Exponential converter: Difference between revisions

 

Early [[synthesizer]] [[VCO]]s (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.<ref>[http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html Thomas Henry VCO-1]</ref>