Environmental Engineering Reference
In-Depth Information
the input, they can be combined into a single voltage-noise generator,
i.e. provided that the correlation is small. This leads
to a simplified noise equivalent circuit of the operational amplifier
shown in Fig. 11.28(b) [115, 116, 117]. If the operational amplifier is
realized using BJTs, then consists of the thermal noise of the base
resistances, shot noise in base current and the input-referred noise
originating in the collector currents of appropriate transistors in the
differential stage. If the input stage is realized using MOSFET tran-
sistors, then is made of the input-referred thermal and flicker noise
originating in the inversion layer of the appropriate MOS transistors
in the stage. It should be noted that when the operational amplifier
is operated at low frequencies, is independent of frequency. How-
ever, at high frequencies, since the gain of the differential input stage
varies with frequency, the input-referred noise source also changes
with frequency. Consequently,
is frequency-dependent.
4.4
The Algorithm
The equivalent noise band width of periodically switched linear cir-
cuits usually exceeds the clock frequency by orders of magnitude. The
under-sampling of the wide band noise results in a strong aliasing effect.
The noise power folded over from the side band components of the noise
sources dominates the output noise power. In the preceding section, we
have shown that the response of periodically switched linear circuits with
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