AW: CA3046 exponential converter

jhaible jhaible at
Sun Jan 3 17:30:46 CET 1999

> There's also Moog Rogue,but it uses just one transistor from the array
> (like diode ?) and directly affects emitter current of expo pair
> (someone can explain it better than me) and uses a PNP for cap reset.

I've seen this on some Moog synth (maybe the Rogue), too.
You don't really need an extra transistor for temperature sensing, if you
monitor the voltage across the transistor of the expo pair with the
current (the one inside the opamp feedback loop). This works perfectly
if you tie its base to GND and use the base of the "output" transistor for
the CV input. The only drawback of this method is that you will need a very
low impedance resistor divider for the CV in that case (best would be an
active divider, i.e. inverting omamp with gain << 1). That's because the
accuracy of the exponential function is very sensitive to a resistance in
base of the "output" transistor, while it's not so critical on the
transistor (which operates at a fixed current anyway).

The good thing is that you can really build three tracking expo converters
from a single 3046: One reference transistor, 3 output transistors (with
individual CV's on their bases), and one heater transistor.

One Moog synth (The Rogue or the Micro ?) has a very well tracking VCF
built with this method, so it can serve as an additional, tuned VCO.


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