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Hello,
say you have a single supply (+3.3V) and a bipolar AC input, which doesn't =
have
to be referenced to the main circuit's ground (i.e. it is isolated). I'd li=
ke to
know if this input goes above +2V or below -2V or is inbetween. After some =
work
I came with this scheme in the attached GIF.

Which works. The signal source output swing is within the base breakdown li=
mits
of the transistor, so no need to protect the base-emitter junction with dio=
des.

I could omit a couple of resistors, but I wanted to keep the symmetry for n=
ow.

If I understand correctly, I almost reinvented the differential amplifier, =
but
although I then studied it, there are differences (mostly the emitter resis=
tances)
that I cannot interprete.

However, the reason I'm writing is that I hoped that with this circuit I co=
uld
also improve noise immunity vs a "one leg of the source is grounded" input =
circuit.

But my simulations, where I add common mode voltage to both bases, do not s=
how
wonderful things. Perhaps its common mode rejection is very poor?

Will this circuit have any advantages vs one where one of the two signal so=
urce's
leads is grounded? I have already built it so I just wonder if it would be =
wasted
work to make it differential instead, as I'd have to modify several existin=
g boards.

Last but not least.. using a single +3.3V supply (a constraint which I cann=
ot
relax), and having this isolated signal source, would it be possible to use=
 a
off the shelf comparator IC (again the goal is to get 2 digital flags, one =
to
say if the signal source is >=3D+2V and the other if it is <=3D-2V), or it =
will not
be possible unless I use a dual power supply (which I cannot)?

Thanks!
Mario

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