Feedback

[Don Tillman]

   Date: Sun, 02 Jul 2000 01:45:32 +0200
   From: Magnus Danielson <cfmd at swipnet.se>

   I just wanted to comment some on this feedback. For some of you it
   migth not be obvious on how it work, it's a kind of non-linear thing
   so let's make a model just for the sake of it.

[...]
Wow!

   So, where this little exercise enougth enligthening to understand why
   negative feedback in high gain op-amps may not be so beneficial to
   distorsion?

Yes indeed.

Ten years ago I hacked together a program to perform an FFT on the
output of any arbitrary transfer function with feedback so I could
watch how the harmonic distortion products changed with the amount of
feedback.  The results were enlightening.  Really enlightening.

Sure enough, if you take a transfer function with simple harmonic
distortion products and add feedback, the total quantity of distortion
goes down but lots of new harmonics get created, and thus the
character of the distortion moves to higher harmonic content.  And
that translates to more intermodulation effects.

My original program was written in Lisp, so it wouldn't be very
practical to run assuming I could find it.  Some day I'll put together
a Java applet to do this.  It's a great demo.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com