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I am having issues solving the gain stage section of my bandpass filter design. I used a multiple feedback 6th order lowpass and a 6th order sallen-key high pass to get a bandpass filter with cutoff frequencies 700,4200 Hz respectively. I built and tested it using the Analog Discovery 2 (AD2) device since school labs are closed right now due to the quarantine. My filter works great before I implemented any gain stage. Almost perfect cutoff frequencies at the measured decibel range. For the record, I am using TL084 quad op-amps Now, when I tried to make a non-inverting op-amp for the gain, I came across odd and frustrating behavior. When I try increasing the gain, the inband gain will stop at ~10.9dB (~3.5 Volts). Now only this, but this will also stretch the bandwidth so I no longer have my desired cutoff frequencies. It is weird because the AD2 can supply 5 volts (V+ and V-) and when I tested with an external 9 volts it still will peak at the same in-band gain.

I am seeking guidance on why this is happening and what I can do to rectify it and have proper gain behavior that matches my LTSpice simulations. I greatly appreciate any help with my issues. Thank you all and please feel free to ask me any clarifying questions.

Below is my AD2 network analyzer (or Bode Plot) screen capture before and after the gain stage.

Before gain stage. Almost perfect and expected response Before the gain stage[![After gain stage. In-band gain always tops off at ~10.9 dB and the bandwidth is screw up when that happens]3

UPDATE: I have added my LTSpice simulations to clarify my circuit for those who said I should which I should have done. There are many helpful suggestions, but it would be best to show you my schematic and maybe that will clarify any confusion. Also to clarify, my "gain stage" is referring to the non-inverting configuration that will increase the output signal. Right now my AD2 is putting a 1V test though the circuit. Maybe the AD2 cannot actually give complete 5V to the rails, but I do not understand why the cutoff frequencies are stretched when the rails are hit.enter image description here

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    \$\begingroup\$ Add a schematic. Make clear what results are from simulation, and what from real components. Indicate signal levels at all poiints through the chain. Get clear what are relative signal levels or gains (dB, numbers) and what absolute signal levels (volts, dBm, dBV). \$\endgroup\$
    – Neil_UK
    May 19, 2020 at 3:41
  • \$\begingroup\$ I still recommend you find a copy of TR-50 by Sallen and Key and read it. Meanwhile, I find \$f_{_0}=\sqrt{400\:\text{Hz}\cdot 7200\:\text{Hz}}\approx 1700\:\text{Hz}\$ with a fractional bandwidth of \$\frac{7200\:\text{Hz}-400\:\text{Hz}}{1700\:\text{Hz}}\approx 4\$. So you almost certainly need to use a combination of a low-pass and a high-pass filter to achieve this. You haven't added much by way of specifications beyond that. Just a guess on inband gain, maybe? \$\endgroup\$
    – jonk
    May 19, 2020 at 3:45
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    \$\begingroup\$ Often with multiple stages in a filter, one of the interior stages will overload. However, the following stages will filter the clipping and you will not realize that there is an overload at one of the op amp outputs. This is probably happening to you. \$\endgroup\$
    – user69795
    May 19, 2020 at 5:11
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    \$\begingroup\$ You need a schematic. Please also tell us if you have ac coupled the gain stage and if so what capacitor you are using. \$\endgroup\$
    – Robin Iddon
    May 19, 2020 at 5:42
  • \$\begingroup\$ I must admit that I do not understand what you mean with " the gain stage section of my bandpass filter design". The gain stage is the CORE of the filter and there is no filtering at all without this opamp. The same applies to the sentence "The filter works before I implemented the gain stage"...Do you speak about a gain stage BETWEEN both filters? What about a circuit diagram? \$\endgroup\$
    – LvW
    May 19, 2020 at 8:40

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