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 !