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I am considering the following lowpass Sallen & Key filter:

enter image description here

My components are the following

  • \$R_1=10 k\Omega\$
  • \$R_2=10 k\Omega\$
  • \$C_1=150 nF\$
  • \$C_2=1.65 nF\$

Now I have performed a complementary transformation to obtain a Bandpass filter. Here is the schematic on LTSpice

enter image description here

Now, when I simulate this circuit I get a very odd response: enter image description here

It doesn't look at all like a bandpass filter. It looks like a low pass one, but I know that is because of the OPAMP own frequency response. What went wrong with my simulation, why is this happening?

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2 Answers 2

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Solved: When applying the complementary transformation I forgot to swap my infinity resistance r1 and my zero resistance r2. The positive terminal of the OPAMP should be connected to ground.

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    \$\begingroup\$ Correct - now you have a bandpass circuit using the multi-feedback topology. The opamp is assumed to have an infinite gain. \$\endgroup\$
    – LvW
    Commented Nov 13, 2020 at 10:16
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You have positive feedback in your circuit. The output is latched high. The reason for the fall off at higher frequencies is the internal behaviour of the op-amp as you've mentioned.

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