Positive vs. Negative Feedback
Definition: Feedback is an act of balancing the actual system output response with a desired output, generally derived from a reference value. A good analogy is mass beam scale. For instance, if one want to find 500 mg worth of pennies, we would use a reference weight of 500mg on one side of the beam scale, the act of putting more pennies until the beam becomes horizonal; however weights of pennies does not add up to exact 500 mg, so the act of taking one and removing one to keep the beam flat is the essence of feedback system.
Negative feedback: minimize the difference between output and reference. In this above example, the act of balancing to make pennies equal to 500 mg is a negative feedback to keep the weight beam stable.
Positive feedback: maximize the difference between output and reference. In this above example, if the act of adding or removing penny on the scale isn't fast enough, the beam will tilt one side and fall, hence the weight beam becomes unstable.
Feedback is essential to keep a system stable. Almost all design are negative feedback to keep output at a reference value. Disturbance and limited bandwidth can throw off stability of the system balance and make output unstable, generally a large oscillatory response is observed.