expo/VCO equation questions

[macdonald at evenfall.com]

I have been playing with the transistor pair expo converter equation in
gnuplot.  I have been using Iout=Iref*exp(Vin/Vt) (found this in synth-DIY
archives).  Also, in Musical Applications of Microprocessors I found an
equation for calculating the frequency of an integrator-comparator type CCO
which is F=I/C*Vref.

I thought I would put these two equations together so I could calculate
oscillator frequency from the input voltage to the expo converter.  When I
did this I found that I needed negative values of Vin to get the expected
frequencies.  I.e. Vin= -0.163 yields about 16 Hz., Vin= -0.146 gives ~32
hz. etc.  Is it normal to use negative voltages for Vin or are my equations
wrong?

(Iref was .0001, Vt was .026, C was 2.2e-09, Vref was 5.36)

Also, when I graph the frequency variation with temperature using the above
equations the result seems to be backwards.  The plot shows frequency
increasing as temperature increases, doesn't the opposite occur in reality?

Thanks!

-Chris MacDonald