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I have an op amp gain circuit at the output of a 2 stage filter. It seems like at high gain settings (x500), the steady state voltage shifts significantly. I am trying to keep it at ~1.7V and it shifts close to 3V after the amplification stage. In some circuit designs I see the amplification stage between stage 1 and 2 of the filter. Does that keep the voltage from shifting up? How can I keep the voltage consistent while also having high signal amplification?

Thank you!

circuit

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  • \$\begingroup\$ Your circuits don't tally with your words. \$\endgroup\$ – Andy aka Aug 13 at 20:05
  • \$\begingroup\$ This is a poor design. First start off with design specs of LPF HPF -3dB the overall gain and DC output on idle. This can easily be done with 2 Op Amps or 3. With zero drift offset error. But you must give specs. And preferably tolerances. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 13 at 23:42
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C29 should be removing the bias voltage from your input and since you’re running the op-amp on a single supply you should be seeing clipping of the negative half (with respect to the bias). In my mind, the easiest way to get a consistent bias at the output would be to use a summing stage as the last stage; you might be able to lump that in with your amplification stage as well.

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A gain of 500 is pretty aggressive for a single stage. You're essentially multiplying your input offset error and bias current/impedance errors by the gain.

At a minimum, you should be adding a resistor to the input to ensure that both inputs are driven from similar impedances; if you don't do this, your inputs will see different voltage offsets, since they're both drawing roughly the same bias current but through different impedances.

If it were me, I'd use two stages and capacitor couple them (as well as matching input impedances). With a gain of 500, small and often ignored imperfections in the op amp will become far more significant.

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