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I have put the following simple test system together to look at a system closed loop response with varying amounts of feedback in AC analysis with QucsStudio. Unfortunately, unless I am doing something wrong, it doesn’t seem to work? When I apply 50% feedback, I get a gain of 2 as expected but 20% feedback does not give a gain of 5! (It’s down at -80 dBu) Strangely, when the DC supply is increased, things go wrong even more for some reason? (It gives a different gain response which is also incorrect.) I checked whether there are any non-linearity issues with AC analysis – like clipping – but this does not appear to be the case. I can drive the 50% feedback system with a 10 V AC source with a 10 V supply and it still works. Does anyone have any idea what’s wrong?
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Why aren’t you using a real OpAmp like an LM324/358 designed for single rail operation and confirm the transistor and OpAmp are operating properly? You can confirm using DC or Transient analysis.
You need to add a capacitor from R14 to ground called C2 so the OpAmp and transistor are biased properly.
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final schematic…
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