Bias current cancellation is a common technique used in both discrete
and IC amplifier designs. Simple cancellation can be as suggested -
inject a current into the summing node from an adjustable source. In IC
advantage is taken from the matching of components so that collector or
drain currents is monitored and mirrored in a parallel path. For
example:

       --------+-----+
       |e    e |     |
        \|   |/      |
         |-+-|       |
        /| | |\    |/
       |   +---+---|
       |    ->~Ib  |\
       |            e|
       |             /
       |~Ib          \Rload
       |             /
       |             \
       |             |
   Vin |   ->Ib    |/
      -+-----------|
                   |\
                    e|


- Wayne Foletta
        BMI - Saratoga, CA