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analogue mathematics for control voltages
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JingleJoe



Joined: Nov 10, 2011
Posts: 878
Location: Lancashire, England
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PostPosted: Thu Sep 06, 2012 8:16 am    Post subject:  analogue mathematics for control voltages
Subject description: inverse logarithm circuit
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For a VCF I made recently I needed to make exponentiator circuits for the CV (as the filter has an exponential response) I've made both a low pass and high pass filter, working from a single supply so the other circuitry must also work from a single supply. From looking at standard diode based exponentiators I devised my own circuits.

The low pass CV exponentiator is fine however my suspicions about the high pass filter were proved right when I tested today; it has an inverted response to control voltage!
Herein lies my problem.
Inverting the exponential gives you the log curve phase shifted 180 degrees which would only worsen the filter response.

Attached below are diagrams of my solution circuit, along with the logarithmic and the exponential amplifiers.

My solution is the first thing I thought; which was to simply stick an inverting amplifier with a gain of 1 on the end of a log amp. That works fine, however I know there must be a way to make an inverting logarithmic amplifier which works from a single supply, but I can't seem to fathom it today Sad I've had a pretty comprehensive google for answers but can't find anything, can anyone provide input or a solution to my quandry?


log amplifier.png
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log amplifier.png



exponentiator or anti-log amplifier.png
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exponentiator or anti-log amplifier.png



inverse log amplifier.png
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inverse log amplifier.png



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