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

Viewing 4 posts - 1 through 4 (of 4 total)
  • Author
    Posts
  • #8050

    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?

    #8166
    tomhajjar
    Participant

      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.

      • This reply was modified 3 weeks, 2 days ago by tomhajjar.
      • This reply was modified 3 weeks, 2 days ago by tomhajjar.
      • This reply was modified 3 weeks, 2 days ago by tomhajjar.
      #8170
      tomhajjar
      Participant

        You need to add a capacitor from R14 to ground called C2 so the OpAmp and transistor are biased properly.

        • This reply was modified 3 weeks, 2 days ago by tomhajjar.
        • This reply was modified 3 weeks, 2 days ago by tomhajjar.
        #8175
        tomhajjar
        Participant

          final schematic…

        Viewing 4 posts - 1 through 4 (of 4 total)
        • You must be logged in to reply to this topic.