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I have a circuit that amplifies a one-shot inductive trigger pulse of about 1.3Khz (@780 micro sec period) from about 10-200mV to about 3-4V. Between C1 and R3 there is a 0.13uH inductor coil with 11 ohms resistance that is being triggered externally (magnetically) and eddy currents are created as a result. I did not know what symbol I should use for the inductor coil which is actually that IG1 sine-wave source in my circuit. So there is no ground there, just a coil between C1 and R3.

I am using the op amp as a non-inverting amp. The inverting amp (using the negative input of the amp) was too unstable. It basically acted like a comparator and with every tiny imbalance between the + and - it fired the Output. The circuit is single supply 3.3V, and only the op amp's power pin is powered.

It works good, but seems to have problems with weaker signals. The oscope shows either a lower amplitude first square wave of the pulse-period, or a lower amplitude second square wave. From the 3-4V it goes to 1-2V. Weaker, not only means lower mV amplitude, but it seems that at frequencies lower/higher away the signal is being cut off...by the circuit's inherent low/high-pass (band-pass)?

I used Texas Instrument's online program to design an op amp circuit with the correct band-pass I need. It's 500Hz-10Khz (a range of about 100 micro sec - 2000 mico sec periods). I used a single stage op amp, for now.

But, I hope to see were the problem is with my current circuit before I make this other one suggested by TI. The TI circuit is designed to cut-off at about -40DB by default as part of the TI design. Is that minimum recommended cut-off for band-pass filters?

Here is the TI-designed circuit: enter image description here

And here is the one I built that seems to have intermittent cut-off(?) issues:

enter image description here

This is the oscope when reading a strong/good signal: enter image description here

Reading a weaker signal. Seem like the first wave is spiked instead of being more like a square: enter image description here Reading a weaker signal. Seem like the first wave is spiked instead of being more like a square: enter image description here

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  • \$\begingroup\$ Your design has the differential input across 3uf + 10R and the impedance of the voltage source is not known and you have no design specs. The TI filter is a HPF & LPF and no comparison to yours. Also the 576k does nothing.(R2) \$\endgroup\$
    – Tony Stewart EE75
    Jun 3, 2020 at 0:53
  • \$\begingroup\$ The circuit you built has no negative feedback so maybe you have its opamp inputs swapped? \$\endgroup\$
    – Audioguru
    Jun 3, 2020 at 1:04
  • \$\begingroup\$ Between C1 and R3 there is a 0.13uH inductor coil with 11 ohms resistance that is being triggered externally (magnetically) and eddy currents are created as a result. I did not know what symbol I should use for the inductor coil which is actually that IG1 sine-wave source in my circuit. So there is no ground there, just a coil between C1 and R3. \$\endgroup\$
    – TommyS
    Jun 3, 2020 at 1:25
  • \$\begingroup\$ I am using the op amp as a non-inverting amp. The inverting amp (using the negative input of the amp) was too unstable. It basically acted like a comparator and with every tiny imbalance between the + and - it fired the Output. The cicruit is single supply 3.3V, and only the op amp's power pin is powered. \$\endgroup\$
    – TommyS
    Jun 3, 2020 at 1:29
  • \$\begingroup\$ What you describe your circuit doing is a lot different from what we normally mean when we say "band-pass filter". So it might help to explain what problem this circuit is meant to solve. Then we can suggest whether your circuit is a good solution, or whether a band-pass filter is a good solution, or whether something else is needed. \$\endgroup\$
    – The Photon
    Jun 3, 2020 at 1:48

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