I know that a simple circuit like this in an ideal case will have a frequency response of a single pole LPF, where the cutoff frequency is given by:

enter image description here

cutoffFreq = 1/(twoPi*L*2R)

This is reviewed here.

But what would the frequency response look like if you add a capacitor in parallel to the inductor like this:

enter image description here

Would it still be a low pass filter? What would it look like? How would it change based on the capacitance?

I tried running this on circuitlab for simulation but it is giving me an error and I don't know why:

enter image description here

It gives info on this error here but I don't understand it: https://www.circuitlab.com/docs/faq/#q_bode_opsolve_failed

No matter what I do it won't simulate with the inductor in there. But if I replace it with a capacitor it will simulate. Is there something else I'm supposed to be doing?

If it's not hard, what would be the transfer function of this circuit? Thanks.

  • \$\begingroup\$ I would recommend downloading the free LTspice and play around \$\endgroup\$
    – Huisman
    Dec 24, 2019 at 10:19
  • \$\begingroup\$ Regarding the transfer function: give it a try yourself first as you learn the best by trying. Edit your question to show your approach and people will be happy to help you along. \$\endgroup\$
    – Huisman
    Dec 24, 2019 at 10:25
  • \$\begingroup\$ You need to specify the input and output signals. It matters. \$\endgroup\$
    – Chu
    Dec 24, 2019 at 10:35
  • \$\begingroup\$ @Huisman I tried running it on circuitlab.com but it's giving me an error. Do you know what I might be doing wrong? \$\endgroup\$
    – mike
    Dec 24, 2019 at 10:47
  • \$\begingroup\$ It becomes a notch filter. \$\endgroup\$
    – Andy aka
    Dec 24, 2019 at 11:29

1 Answer 1


Well looks like I was able to get it running on https://www.falstad.com/afilter/

Looks like the capacitor turns it into a notch filter. Kind of interesting.enter image description here


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