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I've marked the circuit in red in the schematic. I'm also seeing this circuit in different schematics but I can't figure out what it's being used for. It seems to be an SCR made with transistors and a diferential pair.
I'll attach the schematic.
I think it's a 'long tailed pair' inverter with a constant current source in the combined emitter connection so it gives maximum voltage gain. Exactly why that is necessary isn't clear unless it's to ensure the drive signals to the IR2110 switch abruptly between logic levels.
Seems like its purpose is to act as a level shifter. The LM311 will swing +/-3.3V, but must control the IR2110 which is biased at -65V. I presume the resistor values in that circuit are chosen specifically to give the proper voltage swing on the outputs, so as to not damage the IR2110. The voltage swing will be 2200/120*Vbe, which is probably around 10-12V, which sounds about right for driving the IR2110.
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