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I am trying to build the following exponential converter circuit from the Thomas Henry "Making Music with the 3080 OTA" document.

schematic

simulate this circuit – Schematic created using CircuitLab

I understand from his text that this circuit may be ok for VCAs and VCFs but isn't ideal for tuning VCOs unless you can find an NPN-PNP Matched pair, which is very difficult to do. I'm not quite yet at the skill level to understand the matched pair expo converters using a differential amplifier setup of two matched PNP or NPN transistors, but I do understand this circuit and would like to see if I can use it for a simple VCO of about 4 octaves before I continue studying the more standard matched pair expo converters.

Question: Can I use two different kinds of NPN/PNPs to match, such as a 2N3906 and a 2SC945, or would the two transistors have to be complements of the same kind like the 2N3906-2N3904? I managed to find a 2N3906 and 2SC945 who's Vbe deltas are just about 2mV. Can I use those in this circuit?

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  • \$\begingroup\$ Try using a sim tool and altering the ambient temperature to see what happens to VCO frequency stability. \$\endgroup\$ Commented Apr 5, 2016 at 17:33
  • \$\begingroup\$ Huh, didn't even know Sim tools could do that. I will try this \$\endgroup\$ Commented Apr 5, 2016 at 17:41

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