I'm looking at the basic operation of a current shunt monitor*, but I don't quite understand the 'operation' description given by it's manufacturer.
Figure 1 shows the basic circuit diagram for both the INA139 and the INA169. Load current, IS, is drawn from the supply, VS, through the shunt resistor, RS. The voltage drop in the shunt resistor, VS, is forced across RG1 by the internal op amp, causing current to flow into the collector of Q1. The external resistor, RL, converts the output current to a voltage, VOUT, at the OUT pin.
Why does the voltage forced over RG1 cause a current to flow through Q1, and what is the purpose of RG2?