Home Forums Usage Circuit fails in AC analysis with 20% applied feedback but not with 50%?

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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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