Environmental Engineering Reference
In-Depth Information
highly symmetrical topology, the rms values of the voltage (current)
generators are equal.
If operated under negative feedback, the well-known virtual short circuit
exists between the two input terminals. It allows the two noise voltage
sources to be summed, indeed, they appear as connected in series. Therefore,
the ultimate opamp noise model is that depicted in Fig. 8.6b in which the
PSD is given by the sum of each component,
Finally, observe that in all those configurations in which the noninverting
input terminal is grounded (e.g., the inverting opamp configuration), the
noise current has no effects and can be left out of the circuit.
More details on operational amplifiers noise can be found in [TO89],
[XDA00], [L95], and [A95].
8.4 EFFECT OF FEEDBACK
To see how noise is affected by feedback, consider the block diagram
shown in Fig. 8.7, representing a two-stage amplifier. Gains H1 and H2 are
the gain of the amplifier stages and f is the feedback factor. The input signal
is
and the
are noise voltages injected at various critical nodes, with
PSD
