I'm trying to understand the exponential converter in the MFOS VCO. Here is a link to the complete MFOS Noise Box synth schematic. http://musicfromouterspace.com/analogsynth_new/NOISETOASTER/NOISETOASTER.php However I tried to isolate just the exponential converter of the VCO here.
https://www.circuitlab.com/circuit/8emjqv7bj65n/mfos-exponential-converter/ (warning, when simulated, this circuit does not work. I'm new to circuitlab so I'm not sure why)
Is this what going? The input (at node in) coming from the VCO mixer goes through an inverting opAmp (not shown), so represents a negative DC voltage at the base of transistor Q1. The - input of OpAmp OA1 is positive but near ground because + input is at ground. This drive the output of the OpAmp very negative. This turns on both PNP transistors. Because current is exponentially related to voltage across a transistor, the output is an exponentially related negative current. ?
(Bonus question, why doesn't the circuitlab simulation work?)