Bird's-Eye View

Author: J.B. Hoag

It is interesting at this point to glance back for a moment to several subjects, scattered in this book, but related in basic thought (the feedback principle). For example: with a single tube we used feedback to increase the gain in regenerative and super-regenerative circuits and also to produce self-sustained oscillations; with several tubes we have used feedback of the degenerative type to produce stability in amplifiers and, combined with regeneration, to create planned amounts of equality or inequality of gain over a range of frequencies.

Then, too, we have studied filters, made of LCR combinations, which can separate one class of currents from another: d.c. from a.c; a.f. from r.f.; or even pass or eliminate a narrow band of frequencies. In our studies of Detectors we have seen how to " pull-out " a.f. from a mixture of a.f. and r.f. (modulated carrier). With Frequency Converters, we have supplanted one carrier frequency with another. With Modulators we have injected one frequency (a.f.) upon another (r.f.). And we have combined these basic processes into our final transmitters and receivers.

We now look ahead to the possibility of applying the feedback principle to the combinations of r.f., i.f., and a.f. units which make up our receivers (and transmitters). We are confronted with the interesting possibilities of feeding back i.f. onto the r.f. stages; or a.f. to the previous i.f. or r.f. circuits; or even of rectified average values of a.f. and i.f. to the earlier stages.

Nor is this the end: we must consider the inverse possibilities of feeding from the front end of a receiver toward its output end; of diverting some of the modulated carrier to a special circuit which removes all but a certain component, and of re-injecting this component beyond the diversion point. To cover this general class of circuits we shall use the term feedahead principle.¹

There are many useful feedback and feedahead circuits. We proceed to a detailed discussion of only one, automatic volume control, and shall then but briefly outline some of the others.

1	The term " feedahead " has been coined by the author.