonance circuit (using inductor and capacitor) to amplify the microphone output ONLY at 10kHz (simply hooking up the microphone to an op-amp did not work because of poor signal/noise ratio).

    The problem is, finding the right inductor is not easy, so I thought about using a "simulated" inductor:

    So my question is, will this work? I don't know if this fake inductor can do the job in this case, so I'm looking for advice. Thank you!
  2. jcsd
  3. Feb 23, 2014 #2


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    There is no reason why it shouldn't work, provided you realize that one end of the virtual inductor is effectively grounded. That limits your circuit design options a bit.

    On the other hand, maybe a high-Q bandpass filter would do the job just as well: http://en.wikipedia.org/wiki/Sallen–Key_topology#Application:_Bandpass_filter
    You can find component value calculators for Sallen & Key filters on the web.
  4. Feb 23, 2014 #3

    jim hardy

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    Very early touch-tone telephones had actual inductors inside; one from a thrift shop would be a source . You can identify those phones easily - they weigh two or three times more than all electronic ones. There'd be an element of nostalgia to that approach.

    But, Yes, a filter of some sort seems very natural.

    I have used the biquad filter in this datasheet with good results at around 25khz, Q around 50.
    It should do fine if you dont mind old school design.

    See figure 52 here:

    pretty sure i built the type1, single supply probably 5 volts - it was about twenty years ago.

    I used 5% resistors but hand selected them to match one another and fine-tuned the filter with a frequency generator.

    old jim
  5. Feb 23, 2014 #4


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    ALephZero is right. What you want is a band-pass filter (the highest Q the better). Read up on them. You can implement them with R, L, C passives, with opamps, Rs, and Cs, or buy them prepackaged.

    Getting an old radio or phone and harvesting passives could be fun too. Or you could just design an LC ladder filter (look it up) and buy the passives from Digi-Key or something.
  6. Feb 23, 2014 #5


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  7. Feb 23, 2014 #6
    Thanks guys, you are right that a band-pass filter is what I should use for my situation.

    Here's a dumb question, but for the Sallen-Key type filter, I cannot use electrolytic capacitors correct? Should I buy the thin-film caps for this purpose?
  8. Feb 23, 2014 #7


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    You can't use polarized capacitors, for the obvious reason. You could use non-polarized electrolytic caps.

    But for a 10 kHz frequency, I'm surprised you need capacitor values big enough to be even thinking about electrolytics, if you use sensible resistor values, i.e. in the 10K to 100K range.

    In any case, if you want accurate frequencies, electrolytics often have HUGE tolerances (like +100% -20% of the nominal value)
  9. Feb 24, 2014 #8

    jim hardy

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    Look for electronic surplus. I found, at my metal recycle yard, an old telephone exchange. I got several boards chock full of 1% capacitors, and they had leads long enough to salvage.

    Aleph is right about electrolytics. They're most often used for filter capacitors where 'more' is almost always 'better'.

    You'll want some sort of film. Some ceramics have a hefty temperature coefficient, so read the datasheet carefully.
  10. Feb 28, 2014 #9
    Film capacitors. Polyester for cheap and compact through holes. Polysulfone for surface mount.
    Gyrators are okay, but their Q is limited by their feedback resistor. It's difficult to set the resistor less than about 30 ohms and still get stable performance.
    For a bit of fun, you can use your common BJT as an emitter follower to replace the op amp.
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