Positive Feedback

In the circuit shown the voltage at the non-inverting terminal (V_n) 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_p) is V_out/2; i.e., there is a voltage divider between V_out and V_p.

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t<0

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

Note: in the real world this would not happen because any small disturbance, or noise, in the circuit starts the positive feedback cycle describe below.

t slightly greater than 0

Immediateyl after the step input on V_n (red), the output starts to go down (yellow).
This causes the input on V_p (blue) to decrease.
This increases the difference between V_p and V_n (negative feedback)

* To reiterate: the important point is that with positive feedback V_out changes to increase the difference between V_p and V_n

t even greater than 0

V_out and V_p continue to decrease, but V_out drops more quickly as the difference between V_p and V_n increases.

t is large

There is no equilibrium, V_out decreases until it reaches its minimum possible value (-10V in this case).