May 1956 Popular Electronics
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Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
In this episode of John T. Frye's "Carl & Jerry" series, the intrepid pair of teenage electronics hobbyists and Ham radio operators are experimenting with an audio amplifier rig that uses a parabolic dish for concentrating sound waves at a focal point where they have a microphone mounted. Aside from picking up bird noises and a neighbor lady scolding her husband for not properly washing the windows during a round of Spring cleaning, Carl imposes upon Jerry for a lesson in feedback techniques - both positive and negative - and the reasons one is preferred over the other. The story winds up with a clever double entendre comment referring to 'osculation.'
Carl & Jerry: Feedback
By John T. Frye
You might have thought - if you were a careless observer and didn't knew the boys very well - that Carl and Jerry were doing nothing. It must be admitted that they looked idle as they sprawled on the turf of Jerry's back yard under the relaxing rays of the warm May sun. However, you had only to look a little closer to notice the slowly revolving reels on the tape recorder beside them and to see the cord leading from it to a slender microphone mounted in the center of a metal object shaped like a huge shallow dish some three feet in diameter. This was propped up on edge so that its concave side carrying the microphone faced away from the boys toward the hedge separating Jerry's yard from the one next door.
"I think that's a lot of stuff about that king-size popcorn bowl picking up sounds we can't hear," Carl muttered in low tones with a somewhat disparaging glance at it.
"Have it your way," Jerry answered, "but when we play back the tape, you'll hear these birds that are playing around in the hedge now cheeping and twittering as though they were right in front of the microphone. That parabolic reflector focuses the sound waves on the microphone the same way the concave mirrors we play around with in the physics lab at school focus light rays down to a single bright spot. People who collect bird calls and insect sounds use this technique all the time. Just keep your voice down so you don't scare the birds and so the microphone doesn't pick it up."
"Mrs. Selden is the one who ought to keep her voice down," Carl remarked, as the shrill complaining of the woman next door came through the hedge.
"They're taking out the storm sash and washing the windows," Jerry reported, after rising to his knees so that he could see over the hedge.
"Hew come she bends Mr. Selden's ear that way all the time?" Carl asked.
"Habit, I guess. Leastways, that's what Mom thinks. She says Mrs. Selden has been scolding so long she doesn't know she's doing it," Jerry explained with a yawn as he stretched out on the grass again.
"Hey, Jer," Carl said lazily, toying with the rubbery stem of a plucked dandelion, "how's about briefing me a little on negative feedback while we're eavesdropping on the birds? I'm cooking up a new speech amplifier for my ham rig, and I don't know whether to use feedback or not."
"Hokey-dokey," Jerry agreed. "To begin with, feedback is simply the taking of some of the output of a device and feeding it back into the input. As far as an amplifier is concerned, feedback comes in two different flavors: if the portion of the output signal fed back is 'phased' or timed so that it aids or increases the swing of the input signal, it's called 'positive' or 'regenerative' feedback. Positive feedback increases the amplification and results in greater output. But an amplifier with positive feedback has a strong tendency to favor one frequency over the others and so produce harmonic distortion and non-linear amplification. Worse yet, when enough positive feedback is applied, the circuit breaks into oscillation at essentially this favored frequency, and the circuit then becomes useless as an amplifier. Generally speaking, positive feedback is a darned nuisance as an audio amplifier - it leads to howling, motorboating, and poor performance; but don't forget that when it comes to oscillators, we must have positive feedback or we don't have any oscillation.
"'Negative,' 'degenerative,' or 'inverse' feedback is phased so that the energy returned to the input circuit from the output actually opposes the signal voltage acting on the grid and reduces the amplification. This contrariwise relationship between the plate and grid circuits works to advantage, for any hum or noise or distortion generated in. the plate circuit tends to 'buck' itself out. The distorting 'zig' in the plate circuit produces an opposite-going 'zag' in the controlling grid voltage that sends through a correcting signal to help iron out the distortion in the output."
... The huge shallow disc was propped up on edge so that its concave side faced toward the hedge separating Jerry's yard from the one next door ...
"Hey, that's pretty cute: it's as though the circuit were correcting all its own faults!"
"Yes, negative feedback has several advantages. It reduces output circuit hum, noise, and harmonic distortion by the same percentage that it reduces the gain. When it's applied as it should be, negative feedback evens up the amplification given to different frequencies and makes an amplifier more nearly 'flat' in its frequency response."
"Negative feedback seems to have everything. What's the catch?" Carl wanted to know.
"The only catch is that it reduces the gain by the same amount that it reduces the distortion. It can only be applied when you have sufficient surplus of gain to sacrifice amplification in order to obtain the other advantages."
"How do you put negative feedback into a circuit?" was Carl's next question.
"An easy way to do it with a single-tube amplifier is simply to leave the cathode resistor unbypassed. In this case, the cathode resistor becomes a part of the plate load, and a portion of the plate signal voltage appears between the cathode and ground. The voltage across the cathode resistor also appears in series with the signal voltage on the grid, but it's of a polarity which opposes that signal voltage. Take a 'ferinstance.' Say that the signal swings the negatively biased grid less negative. This increases both plate and cathode current. Increased cathode current increases the voltage drop across the cathode resistor, making the cathode more positive with respect to ground. Since the grid is connected to ground, it also makes the cathode more positive with respect to the grid, or the grid more negative with respect to the cathode. This last action opposes the original signal voltage that was driving the grid less negative."
"The amount of feedback in such a case would be determined by the ratio of the cathode resistor to the plate load resistor, I suppose."
"That's right. An interesting example of 100% negative feedback occurs in the cathode follower circuit in which the plate is grounded, as far as signal voltage is concerned, and the cathode resistor is the entire plate load. In this case, the voltage gain or amplification of the stage is reduced to less than one, but distortion is practically nil."
"I suppose there are other ways of introducing negative feedback."
"Oh, sure.' Quite often a lead is run from one side of a speaker voice coil back to the grid or cathode of a preceding stage. By selecting the proper end of the output transformer secondary, you can get a voltage that will constitute 'negative feedback' for any preceding stage. Remember that every time a signal passes through a tube it undergoes a 180° phase shift; so a voltage that would be 'negative feedback' at the grid of one tube would be 'positive feedback' at the grid of a preceding or following stage."
"Then a feedback loop may embrace more than just one tube."
"That's right. It's quite common to feed back for two or three stages."
"I notice you speak about negative feedback being applied to a 'device: Did you mean to say that?"
"Yes. Feedback is found in a lot more places than audio amplifiers. For example, in a public address system, when the volume is boosted too high, you get positive acoustic feedback from the speaker to the microphone that results in a howl or oscillation. It's interesting to note, incidentally, that this howl usually occurs on a particular note for a given system. Remember we said that positive feedback favored one frequency?
"A fine example of negative feedback applied to a mechanical system," continued Jerry, after taking a deep breath, "is in the governor of a steam engine. This governor consists of two metal balls attached by hinged rods to a vertical shaft that is rotated by the steam engine. As the balls swing toward or away from the shaft, they control a valve that regulates the amount of steam admitted to the engine. When they're resting next to the shaft, the valve is wide open; and the farther they swing out, the more this valve closes. When the engine tries to speed up, the vertical shaft is rotated faster and centrifugal force causes the balls to swing out, cutting down on the steam and slowing down the engine. If the application of a heavy load reduces the speed of the engine, the balls swing in and open the valve, which restores the speed.
"Feedback even plays an important part in our physical actions. For example, notice what happens when I decide to pick up that twig. My brain sends a message to my hand that starts it moving toward the twig. As my hand moves, my eye keeps measuring the distance that still separates my hand from the little branch and constantly reports this information back to the brain. As the distance grows less and less, the information fed back is acted upon to cause my hand to slow down and finally stop directly over the twig."
"In fact," Jerry concluded, as he rolled over to stop the tape recorder and start rewinding the tape, "feedback plays a most important part in electronic brains, guided missiles, and so on. In all these devices, the data, direction, or movement is constantly being sampled and tested and fed back to the controlling mechanism to answer its unceasing need to know 'How am I doing?'"
As he finished speaking, he started the tape playing through. the recorder. At first, the only sound was that of the birds chirping away with amazing volume - and lifelike clarity. They sounded as though they might have been perched right on the microphone. Suddenly, though, the shrill complaining voice of Mrs. Selden burst through with a "presence" that made both boys jump. She kept up a constant tirade at her husband: he was clumsy; he was going to break the storm sash; he was not washing the windows clean; etc. All he was heard to say in reply was a patient, "Yes, Martha; no, Martha."
Listening to the voices that scarcely could have been more distinct if they had been talking directly into the mike, Carl's face suddenly took on a very thoughtful look. He peeped over the hedge at Mr. and Mrs. Selden, now sitting in their porch swing, and then turned to Jerry.
"Didn't you say applying negative feedback corrected imperfections in the output?" he demanded in a whisper.
"That's right, but so what?"
"Wait here. I'll be right back," Carl ordered, as he left on a stooping run for Jerry's basement laboratory.
He was, back very shortly carrying a small extension speaker for the tape recorder. After plugging one end of its long cord into the external speaker jack of the recorder, Carl started crawling with it over to the hedge. Here he set up the speaker so that its cone pointed at the couple in the porch swing only a few feet away. Then he, directed Jerry - by means of elaborate motions - to rewind the tape and start it playing again. Jerry carried out the pantomimed instructions and then crawled over to his chum.
"Those birds in the hedge certainly are happy today," Mr. Selden remarked, when the first part of the tape started playing. In a few minutes, Mrs. Selden's voice issued through the speaker.
"Where can that woman with such a mean voice be?" Mrs. Selden wanted to know, as she listened to the constant scolding. "If I were her husband I'd tell her off - why, Jim, that sounds like your voice!"
As she continued to listen, a slow flush crept over the face of Mrs. Selden. In the beginning, she had not recognized her own voice; but the familiar words and phrases soon left no doubt in her mind as to who the speaker was. She turned to her husband - whose face was wearing a look that was apprehensive, embarrassed, and reassuring all at once - and looking at him with eyes brimming with tears, she said gently, "Jim, I never realized I sounded like that. I don't see how you put up with me."
"Don't say that, Martha," he replied gently, as he placed an arm about her quaking shoulders. "I don't really mind at all. I know you don't mean it. It's just your way of talking."
"It WAS my way of talking," she corrected, snuggling against his shoulder. "As long as I live, I'll never, never talk to you like that again."
As she finished speaking, she lifted a tearstained face to her husband's, and the boys beat a hasty, wriggling retreat to Jerry's basement, carrying the extension speaker with them.
"Say," Carl demanded, "are you sure that was negative feedback we were using on Mrs. Selden?"
"It must have been," Jerry said, with a broad grin. "You heard for yourself that it was going to improve her performance. Why do you ask?"
"Well, it looked to me as if they were about to break into osculation when we left, and I thought you said only positive feedback caused -"
He was not able to finish because Jerry" who hated puns, flipped a loop of the extension speaker cord over his chum's neck and pulled the ends taut.
Posted July 15, 2015
Carl & Jerry: Their Complete Adventures is now available. "From
1954 through 1964, Popular Electronics published 119 adventures of Carl Anderson and Jerry Bishop, two teen boys with
a passion for electronics and a knack for getting into and out of trouble with haywire lash-ups built
in Jerry's basement. Better still, the boys explained how it all worked, and in doing so, launched countless
young people into careers in science and technology. Now, for the first time ever, the full run of Carl
and Jerry yarns by John T. Frye are available again, in five authorized anthologies that include the
full text and all illustrations."
Carl & Jerry Episodes on RF Cafe
- Pi in
the Sky and Big Twist, February 1964
Bell Bull Session, December 1961
Cow-Cow Boogie, August 1958
Picture, June 1955
- Electronic Eraser,
- Electronic Trap, March
- Geniuses at Work, June
- Eeeeelectricity!, November
- Anchors Aweigh, July
- Bosco Has His Day,
- The Hand of Selene,
- Feedback, May 1956
- Abetting or Not?, October
- Electronic Beach
Buggy, September 1956
- Extra Sensory
Perception, December 1956
- Trapped in a Chimney,
- Command Performance,
- Treachery of Judas, July
- The Sucker, May 1963
- Stereotaped New
Year, January 1963
- The Snow Machine, December
Education, July 1963
- Slow Motion for
Quick Action, April 1963
- Sonar Sleuthing, August
- TV Antennas, August 1955
- Succoring a Soroban,
- "All's Fair --", September
- Operation Worm Warming,
Santa's Little Helpers - December 1955
Two Tough Customers - June 1960
Transistor Pocket Radio, TV Receivers and Yagi Antennas, May 1955
Stomping, March 1962
- The Blubber Banisher,
- The Sparkling Light, May
- Pure Research Rewarded,
- A Hot Idea, March 1960
- The Hot Dog Case, December
- A New Company is Launched,
- Under the Mistletoe,
- Electronic Eraser,
- "BBI", May 1959
- Ultrasonic Sound Waves,
- The River Sniffer, July
- Ham Radio, April 1955
- El Torero Electronico,
- Wired Wireless, January
- Electronic Shadow,
- Elementary Induction,
- He Went That-a-Way,
- Electronic Detective,
- Aiding an Instinct,
- Two Detectors, February
- Tussle with a Tachometer,
- Therry and the Pirates,
- The Crazy Clock Caper,