Fri Dec 24 01:36:27 EST 2010

Exponential converter

The circuit in Figure 3 [1] can be simplified by removing A2 and using
the collector current of Q2 as the output to set the bias current of a
differential pair, or in reverse polarity, to drive the current input
of an OTA.

( The circuit can be simplified a great deal by removing the
capacitors and input and output resistors that don't seem to be
necessary for a modern opamp.  They are not mentioned in the AN
either.  Note, this is dated November 1969! )

Note that this circuit is really just a differential pair where one of
the inputs is at ground, and the total setup current is regulated such
that the current in one of the legs remains constant.  The baseline is:

     I1 / I2 = exp[(V1 - V2) / V_T]

This circuit computes a negative exponential (current reduces as
voltage rises).  It can easily be inverted by swapping the inputs.
However it seems simplest to keep it as is, since that allows for
multiple current mirroring transistors to be driven.

The circuit in [2] also has C2 and R4 from [3], figure 3.  Maybe C2 is
necessary to compensate the feedback loop for higher frequencies.  I
don't get R4 though.  It seems to limit the output impedance.

Anyway, this circuit has two inputs: input CV and reference voltage
wich sets the 0 current.  This gives enough information to perform

[1] http://www.national.com/an/AN/AN-30.pdf
[2] http://www.jhaible.de/tonline_stuff/jh2040.gif