Electronic – Non-inverting op-amp amplifier gain as open-loop gain goes to infinity (limit calculation)

operational-amplifier

Why is the following true?

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I can't seem to understand why. Shouldn't it just be infinity/1+infinity which is approximately 1?

It's comparing the vo/vin of a non-inverting amplifier with finite gain with that of one with infinite gain.

Best Answer

Just follow the following approach:

$$\begin{align*} &= \lim_{A\rightarrow\infty}\frac{A}{1+A\frac{R_1}{R_1+R_F}}\\\\ &= \lim_{A\rightarrow\infty}\frac{A}{1+A\frac{R_1}{R_1+R_F}}\cdot\frac{\frac{1}{A}}{\frac{1}{A}}\\\\ &= \lim_{A\rightarrow\infty}\frac{1}{\frac{1}{A}+\frac{R_1}{R_1+R_F}}\\\\ &= \frac{1}{\frac{R_1}{R_1+R_F}}\\\\ &= \frac{R_1+R_F}{R_1}\\\\ &= 1+\frac{R_F}{R_1} \end{align*}$$