Environmental Engineering Reference
In-Depth Information
Observe that each noise source, when referred to the input, is reduced by the
(squared) gain between the input and the point where noise has been
injected. This means that the principal noise contribution in cascaded
amplifiers is due to the first stage, provided that the gain of this stage is
sufficiently large. This fact also justifies the use of low-noise preamplifiers
to improve the performance of noisy amplifiers (for example, in power audio
amplifiers affected by power-supply hum). In addition, comparison of
(8.12a) and (8.12b) allows important considerations to be made. First, if
noise is added to the amplifier's input or within the direct amplifier path,
feedback does not have any effect on the equivalent input noise. Second,
noise injected before or after the feedback summing node has the same
effect. In contrast, the effect of noise injected at the output (for instance due
to the noise of an additional load element) depends on whether or not
feedback is applied. Finally, it should be pointed out that if the feedback
block is implemented with noisy components, the additional source of noise,
which is absent in the original open-loop amplifier, contributes to increasing
the equivalent input noise. This aspect is better explained by considering the
effect of feedback in an amplifier model with real feedback networks, as
discussed below.
Consider the circuit in Fig. 8.8. It shows a voltage amplifier
(characterised by input impedance and by a large open-loop gain with
parallel-mixing feedback at the input. Amplifier noise is modeled via
generators
and
, and
accounts for the resistor noise.
