The heated CA3046, part 1

A reliable VCO needs a steady exponential converter. Exponential converter are, because of their transistor based design (needed for the exponential functions), very temperature sensitive. A number of solutions to this problem exist. A solution that looks very interesting from a technical point of view is the solution found in (among others) the Moog Prodigy. The exponential converter only needs two transistors (the matched pair) from the five transistor available in the CA3046 chip. The Prodigy uses one of the left over transistor to heat the entire chip. An other transistor is deployed to measure temperature (the CA3046 transistors have a -2mV per centigrade temperature sensitivity). This measurement is compared by a comparator to stop or start the heating and keep the chip at a 55 degrees temperature.

First simulation: The temperature sensitivity of 1 of the transistors in the CA3046


Result: The transistor shows a temperature sensitivity of around -2mV per centigrade (first 5 temp.steps shown):


According to the simulation, the voltage at 20 degrees Celcius is 621mV and the voltage at 55 degrees Celcius is 553mV