A single op-amp antilog amp?

[Jim Johnson]

I just picked up the last two VC Analog Delay semi-kits from Midwest Analog
Products, and I'm thinking about redoing the CV section. The design uses a
linear VCO as the clock, which will result in sweeps that are noticeably
faster in the low frequency range (when swept with a sine or triangle). It
seems to me that adding an exponential converter in front of the VC should
do this. In order to simplify the mods to the PCB layout, I'd like to do
this using the op-amp that the circuit currently uses for CV scaling (I'll
build an external CV mixer to drive the input of the antilog amp.

I have seen the basic topology for an antilog amp - diode in place of the
input resistor in an otherwise standard op-amp amplifier - but I don't
remember how to do the math to calculate the resistor values. Obviously the
input voltage to the antilog amp must be limited to a range of 0 to about
0.7 volts - but how do I calculate the current through the diode (and hence
the feedback resistor)?

Jim Johnson 
Metaphoric Software
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Software for Electronic Music
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