Negative Feedback

In the circuit shown the voltage at the non-inverting terminal (V_p) is a unit step (V_in(t)=0 for t<0; V_in(t)=1, for t≥0).

As shown, R₁=R₂, so the voltage at the inverting terminal (V_n) is V_out/2; i.e., there is a voltage divider between V_out and V_n.

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t<0

For t<0, V_p=V_n=V_out=0.

t slightly greater than 0

Immediately after the step input on V_p (blue), the output (V_out) starts to go up (yellow).
This causes the input on V_n (red) to increase.
This decreases the difference between V_p and V_n (negative feedback).

To reiterate: the important point is that with negative feedback V_out changes to decrease the difference between V_p and V_n

t even greater than 0

V_out and V_p continue to increase, but V_out rises more slowly as the difference between V_p and V_n decreases.

t is large

At equilibrium V_p≈V_n. In other words, the negative feedback has acted to make the two inputs equal.
V_n = V_out/2, so V_out = 2·V_n = 2·V_p = 2·Vin.