my impression is that the 100K is setting the output voltage
and the highlighted circuitry is the output stage.
. I put it in post #3 to help , it was along the line of what i was thinking initially. I just needed someone with more experience to explain it in more detail to me. This then leads to my next question. Why would you want a high voltage gain when there are current spikes?Hard to tell without the full diagram
Hard to tell without the full diagram, but this open collector IC output regulates the opto isolator feedback to maintain equal V+,V-
Vout is ac coupled to a common emitter device with a Shottky diode clamp , so that if there is current spike that exceeds 0.6V/330R it also saturates the collector to turn on the Opto feedback, while the 100K to gnd reduces the turn off time. Since Re is small (47R) and Opto diode pullup is large (1k) it operates as a comparator with high voltage gain for positive spikes only.