Op Amps for Everyone - The Repeater Builder's Technical ...

Op Amp Noise
can be represented better by a voltage source, and some by a current source. Input voltage
noise is always represented by a voltage source in series with the noninverting input.
Input current noise is always represented by current sources from both inputs to ground
(Figure 10–7).
_
_
+
inn
inp
en
Noiseless
Op Amp
+
Figure 10–7. Op Amp Circuit Noise Model
In practice, op amp circuits are designed with low source impedance on the inverting and
noninverting inputs. For low source impedances and CMOS JFET inputs, only the noise
voltage is important; the current sources are insignificant in the calculations because they
are swamped in the input impedances.
The equivalent circuit, therefore, reduces to that shown in Figure 10–8:
_
_
Noiseless
Op Amp
+
en
+
Figure 10–8. Equivalent Op Amp Circuit Noise Model
Op Amp Noise Theory and Applications
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