I built a Multiple Feedback low pass filter on the breadboard using an LM7171. Below is the schematics: enter image description here

While doing measurements, I noticed two things:

  1. Large DC bias at the input R3 and the output of the op amp.
  2. The signal at the output is severely attenuated even at low frequencies.

I ran a frequency response analysis and below are the results obtained: enter image description here

The response is not for a low pass filter. The gain of the signal seems to increase as frequency go up.

Any Idea on what I'm doing wrong?

  • \$\begingroup\$ Are the supply voltages to the Op-amp connected in the right way? \$\endgroup\$ Commented Nov 6, 2021 at 4:00
  • \$\begingroup\$ The supply voltages are correct. I did check that. Besides if they were wrong, the op amp would have been destroyed by now. LM7171 pin 4 is negative and pin 7 is positive. \$\endgroup\$
    – Analog
    Commented Nov 6, 2021 at 4:29
  • 1
    \$\begingroup\$ If bias is the problem, try adding a series capacitor of a few uF, to block the DC. \$\endgroup\$ Commented Nov 6, 2021 at 8:11
  • \$\begingroup\$ Have you calculated what behaviour you expect from this circuit, using math? \$\endgroup\$ Commented Nov 6, 2021 at 12:31

1 Answer 1


One problem is that the power supplies are backwards. Pin 4 is for - and pin 7 is +.

When working with op amps first make sure the DC voltages are correct. If the biasing is wrong, the op amp is not operating correctly and frequency response means nothing.

  • \$\begingroup\$ I apologize for the schematics. I just put it quickly for demonstration. I could assure you the supply voltages on the breadboard are correct. \$\endgroup\$
    – Analog
    Commented Nov 6, 2021 at 4:32

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