Feedback

The principle of feedback is very commonly encountered in daily life. Feedback refers to the transfer of energy or information from the output of an amplifying or controlling device back into its input, so that the operation of the device is affected in some manner. Take as an example the control system consisting of a man steering his car along a street; his eyes are extracting information from the output (the direction of travel) and feeding this into his brain, which controls his arm muscles, which in turn control the direction of travel. In this manner errors in the heading of the car are corrected by the driver's moving the steering wheel in the proper direction, so that the car travels on a smooth and steady course. This type of feedback is variously called corrective, negative, inverse, or degenerative.

Now let us suddenly put on this driver a special pair of glasses which will reverse his perception of left and right. We have now reversed the relative phase of the feedback, so that he reacts to an observed error in heading by turning the steering wheel in the wrong direction. His attempts to keep the car on course will result in wild oscillation from curb to curb. The system is now experiencing positive, or regenerative, feedback.

Feedback is quite often present in r-f amplifiers. If r-f energy from the plate circuit of a tube finds its way back into the grid circuit and causes a voltage swing on the...