The input common-mode signal vIcm is amplified in the first stage by a gain equal to
that experienced by the differential signal vId. This is a very serious issue, for it could
result in the signals at the outputs of A1 and A3 being of such large magnitudes that the
op amps saturate (more on op-amp saturation in Section 2.8). But even if the op amps
do not saturate, the difference amplifier of the second stage will now have to deal
with much larger common-mode signals, with the result that the CMRR of the overall
amplifier will inevitably be reduced.