Effect of negative voltage at NPN transistor base (interesting example)

Question

I'm trying to understand the behaviour of an NPN transistor and two diodes connected to the output of an instrumentation amplifier [image below]. The transistor appears to switch on when there is a negative voltage applied to the base, as well as when there is a positive voltage applied. Can anyone explain what is happening here?

I would also like to know how I could tailor this circuit to work with a different voltage range and different supply voltage, in particular, for +-1.4V input to base with a 3.3V supply.

Thanks.

The supply is dual rail +-5V and the instrumentation amplifier gain is just under 10.

The text and images are from the book "Medical instrument design and development: From requirements to market placements" by Claudio Becchetti and Alessandro Neri.

Answer 1

It doesn't turn on when there is a negative voltage at its base, it turns off. The output "Sat_Test" also goes high when QS1 base goes high enough to cause the emitter voltage and collector voltage to get close to the +5V rail. It also goes high when the base goes close enough to the negative rail to cause the current in RS2 (and also RS1) to drop to a low level and thus allow the output voltage to rise.

The output is only low for a region around zero volts where there is enough current flowing through RS2 (and therefore RS1) to bring the output down to about 1V.