A noninverting op amp is shown in Figure 6–3. The dummy variable, VB, is inserted to make the calculations easier and a is the op amp gain.
Figure 6–3. Noninverting Op Amp- Non Ideal
Equation 6–8 is the amplifier transfer equation.
The output equation is developed with the aid of the voltage divider rule. Using the voltage divider rule assumes that the op amp impedance is low.
Combining Equations 6–8 and 6–9 yields Equation 6–10.
Rearranging terms in Equation 6–10 yields Equation 6–11, which describes the transfer function of the circuit.
Equation 6–5 is repeated as Equation 6–12 to make a term by term comparison of the equations easy.
By virtue of the comparison we get Equation 6–13, which is the loop-gain equation for the noninverting op amp. The loop-gain equation determines the stability of the circuit. The comparison also shows that the open loop gain, A, is equal to the op amp open loop gain, a, for the noninverting circuit.
Equation 6–13 is also derived with the aid of Figure 6–4, which shows the open loop noninverting op amp.
Figure 6–4. Open Loop Noninverting Op Amp
The test voltage, VTEST, is multiplied by the op amp open loop gain to obtain the op amp output voltage, aVTEST. The voltage divider rule is used to calculate Equation 6–15, which is identical to Equation 6–14 after some algebraic manipulation.
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