Feedback is a very important concept in electronics, or systems, or biology, or … Well, feedback is important. Period. And even if we do not intentionally design with feedback – there will be feedback anyway. Through parasitic paths or through bias wires or common rails of different kinds.
Explaining (negative) feedback in detail – negative feedback is a positive thing – positive feedback is a negative thing – does not make sense here. There are essentially millions of resources out there going through all this.
Instead, in this post, I just present a picture that I did the other day. It compiles the four passive (well, …) fundamental feedback modes that you can expect to encounter. The four cases are essentially given by the dualities we have: Voltage/Current and Input/Output, i.e., 2 times 2 = 4 cases. The picture is there to illustrate the fundamental properties on a “low” level. As mentioned: there is so much, much more to dig into when it comes to feedback.
One can understand that H.S. Black had some work to do after he got inspired by Steinmetz. Steinmetz, btw, was also involved in the group pictures post the other day. They all seemed to know each other. (Speaking of that Mr. Black was also part of the team that coined the term “transistor”).
Anyway, for future quick-reference, here are four examples of feedback with some simple properties compiled in a two-by-two matrix. Beta is the feedback factor and H is the system transfer function. Some of the examples contain IV and VI conversion which in some sense obscures the feedback. I have indicated those – I hope.