Help with narrow bandpass filter FilterDesign

Thread Starter

efsane61

Joined Nov 19, 2020
2
Hi everyone,
I need a narrow bandpass filter, I m using opamp(LM358) and I calculated, simulatied in lt spice and TINA ti. My problem is although I calculated but simulation is wrong and real circuit is not working. I use multiple feedback bandpass filter topology. if there is somebody designed filter or work about this, can help me.
 

Papabravo

Joined Feb 24, 2006
15,779
That is most definitely NOT a narrow bandpass filter; The curvature is all wrong. It looks like you cascaded a soft high-pass filter with a soft low-pass filter.
Why isn't the source V19 AC coupled to U2? This is a single supply design. AC ground should be at 2.5 V or Vcc/2
U1 and U3 have no DC path to ground.
IN+ is connected to ground on U4, and to U2+, R6, and D1 -- what nonsense.
This whole circuit is just a hot mess
 
Last edited:

Audioguru again

Joined Oct 21, 2019
2,992
The bandpass filters are called Multiple Feedback Bandpass filters. Look it up.
Look at the output level: -145dB!!!! That is less than nothing because the opamps are biased completely wrong and are not working.
A lousy old LM358 opamp works poorly above 2kHz. Use a modern audio opamp instead.

The frequency is wrong and the bandpass is also wrong.
Here is the bandwidth of one opamp stage when a good opamp (not a lousy old LM358) is biased correctly:
 

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