An exponential converter is a pair of matched PNP transistors, forming an exponential voltage-to-current converter.
Early synthesizer VCOs (voltage controlled oscillators) were difficult to keep in tune. For a stable analogue audio VCO, the design solution in use since the 1970s has been to use a pair of matched PNP transistors, to form an exponential voltage-to-current converter. The input base-emitter voltage is exponentially related to the collector current. By having the second transistor configured, so that its current is in the opposite direction to the first, most of the temperature dependence is cancelled out. Any remaining temperature dependence is dealt with by using a positive temperature coefficient thermistor (tempco) in thermal contact with the matched transistor pair, in the feedback path of an op-amp voltage summer at their input.
- Electronic Music Circuits by Barry Klein, The Exponential Converter, pp. 83-87
- Electronotes Supplement S-019 Exponential/Log
- A tutorial on exponential convertors and temperature compensation by René Schmitz
- Thomas Henry VCO-1
- Temperature Compensated VCO
- yusynth VCO
- Oscillators – At The Source
- Tempco Equations
- A couple of pages about tempcos linked from Synthesizer Projects and Schematics by Ian Fritz
- Thermal Compensation of Analog Exponential Converters, openmusiclabs, March 2015
- On Oscillator Linearity and Musicality by Neil Johnson
- temp. comp. with transistors rather than thermistor?, Mod Wiggler forum, Oct 2014
- Exponential converters and how they work