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.


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

Negative voltage to NPN base

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


1 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.


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