3.2: What Negative Feedback Is and Why We Use It

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People use negative feedback every day of their lives. In fact, we probably couldn't get along without it. Simply put, negative feedback is a very rudimentary part of intelligence. In essence, negative feedback lets something correct for mistakes. It tends to stabilize operations and reduce change. Negative feedback relies on a loop concept. In human terms, it is akin to knowing what you are doing and being able to correct for mistakes as they happen. You are constantly evaluating and correcting your actions in order to achieve a desired goal. This may be stated as letting the input know what the output is doing. A good example of this is your ability to maintain a constant speed while driving along the highway. You have a desired result, or set-point, in mind, say 60 MPH. As you drive, you constantly monitor the speedometer. If you glance down and see that you're zipping along at 70 MPH, you think “Oops, I'm going a bit too fast” and lift your foot slightly off of the gas pedal. On the other hand, if you're only going 40 MPH, you will depress the gas pedal further. The faster and more accurate your updates are, the better you will be at maintaining an exact speed.

In contrast to negative feedback is positive feedback, which reinforces change. If you were to correct your speed by saying “Hmm, I'm going 70 MPH, I'd better step on the gas”, you'd be using positive feedback. Other examples of positive feedback include the “acoustic squeal” often heard over public address systems, and thermal runaway effects seen in discrete devices. When positive feedback is applied to normal amplifiers, they oscillate. That is, they produce their own signals without any input applied.