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?
Best Answer
I've never used this device so I'm only answering half the question - the bit that relates to the gain control input. It's always worth a look in the data sheet. Here's what the equivalent circuit is: -
On pins 1 or 16 the maximum voltage can only be the Vbe of Q2 plus the diode drop of D1. This is going to be somewhere between 1V and 2V relative to the negative rail.
The resistor in series on your circuit is to attempt to convert a variable voltage to a variable current so yes, it is possible to deliver the variable constant current drive from a much lower voltage, probably from as low as 3.3 volts above the negative rail.