Inverting Op Amps - Non Ideal

The inverting op amp circuit is shown in Figure 6–5. The dummy variable (VA) is inserted to make the calculations easier, and a is the op amp open loop gain.

Figure 6–5. Inverting Op Amp - Non Ideal

The transfer equation is given in Equation 6–16:

The node voltage (Equation 6–17) is obtained with the aid of superposition and the voltage divider rule. Equation 6–18 is obtained by combining Equations 6–16 and 6–17.

Equation 6–16 is the transfer function of the inverting op amp. By virtue of the comparison between Equations 6–18 and 6–14, we get Equation 6–15 again, which is also the loop gain equation for the inverting op amp circuit. The comparison also shows that the open loop gain (A) is different from the op amp open loop gain (a) for the noninverting circuit.

The inverting op amp with the feedback loop broken is shown in Figure 6–6, and this circuit is used to calculate the loop-gain given in Equation 6–19.

Figure 6–6. Inverting Op Amp: Feedback Loop Broken for Loop Gain Calculation

Several things must be mentioned at this point in the analysis. First, the transfer functions for the noninverting and inverting Equations, 6–13 and 6–18, are different. For a common set of ZG and ZF values, the magnitude and polarity of the gains are different. Second, the loop gain of both circuits, as given by Equations 6–15 and 6–19, is identical. Thus, the stability performance of both circuits is identical although their transfer equations are different. This makes the important point that stability is not dependent on the circuit inputs. Third, the A gain block shown in Figure 6–1 is different for each op amp circuit. By comparison of Equations 6–5, 6–11, and 6–18 we see that ANON–INV = a and AINV = aZF ÷ (ZG + ZF).