discrete audio opamps ? (was: Audio Op-Amps)

[Tony Clark]

> I'm curious: have you tried any A/B test with it and IC opamps? May I see
> one or two schems from this book? 

   No, I never did because I only had it on the breadboard for a short 
time.  I did not extensively test it for noise, performance, feedback 
characteristics, etc.
   The book doesn't really contain _full_ circuits but only pieces.  I 
was able to use various circuits to make a completed opamp.  I have drawn 
out what should be close to what I breadboarded:

http://aupe.phys.andrews.edu/~clark/opamp.gif

   Here you can see it has a differential amplifier made up of Q1-Q4 with 
an amplifier Q5 and a buffer Q6.
   The transistor Q7 and subsequent transistors Q8-10 make up three 
stable bias currents.  R2 should be chosen to be equal to kT/q and R1 
chosen to determine the bias current.
   In the drawing, Q10 determines the max output current, so if you want 
higher current, you'll have to make a separate bias current generator.
   For small errors, make sure that the betas of the PNP transistors are 
as close as possible (from the book).  And I assume that it'd be prudent 
to match Q3 and Q4 as well.  :)
   I'd be interested to find out how well it works (IF it works...), it's 
nice and simple.  Anyway, this stuff can be found in the Analog Circuit 
Design book in chapter 16, "Building Blocks for the Linear IC Designer".

   Tony

------------------------------------,----------------------------------
I can't drive (my Moog) 55!         |     The E-Music DIY Archive
------------------------------------|  
Tony Clark -- clark at andrews.edu     | aupe.phys.andrews.edu/diy_archive
http://aupe.phys.andrews.edu/~clark |     Contributions welcomed!
------------------------------------'----------------------------------