# Intuitive explanation for this hysteresis circuit?

simulate this circuit – Schematic created using CircuitLab

I am trying to get an intuitive understanding of this small circuit. I have both breadboarded it to look at behavior on a scope and simulated it on LTSPice, and it does display a clear hysteresis (see LTSpice generated V-I curve below).

What is less clear is why. I have a very hard time understanding this circuit intuitively.

First question : when the voltage at the emitter of Q1 rises, it reaches a level around 1.02Volt where everything seems to abruptly open wide. But then, as voltage rises, the intensity rises with it, in an almost linear fashion. I was expecting the whole thing to behave more like a diode and open very wide very abruptly, but its behavior once open resembles more that of a resistor.

Not sure why. I suspect R1 is some sort of feedback loop, but an explanation on the resistor-like behavior would be very welcome.

Second question: where does the hysteresis really come from ? As voltage generated by the triangle voltage source goes back down, why doesn't the whole thing turn off again exactly where it turned on ? What is is that is asymmetric in these two BJTs (I assume the "seed" of the asymmetricity is not in R1) that makes this thing finally turn off at a much lower voltage that it turned on at ?

Thanks in advance for pointers to help me understand !

Keep in mind that the 'constant Vbe above which current flows' is kind of a convenient falsehood- some current will flow with any voltage across the junctions. In fact there would be no hysteresis (the thing would switch on immediately with any positive voltage) if the transistors had a constant $\beta$ with collector current , but they don't- the gain drops as the collector current drops, and at some point the loop gain is less than 1 and the circuit cannot latch on. So as the voltage rises you will see the (small) collector currents rise as well, tickling the dragon, until at some point (perhaps with a touch of noise) the loop gain exceeds 1.0 and the pair switches on.