The previous section described different types of noise generated by the op-amp. As discussed, the internal noise of an op-amp is defined as equivalent input noise voltage. Figure 3-12 shows a simplified equivalent circuit. The op-amp represented by the triangle is the ideal one. Since Figure 3-12 expresses the equivalent input noise voltage (VNI) as a difference in voltage between VIN(+) and VIN(-), it is not significant whether VNI is inserted in series with VIN(+) or VIN(-).
Next, let’s consider noninverting and inverting amplifiers using this model.
Figure 3-13 shows an inverting amplifier, and Figure 3-14 shows a noninverting amplifier. Both the inverting and noninverting amplifiers have an equivalent input noise source (VNI) inserted in series with the Vin(-) input of the ideal op-amp. These amplifiers have a signal gain of AV as discussed in Sections 2.4 and 2.5.