Is my analysis of TI TL431 schematic correct?

[annebrt]

1. The schematic (taken from the TI TL431's datasheet) is divided into three blocks as in the image: block 1 is a band gap reference, block 2 is a differential pair, and block 3 is a Darlington configuration. Is it correct?
2. If my division is incorrect, how to divide it? If my division is correct, why is it still a valid differential pair since the left and right branches are not symmetrical? What is the impact of the asymmetry?
3. What is the function of Q1? What if connect directly without Q1?

 

[andre_luis]

What is the function of Q1?

My guess is that, since this transistor is inserted in an emitter follower configuration, it has the characteristic of making the load effect of the entry stage of the circuit smaller for whatever is connected to the reference input, that is, acting as a buffer.

 

[crutschow]

Is this homework?

 

[BradtheRad]

block 2 is a differential pair

It looks like a current mirror. Left-hand PNP current is influenced by the network in the area of your blue numeral 1.
It has bias current in common with the right-hand PNP, thus similar current goes through the right-hand PNP.

If it were a long-tail pair (commonly used as a differential detector), the two emitter legs would share one resistor or transistor or current source.

Transistor Long Tailed Pair » Electronics Notes

The transistor long tailed pair circuit used to provide a differential amplifier function within many circuits.

www.electronics-notes.com