I'm trying to build the circuit shown below. This is a VCA, application taken from Nuts & Volts magazine I think (don't have the article). I know very little about OTAs in general but from what I could understand the limits of the control voltage are the supply voltages so the max control current is about 0.4mA. How can calculate the output from that given the circuit below and given a known input voltage? Also, would it be possible to supply control current via a lower voltage and smaller resistor?