0
\$\begingroup\$

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?

\$\endgroup\$
4
  • \$\begingroup\$ Are the supply voltages to the Op-amp connected in the right way? \$\endgroup\$
    – Jonathan_the_seagull
    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
    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\$
    – a concerned citizen
    Nov 6, 2021 at 8:11
  • \$\begingroup\$ Have you calculated what behaviour you expect from this circuit, using math? \$\endgroup\$
    – Jan Eerland
    Nov 6, 2021 at 12:31

1 Answer 1

Reset to default
1
\$\begingroup\$

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.

\$\endgroup\$
1
  • \$\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
    Nov 6, 2021 at 4:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.