Environmental Engineering Reference
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the noise behavior of MOSFET transistors at high frequencies, both
the intrinsic and parasitic capacitances must be included. A noise
equivalent circuit of MOSFET transistors in saturation is given in
Fig.11.27. At very high frequencies, the thermal noise generated by
the substrate resistance should also be taken into consideration as it
contributes nearly 20% of the total noise of MOS transistors [112].
It is also worth noting that the so-called gate-current fluctuation
[56, 100, 113] is d u e to t h e thermal noise originating in the channel
and coupled via the gate-channel capacitance at high frequencies. It
should not be considered as an independent nor a correlated noise
source.
Operational Amplifiers
: Noise generated by an operational ampli-
fier is mainly due to the noise generated in the differential input
stage. The contribution of the noise generated in the following stages
is usually negligible. To model the noise of the differential input stage
in either open- or short-circuited cases, two pairs of noise-current
generator and noise-voltage generator are needed at each input ter-
minal of the operational amplifier and the operational amplifier is
thereby treated as a hypothetical noise-free device, as shown in Fig.
11.28(a) in which and are current-noise generators, and
are voltage-noise generators [114]. Because and represent
common-mode signals, they produce virtually no differential output
if the common-mode rejection ratio of the operational amplifier is
high. Also, for MOSFET input stages, is negligible. They can be
removed from the equivalent circuit without introducing large errors
[2]. Also, because the two voltage-noise generators are in series with
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