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Feedback: The very, very basics

The functioning of the whole of our natural and artificial worlds depends on feedback which takes place either by deliberate intent, or by accident and surprise. Taking a really close look at this elementary concept is worth a few moments of our attention.

“Feedback” is a word we use quite glibly, but it is our key tool for making an amplifier that has just the right amount of gain, it is our key tool for making a power supply’s output voltage stay put and so on. There is a YouTube video in which this is discussed quite nicely.

However, the basic concept of feedback has far more general applicability and sometimes, it sneaks up on us unawares.

Leaving “electronics” totally out of it for now, if you see a glass of water on the dinner table and you want to take a sip, you move your hand toward that glass along a trajectory that you are constantly monitoring and correcting by way of feedback via your eyesight and muscle control. If you didn’t have that feedback, if you were visually disabled for example, accessing that water glass would be more difficult. Similarly, you would use feedback when you steer an automobile straight down the street and not up onto a sidewalk.

Feedback can be “positive”, or feedback can be “negative” and without prejudice, it can yield either beneficial or detrimental results as in these examples:

Figure 1 Examples of negative and positive feedback.

Sometimes the outcomes of feedback can be difficult to predict. Sneak feedback paths which are totally unanticipated by the responsible engineer, can play unexpected havoc as in the case of the inadvertent positive feedback situation previously described in my sneaky feedback paths piece.

Think of the “law of unintended consequences”.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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