Saturday, January 11, 2014

Logarithmic Converter, Control Voltage to Logarithmic Current, Part 2, Implementation

Here's an image of the logarithmic converter built on a copper plane.
Logarithmic converter
The measurement cables measures IC2, from converter to ground. The two PNP transistors are glued together for better thermal stability. The IC is a dual op amp, LM358P.
The following table shows IC2 as a function of VC.

VC(V) Simulated IC2 (mA) Measured IC2 (mA)
0 0.027 0.031
1 0.055 0.064
2 0.110 0.130
3 0.220 0.268
4 0.443 0.551
5 0.871 1.02
I think simulated and measured data compares pretty good, considering that simulation is done using general op amp model, and not the right transistors. IC2 doubles pretty well with every voltage step. We really don't need exact values, since a calibration pot will be added as soon as the VCO is up and running. The next post will be about building the VCO.

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