Exponential converter
An exponential pair refers to 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 thermistor in thermal contact with the matched transistor pair, in the feedback path of an op-amp voltage summer at their input.[1]
See also
References
External links
- 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