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It consists of an npn cascsode pair of transistors, the top one connected (collector to base) as a diode, and the two bases tied together.
I have no idea how this works, and am going slightly bonkers trying to work it out – not helped by the lack of comprehensive transistor data sheets.
Running the arrangement through LTspice (sadly, there is no Pspice-for-TI or Tina-TI in Wonderland) with another smaller-die npn substituted for the 2N3708, suggests it works well as a simple low-voltage reference.
Its output varies only about ±1mV either side of 20mV as the ‘5V’ is varied from 2 to 10V – so 10% variation for 500% variation.
Making them both 2N2222 also worked, at a lower output – 16mV, I seem to remember.
Someone please tell me what is going on here !?!?!
Find the LM311 data sheet (version SLCS007K – September 1973 – revised March 2017) here
The bottom transistor is in saturation, and the top transistor sets the base voltage as that needed to just set the bottom transistor into saturation, but no more. Vsat at a specific current is specified for some transistors, not for others.