Who’s Afraid of Positive Feedback? [Shop Talk: What you didn’t Learn in School]

Circuit designers have a blind spot when it comes to positive feedback. We’re okay with positive feedback in things like latches and oscillators. Those aren’t supposed to be stable. But for normal control loops with positive feedback, we either believe it can’t be done or believe it shouldn’t be done. Most textbooks on circuits, and even on control theory, tend to dismiss the topic with a few sentences about growing exponentials and certain death (seeFigure 1). Where did these mental roadblocks come from? Despite all the bad press, positive feedback is far more common—and useful—than you might think. So, stop cowering under your desk and let’s face this like adults.