  When considering the effect of quantum noise (QN) in a phase-insensitive
linear amplifier or attenuator, it is customary to use the single channel Caves
model (SC-CM). Although this model is valid in simple situations, such as the
presence of a beam splitter, it is not necessarily valid when a system with
many degrees of freedom is involved. In order to address this issue, we
consider in this paper various atomic transitions corresponding to
amplification or attenuation using the master-equation- (ME-) based approach to
model the QN and to compare the results with the SC-CM. For a four-level system
that consists of a transition producing a broad gain peak and a transition
producing an absorption dip, which results in perfect transparency at the
center, we observe a catastrophic breakdown of the SC-CM. We also show that for
a general two-level atomic system, the SC-CM does not apply, except in the
limiting case when only either amplification or attenuation exists. A special
case where the two models predict the same result is a {\Lambda}-type
three-level electromagnetically induced transparency (EIT) system in which the
QN at zero detuning vanishes while the system is in the dark state. We also
study an optically pumped five-level gain EIT system which has a perfect
transparency dip superimposed on a gain profile, and yields the negative
dispersion suitable for use in enhancing the sensitivity-bandwidth product of
an interferometric gravitational wave detector. In this case, we find that, for
some set of parameters, the QN is vanishingly small at the center of the dip,
and the SC-CM agrees closely with the ME model. However, we also find that for
some other set of parameters, the SC-SM model disagrees strongly with the ME
model. All these cases illustrate a wide range of variations in the degree of
disagreement between the predictions of the SC-CM and the ME approaches.
