matched expo source / sink idea

[terry michaels]

Hi Juergen:

A couple thoughts.  The current through Q4 is the base current of Q1, 
typically a very small amount of current.  The Vbe of Q1 and Q4  will
therefore be very different,  to fix that you should source some current
into the collector/base of Q4 by adding a resistor to +15V.   Also, I don't
know how much compliance you need from this to drive another circuit, but
you might find that voltage variations on the collector of Q3 (or Q2) will
probably cause small changes in the collector current of Q2 (or Q3).  The
scale factor will be 36 millivolts/octave, twice the usual value.

Terry Michaels