The first aid to analyzing op amps circuits is to differentiate between

*noise gain*and*signal gain*. We have already discussed the differences between non-inverting and inverting stages as to their signal gains, which are summarized in Equations 1-2 and 1-4, respectively. But, as can be noticed from Fig. 1-6, the difference between an inverting and non-inverting stage can be as simple as just where the reference ground is placed. For a ground at point G1, the stage is an inverter; conversely, if the ground is placed at point G2 (with no G1) the stage is non-inverting.
Note however that in terms of the feedback path,

*there are no real differences*. To make things more general, the resistive feedback components previously shown are replaced here with the more general symbols Z_{F}and Z_{G}, otherwise they function as before. The feedback attenuation, β, is the same for both the inverting and non-inverting stages:
Noise gain can now be simply defined as:

*The inverse of the net feedback attenuation from the amplifier output to the feedback input*. In other words, the inverse of the β network transfer function. This can ultimately be extended to include frequency dependence (covered later in this chapter). Noise gain can be abbreviated as NG.
As noted, the inverse of β is the ideal non-inverting op amp stage gain. Including the effects of finite op amp gain, a modified gain expression for the non-inverting stage is:

where GCL is the finite-gain stage's closed-loop gain, and A

_{VOL}is the op amp open-loop voltage gain for loaded conditions.
It is important to note that this expression is identical to the ideal gain expression of Eq. 1-4, with the addition of the bracketed multiplier on the right side. Note also that this right-most term becomes closer and closer to unity, as AVOL approaches infinity. Accordingly, it is known in some textbooks as the

*error multiplier*term, when the expression is shown in this form (Some early discussions of this finite gain error appear in References 4 and 5 (References 4 and 5: Frederick E. Terman, "Feedback Amplifier Design," Electronics, Vol. 10, No. 1, January 1937, pp. 12-15, 50 and Julian M. West, "Wave Amplifying System," US Patent 2,196,844, filed April 26, 1939, issued April 9, 1940). Terman uses the open-loop gain symbol of A, as we do today. West uses Harold Black's original notation of μ for open-loop gain. The form of Eq. 1-9 is identical to Terman's (or to West's, substituting μ for A)).
It is useful to note some assumptions associated with the rightmost error multiplier term of Eq. 1-9. For A

This in turn leads to an estimation of the percentage error, ε, due to finite gain A_{VOL}β >> 1, one assumption is:_{VOL}:

**Gain Stability**

The closed-loop gain error predicted by these equations isn't in itself tremendously important, since the ratio Z

_{F}/Z_{G}could always be adjusted to compensate for this error. But note however that closed-loop gain*stability*is a very important consideration in most applications. Closed-loop gain instability is produced primarily by variations in open-loop gain due to changes in temperature, loading, etc.
From Eq. 1-12, any variation in open-loop gain (ΔA

_{VOL}) is reduced by the factor A_{VOL}β, insofar as the effect on closed-loop gain. This improvement in closed-loop gain stability is one of the important benefits of negative feedback.
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