In the op-amp configurations, the output is often connected to one of the 2 inputs by a passive element (resistor, capacitor, …).

When Vs is fed back to the Vinput+, there is a positive feedback.
When Vs is fed back to the Vinput-, there is a negative feedback.

We will study, on an example, the stability of these two types of configurations, starting from an initial state where ε = 0 and Vs = 0.
The two configurations are the same but the + and – inputs are inverted.

Since I+ = I- = 0, then we can write, VR1 =  α Vs,
α = R1/(R1+R2) being the voltage divider given by R1 and R2.

α is also called the output reaction on the input.