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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.

Circuit

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.

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Yes, that's exactly what it is. The current through R2 is CV1/3.3K and that current (minus a small amount of base current) flows from the collector of Q1 into the bias pin.

The bias pin is a couple diode drops above the negative rail so the maximum current is about (1.5V - V-)/8.2K before Q1 saturates.

The diode D1 protects Q1's base-emitter junction against reverse breakdown.

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  • \$\begingroup\$ Nice to know. Thanks, specially for the clarification of the purpose of the diode. \$\endgroup\$ – Adam Calvet Bohl Nov 8 '18 at 15:09

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