Positive and Negative Feedback

Negative Feedback

                          If we connect the output of an op-amp to its inverting input and apply a voltage signal to the noninverting input, we find that the output voltage of the op-amp closely follows that input voltage.
                          As V_in increases, V_out will increase in accordance with the differential gain. However, as V_out increases, that output voltage is fed back to the inverting input, thereby acting to decrease the voltage differential between inputs, which acts to bring the output down.

What will happen for any given voltage input is that the op-amp will output a voltage very nearly equal to V_in, but just low enough so that there's enough voltage difference left between V_in and the (-) input to be amplified to generate the output voltage?
            The circuit will quickly reach a point of stability (known as equilibrium in physics), where the output voltage is just the right amount to maintain the right amount of differential, which in turn produces the right amount of output voltage. Taking the op-amp's output voltage and coupling it to the inverting input is a technique known as negative feedback, and it is the key to having a self-stabilizing system.
                                   

Positive Feedback

                  Unlike negative feedback, where the output voltage is "fed back" to the inverting (-) input, with positive feedback the output voltage is somehow routed back to the noninverting (+) input.

 

                  The inverting input remains disconnected from the feedback loop, and is free to receive an external voltage.

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