I'm experimenting on VCA circuits and found the same (or similar) pattern in many linear VCA projects found in internet using LM13700 OTA. The specific section I'm curious is made by an Op-amp, a transistor, a diode and some passive parts, and is used to feed Control Voltage to OTA's Iabc "amp bias input". In this example image, is composed by parts R2, IC1A, D1 and Q1.
Can somebody explain the purpose of this section? It seems to me like a voltage-to-current converter (it makes sense, since CV is a voltage and Iabc is a current), but I'm not sure.