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An '''exponential pair''' refers to a pair of matched [[PNP]] [[transistor]]s, forming an [[exponential]] [[voltage]]-to-[[current]] converter.
An '''exponential converter''' is a pair of matched [[PNP]] [[transistor]]s, forming an [[exponential]] [[voltage]]-to-[[current]] converter.


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 [[thermistor]] 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>
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>


==See also==
== See also ==
[[Voltage controlled oscillator]]
[[Voltage controlled oscillator]]


==References==
== References ==
{{reflist}}
{{reflist}}


== Further reading ==
⚫
==External links==
* ''Electronic Music Circuits'' by Barry Klein, The Exponential Converter, pp. 83-87
⚫
*[http://www.uni-bonn.de/~uzs159/expo_tutorial/ A tutorial on exponential convertors and temperature compensation] by René Schmitz

⚫
*[http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html Thomas Henry VCO-1]
⚫
== External links ==
⚫
*[http://www.oldcrows.net/~patchell/synthmodules/vco.html Temperature Compensated VCO]
* [http://electronotes.netfirms.com/s019.pdf Electronotes Supplement S-019 Exponential/Log]
⚫
*[http://yusynth.net/Modular/EN/VCO/ yusynth VCO]
⚫
* [http://www.schmitzbits.de/expo_tutorial/index.html A tutorial on exponential convertors and temperature compensation] by René Schmitz
⚫
*[http://www.amsynths.co.uk/Old_Stuff/Oscillators/oscillators.html Oscillators - At The Source]
⚫
* [http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html Thomas Henry VCO-1]
⚫
*[http://home.comcast.net/~ijfritz/sy_cir7.htm Tempco Equations]
⚫
* [http://www.oldcrows.net/~patchell/synthmodules/vco.html Temperature Compensated VCO]
⚫
*A couple of pages about tempcos linked from [http://home.comcast.net/~ijfritz/sy_over.htm Synthesizer Projects and Schematics] by Ian Fritz
⚫
* [http://yusynth.net/Modular/EN/VCO/ yusynth VCO]
⚫
* [https://web.archive.org/web/20150109082220/http://www.amsynths.co.uk/Old_Stuff/Oscillators/oscillators.html Oscillators – At The Source]
⚫
* [http://ijfritz.byethost4.com/sy_cir7.htm Tempco Equations]
⚫
* A couple of pages about tempcos linked from [http://ijfritz.byethost4.com/sy_over.htm Synthesizer Projects and Schematics] by Ian Fritz
* [http://www.openmusiclabs.com/files/expotemp.pdf Thermal Compensation of Analog Exponential Converters], openmusiclabs, March 2015
* [https://web.archive.org/web/20150403064931/http://www.milton.arachsys.com/nj71/index.php?menu=2&submenu=2&subsubmenu=9 On Oscillator Linearity and Musicality] by [[Neil Johnson]]
* [https://www.modwiggler.com/forum/viewtopic.php?t=123352 temp. comp. with transistors rather than thermistor?], Mod Wiggler forum, Oct 2014
* [https://northcoastsynthesis.com/news/exponential-converters-and-how-they-work/ Exponential converters and how they work]


[[Category:Circuit theory]]
[[Category:Circuit theory]]

Latest revision as of 17:37, 15 February 2023

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.[1]

See also

Voltage controlled oscillator

References

  1. ^ Thomas Henry VCO-1

Further reading

  • Electronic Music Circuits by Barry Klein, The Exponential Converter, pp. 83-87

External links

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