Positive Feedback: Edwin “Howard” Armstrong 1st Great Invention!

[@sudarshanbadoni6643]

Anyone who does not Express creates confounding uncertainty is one thing. One who does that know the power of dare to think and gaining from any video. The ZEAL to tell and make new generation aware of history, pain and gains are evident here as love for most dry subjects is daring adventure and thanks for that.

[@jeffharrison1090]

Another brilliant and fascinating story of early inventions! Just love them so much!!!

[@trevorhaddox6884]

It's amazing that someone went from not knowing how winter/climates work, to being an electrical engineer. Makes us all feel a bit more confident.

[@msimon6808]

I became an aerospace engineer (aircraft electronics) sans degree. I was motivated. I loved the stuff. I also designed the IO board that went into the World's First BBS. Helped a little with the software. CACHE Club - Chicago. Support the Revolution Buy A Computer.

[@JimCallahanOrlando]

I remember as an elementary school student assembling a Heathkit going through "labs" of various stages of radio. The Heathkit was a vacuum tube receiver. I remember one stage was just a diode (tube or crystal, I don't remember) and hearing 30 hz hum and then regenerative feedback through a triode tube and a complete superhetrodyne receiver. With boyish enthusiasm, I thought the name "superhetrodyne" sounded really cool. This was with a hot soldering iron on a metal chasis. The difficulty of the point to point wiring gave me a great appreication of printed circuts. I was barely able to follow the labs and never did anything original. But, my dad who was an electrical engineer and worked for a decades later version of the company David Sarnoff founded, RCA was proud of me.

[@santosakowski9846]

This brings back memories of my youth at about age12 to 14, when I first was introduced to crystal radios and transistors via a Remco toy kit. Never thinking to look in a book for answers, I too, rearranged the circuits and experimented for many hours, trying different configurations. I built an AM transmitter and then experimented with increasing its power and range. It was so exciting. I'm glad I never thought to look in a book because learning it on my own was much more fun, even though I didn't know exactly what I was doing. I can really identify with Armstrong's excitement and how ecstatic he must have felt. This was a nicely told story. Thanks.

[@alanbower8175]

Nice to have this young lady fill the gaps of knowledge that everything takes for granted.

[@kentlofgren]

Initially and for a brief moment I thought it was about course evaluation questionnaires ;-)

[@AnbroBR]

Thanks for the video, Kathy! As a ham radio operator, each December I use my ham station to put on a special event in the honor of the birth date of Major Edwin Howard Armstrong. I try to make it a 3-day operation that includes his birth date of December 18. I receive a special callsign and my event is a Morse code (only) operation. In my opinion, Major Armstrong was the GREATEST inventor in radio. Thank you.

[@Thierry78]

Excellent explanations for the layman. Thank you.

[@Kathy_Loves_Physics]

Thierry Dewindt so glad you liked it and it made sense to you.

[@scene2much]

When I was early in High School and onward I got a hold of the American Radio Relay League (ARRL) Manual. It was a 3 inch thick paperback full of instructions and circuits for Radio hobby and professional applications. There were tons of stories by many of the project authors, but I remember nothing about the history of Radio. I learned more about the fundamental workings of Diodes and Triodes from Kathy than from the dry, if precise, explanations --sometimes-- provided in the book. If those books contained the history of people like Armstrong, well... if only they did.

[@jamespowell1442]

You have 6 new fans,my grand children love your channel,keep up the good work.

[@Kathy_Loves_Physics]

That is lovely. Thanks. How old are your grandkids?

[@jamespowell1442]

@@Kathy_Loves_Physics You"re welcome,the youngest is eight,the twins are ten, the others are twelve,fourteen and fifteen.I have computers,chemistry,and physics labs all over the place.Your videos are inspirational to them all.

[@Kathy_Loves_Physics]

@@jamespowell1442 Wow, you sound like a fantastic grandfather. I made these videos for adults (or maybe advanced teens) so I am surprised that kids 8-15 can understand it, they must be super smart like you. My oldest child is 6, maybe I should start showing her the videos? Cheers, Kathy

[@jamespowell1442]

@@Kathy_Loves_Physics Do basic math with applications first ,so you show how important math is to science,then show some computer basics,house old appliances that use electricity then show your videos,the discourse between you and your child will bring you closer to each other.Good luck!!

[@kennethnevel3263]

You explain things in a way that is easy to understand for anyone. My dad was a radio technician , wireless telegrahefer.

[@robertakerman3570]

Is it jus' Me, or does SUPER-HET sound macho?(sexist joke/alpha male-none of the above!)

[@haroldmat13]

Watching your videos is like watching a good Sci Fi movie, I can't tear myself away.

[@johnishikawa2200]

I actually made an Armstrong oscillator receiver that I saw in a magazine full of electronics projects. It uses one tube which is a pentode an this vacuum tube performs amplification, and detection of the radio signal, and it is very sensitive. The power source is batteries-i.e. an assortment of 9-volt and 3-volt batts.I had a lot of fun making it and listening to distant am broadcasts!

[@drlegendre]

Vacuum triodes are rarely ever operated with positive grids. The electron flow is controlled by making the grid more or less negative, without transitioning into the positive. If the grid becomes positive, the tube enters grid current operation, with an associated sharp change in the tube's behavior

[@canuckprogressive.3435]

Right. The grid is biased to a set positive voltage and the signal applied to it is kept small enough to not try to drive it to less than zero volts.

[@dimension2788]

Mahalo Cathy well done. As a general class amature radio Ham I truly thankyou for this simple explanation. These coil and caps are also known as Tank Circuits. I have a trusty MFJ antennae tuner when you look inside you see a spring (coil) and the rotary capacitor. The coil looks kinda like a valve spring pretty thick, with taps at various points. These tank circuits made the under sea telegraph work and soon wireless. Mahalo Cathy! WH6DSF

[@rayoflight62]

Any portable AM radio uses six or seven transistors to amplify the signal from the antenna to the speaker. Older radio required four or five tubes to achieve the same resul In order to be listened properly into a speaker, the signal from the antenna must be amplified from ten thousands to one million times. To achieve such level of amplification, you need from four to six amplifier stages placed one after another, or... a trick. Armstrong used the trick: a superreactive amplifier, which allows a single stage to amplify a signal fro 10,000 to 100,000 times - with some compromises, though. An active stage do amplify a signal - say 100 times. By adding positive feedback to the stage (i.e. by feeding the input with a portion of the output signal kept in phase, or with a 0° degree rotation) the amplification initially increases exponentially, then after few microseconds the stage become an oscillator, and no longer provide amplification. The genius of Armstrong was to interrupt the operation of this stage 50,000 times per second. Right before the amplifier become an oscillator, a signal reset it to the initial conditions. In this way, the superreactive stage worked on the edge of becoming an oscillator, but never achieved self-sustained oscillations. In this way, Armstrong achieved an amplification of 100,000 with a single tube. The resonant circuit at 50 KHz was made with the additional coil in series with the headphone. This frequency, called "dampening frequency" (because it dampened the self oscillation) was well above the range of frequencies which can be perceived by the human ear. In any case, it produced a white noise (an hiss) in the headphone, and was the indicator that the superreactive receiver was up and running. Thank you for the great video!

[@thompsjm]

I had always wanted to build a regenerative receiver, the first project in my father's old AARL handbook, but never found the time. Then, a few years ago, I found a fully assembled Radio Shack kit for a regenerative multiband radio in a thrift store, it was called the Globemaster. Tuning it required two hands, one on the tuner and the other on the feedback control. It was necessary to adjust both simultaneously, bringing the amplifier just to oscillation, then backing it off just a bit. It was extremely sensitive and fun to use, but something like that would not be widely marketable. I hadn't heard of Armstrong's solution, thanks for sharing that information.

[@marcopilati7464]

Amstrong woke her mother up!... AMAZING!! Exactly like Marconi did, who went to her bedroom and cryied: "Mom, come with me to the attic. I've something to show you...." AMAZING! How to say... Mom is Mom!

[@simonowen488]

To a quasi-smart former academic in architecture, heritage, history and forensics turned enthusiast in aIl things electric and electronic, thank you Kathy!!

[@acmefixer1]

Actually, at 4:25 the grid must not be positive in relation to the filament. If it is, it attracts electrons like the plate. If the grid is at zero volts then it has no effect on the electrons flowing to the plate. The grid must be negative, to reduce the electron flow to the plate to a manageable level and thus a small change in negative grid voltage controls a large change in plate current. Thus the audion (vacuum tube) amplifies the weak signal. Thanks for the great video.

[@marcopilati7464]

KATHY, you are a world's resource!

[@Kathy_Loves_Physics]

Thank you!

[@hank1519]

Another great lesson! Thank you!

[@sumonarosner4674]

When I was a teenager I just connected a small wire to the grid of a vacuum tube with a current meter in the plate circuit. To appreciate how sensitive the gird is merely walking around the room would changed the plate current greatly.

[@Kathy_Loves_Physics]

That is amazing

[@noproblem4260]

You where just making a sensitive VTVM with enormous high input impedance.. once you just could "smell" volts

[@jagmarc]

You can repeat this with modern components, any n-channel FET instead of a vacuum tube

[@basinstreetdesign5206]

Good going, Kathy. Keep 'em coming!

[@7c3c72602f7054696b]

I would love to be able to download these, I admire Major Armstrong very highly and your videos are extremely informational. Cheers!

[@Kathy_Loves_Physics]

Glad you liked it but don't keep my videos on my computer as they would take too much room so I can't give you them to download, sorry. I have also become a HUGE Armstrong fan as well. (but his story ends so sadly)

[@7c3c72602f7054696b]

@@Kathy_Loves_Physics Very true. I build and restore old time radios, regenerative and superregenerative recievers are very intriguing.

[@bobconnor1210]

Kudos! You are reminding me of all my favorite teachers who inspired love for learning by their enthusiasm for the subjects they were teaching.

[@raullcalzadilla3541]

Another fantastic presentation: Pure joy!

[@PatrickPoet]

Isn't she great? I too only recently subscribed but man am I enjoying catching up on all the old videos!

[@richardfoster2895]

Armstrong's regenerative circuit made simple 2 tube radios viable for radio reception with an affordable device. Such receivers were popular for both broadcast reception and radio amateurs through the first half of the 20th century.

[@Mike62501]

Kathy thank you I love your explains keep it up. Mike in TN

[@BeachsideHank]

When we were kids my best friend and I would head to the dump (it was wide open and no security back then) about once a month and harvest components from old t.v.'s and radios we dug up for our reuse. Once we took a bucket of tubes to the local supermarket that had one of those courtesy tube testers there, but we were thrown out after about a half-hour of using their electricity to validate our tubes and not buying any of their tubes. ☺ Damn, those were fun times for a coupla broke entrepreneurs!

[@jackpatteeuw9244]

I know this is an old video, but I just recently viewed it. Enlightening ! I have a BSEE (digital electronics) from back in the 70s. Believe it or not, in my 4 years of college I was taught almost nothing about radio. Physic class covered passive components (mostly just resistors and capacitors). My solid state classes covered diodes, BJT and FET (this was before MOSFETs) but only used for amplification. When designing an amplifier, POSITIVE FEEDBACK is very bad thing. I have one class on Fields and Waves, but they concentrated on transmission lines. Lastly, I had a few classes in digital circuits (74xxx TTL) and microprocessors. No one spent any time explaining radio and the use LC circuits that used positive feedback. I certainly missed a lot back then !

[@richardgordon]

Great story! Well explained!

[@videolabguy]

Excellent!

[@PatGBass]

Fascinating. I can't wait for the next episode. I found out your channel recently and immediately subscribed.

[@NEWDAWNrealizingself]

🟦THANKS KATHY 🟦 THE CRUCIAL FINDING OF ARMSTRONG IN RADIO TECHNOLOGY IS UNFORGETTABLE 🟦🟦

[@jimjackson4256]

i like all your videos.A lot of things I already know but you fill in the blanks .A good channel.

[@maximolopezsr9399]

One of thr Must fascinating video or leaning ,I have ever encounter,

[@skybot9998]

Its 5 am and I'm watching this Great video kathy.

[@tadonplane8265]

Edwin Howard Armstrong is one of the greatest inventors of all time, yet he’s hardly known by the public at large. His name can easily stand with Franklin, Davy, Watt, Otto, Diesel, Tesla, Edison, the Wrights, Goddard or anyone else who’s ever invented anything!

[@MrCuddlyable3]

@tadonplane In English the words INVENTED and INVITED are spelled differently because they mean different things.

[@tadonplane8265]

@@MrCuddlyable3 that was a typo, it’s fixed, thanks.

[@MrCuddlyable3]

@@tadonplane8265 Well done Sir, you are a conscientious person.

[@psnpacific]

Great Story! 👍👍

[@mikeadler434]

Great video!, I enjoyed this lecture. 🙂

[@jackd.ripper7613]

I await with abated breath for your next offering.

[@Kathy_Loves_Physics]

Jack D. Ripper awww thanks Jack. 😊

[@hank1519]

Jack D. Ripper I just found out that bated and abated are two forms of the same word!

[@robertjohansen7482]

Very good intro to the history and physics of radio, some errors explaining regeneration, no mention of tickler feedback coil. I have a Crosley pup which is a near copy of the original circuit Armstrong designed I met Jeanne Hammond, Armstrong's niece at a RCA Radio Club of America banquet in NYC 1990's

[@frankartieta7483]

Hello :) I am astonished ! This is really a great video ! Amplification Amplification Application :) The regen circuit a truly Active Circuit ! In a time when there were nothing but passive components ! It made a Triode Tube a Active Device ! The regenerative or reactive circuit did a lot more than what was hoped when first discovered ! A lot more ! The main things were Amplification and Selectivity ! But it also made made higher frequencies usable ! In effect made shortwave usable ! There is a fella :) Norman Field he has a series of KZbin videos named Crystal Radio ! I think eight videos altogether ! He builds starting from simple detection all the way to tune reaction ! He has a wonderful way of explaining the subject ! Best Christmas tu U :)

[@MrMichaelBradfield]

the glass plate in a capacitor serve as a dielectric material, they serve to greatly increase the capacitance of the capacitor (as the material in glass when subjected to a static electric field polarizes and in turn generates a polar static field increasing the charge storage of the capacitor itself) the glass plate is not there to keep charges from flowing from one plate to the other; as long relatively low voltages are used, charges will remain on each plate and a net static electric field will exist between the capacitor plates

[@Singularitarian]

These videos are so good.

[@jonahansen]

I think the quote attributed to Armstrong is actually originally from Mark Twain: "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so." But Nevertheless a good one that bears repeating!

[@jimimaze]

Love this series!

[@Kathy_Loves_Physics]

Thanks, so nice of you to comment on so many videos!

[@shawnmulberry774]

1:39 - love this quote

[@Kathy_Loves_Physics]

I should use it more often

[@opera5714]

That picture reminded me of a friend who watched Armstrong up on the WTRY tower sticking his arm out to make power measurements. He was a daredevil unafraid of heights.

[@Kathy_Loves_Physics]

With a tragic end

[@nigelpearson6664]

To be clear the input bootstrap can be derived from amplifier negative feedback loop as would the constant current source . If I am right Harold Black imagined the use of both negative and positive feedback. Negative feedback will become positive at a critical frequency.. This becomes fascinating when making it wprk. Elegant solutions are great thought experiments.

[@lawrencedoliveiro9104]

5:04 Here’s a question: I recall the older (and now deprecated) name for a “capacitor” was a “condenser”. Why did it initially have that name? Was this another of those hangovers from the 19th-century history of (mis)understandings of the nature of electricity?

[@pirobot668beta]

Capacitors were seen as literally 'condensing static charges' in operation. This mistaken notion was driven by countless early experiments with Leyden 'gathering' static electricity.

[@neilkurzman4907]

Armstrong was probably the first electrical engineer. He actually understood how the circuits worked. While others were adding pieces of metal to their tubes to see if it had any effect.

[@Kathy_Loves_Physics]

Armstrong was the goat of radio.

[@JoeJalopy]

As a kid in elementary school, I read the Scholastic paperback "Man of High Fidelity," about Armstrong.

[@Kathy_Loves_Physics]

Such a good book.

[@NoiseWithRules]

Errm, the patent Pupin sold for lots of money - was the 'loading coil' which made long-distance telephones possible? That idea was invented by Oliver Heaviside. Pupin offered Heaviside a share of the profit, Heaviside refused as he wanted sole credit.

[@richaarrd1]

I understand Kathy's explanation of positive feedback. But since everyone seems to be listening to radio signals, while I understand the amplification... but not the (what i think is) required detection.

[@tobystewart4403]

Great stuff.

[@y_x2]

It is more a discovery than a pure invention... At the begining, feedback was more of a problem that a positive thing.

[@lawrencedoliveiro9104]

10:11 This idea of using radio waves to broadcast music to the general public was a roaring success. Ironically, an earlier inventor had the idea of using his (wire-based) signal-transmission technology in exactly the same way, but that turned out to be a flop. For some reason he couldn’t fathom, people preferred to use his invention to _talk_ to each other, rather than listen to music. Can you guess whom, and what, I am talking about?

[@dareks8000]

Marconi in Cornwall UK?

[@nigelpearson6664]

Bootstrapping is a form of positive feedback that seems like a miracle. Thus it was likened to pulling yourself into the air using your bootstraps. If simple rules followed it works. < 100% feedback is that rule. This is very convenient as getting more than 100% is not the simplest route. A class AB amplifier might offer 94% which is ideal, Lower amounts still useful. One type as in a class AB amplifier makes a form of constant current source. In the same amplifier a bootstrap from the negative feedback loop might raise the input impedance of the amplifier ( See Douglas Self ). Three extra cheap components doing close to magic. Fashion seems to dismiss bootstraps.

[@johngough2958]

Great video! You should do one on Harold Black and the invention of the op amp!

[@donfisher8035]

Whoa. What a leap.

[@noproblem4260]

So that was the "superregenarative receptor... One valve.... Then came the superheterodyne wich mixed a local oscilator to get 455khz ... Then amplify...detect and so on Kathy will explain it better and with pictures...great hostory

[@Vincent_Sullivan]

You forgot the "Reflex" receiver. A very clever idea where the signal was sent through the active device twice - once at radio frequencies and once at audio frequencies. LC filters routed the signals through the correct path. It basically gave the performance of 2 tubes with the costs of only one and without the disadvantages of the (super)regenerative circuit.

[@alikaperdue]

Why LIKE? 4 years in and less than 50k hits? That's not right. Should be 1+ million. I clicked LIKE because others deserve to find this.

[@bulldogbrower6732]

This has to be your best and most informative video, why is the name Pupîn Lost to the ages ?

[@Kathy_Loves_Physics]

I don’t know, he was amazing and delightful. Also, he wrote an autobiography that I really enjoyed and it’s free on the web.

[@msimon6808]

The first electronics I ever designed was a Regenerative receiver.

[@manusudha4269]

Subscribed ! You are awesome.

[@boredout43]

again Great!

[@surendrakverma555]

Very good. How do you get this type of historical scientific information? Thanks for information. 🙏🙏🙏🙏🙏🙏

[@Kathy_Loves_Physics]

i’m so lucky and so much of the stuff not just old scientific papers but also old radio magazines have been digitized and I can just find it from my computer at home.

[@surendrakverma555]

@@Kathy_Loves_Physics Excellent. Thanks a lot

[@goodmaro]

I withheld my thumb up because I don't know what you've diagrammed at 8:25, but it's not a regen receiver. I'm sure that by now I'm not the first to point that out.

[@BarryKort]

Also look up Harold S. Black, a Bell Labs engineer who invented the Negative Feedback Amplifier - a major advance in the use of audio amplifier circuits.

[@marinvidovic763]

You have an amazing series of Videos ! Thanks for Your work , as it really gives Human angle to a purely Technical field. You have a Very interesting and successful Storytelling talent, , that kept me for many hours (*during a COVID Scare), attached to the screen. Thanks !!! (* small remark: M. PUPIN - is pronounced in a different way. Need to put a Stress on a first 2 letters , P!U!- pin Like : .Short stressed " PU",.... , and then Stres goes ... down the hill - " pin" . ... (* ...not the other way around. ) It sounds funny... to us - from his country. But please, dont take me wrong. I like your VIDEOS and this is a minor issue. All the best !!!!!!!

[@ykr786]

People give credit to negative feedback but not enough to positive feedback

[@alangraham8926]

Kathy how does this relate with Nyquist feedback theory?

[@konradswart4069]

Great video! It would be nice, though, if you had added that the resonance frequency of the coil and the capacitor causes the whole circuit to have a high resistance, while at any other frequency the resistance is low. In this way the resonance frequency of the capacitor and the coil _isolates one frequency_ from all the frequencies occurring in the wire, and thus this frequency can be used as a carrier wave for AM modulation. As it stands now, the explanation does not really _explain_ how this combination of a coil and a capacitor is the basis of a radio. It misses this key ingredient.

[@martinmalloy8119]

Thank you for another fantastic lecture. In Germany we have a fantastic video series too by Professor Alexander Gassen called Urknall Weltall und das Leben ( Big Bang Universe and Life ) Viele Grüße Your Martin

[@sbybill3271]

About the discussion on the feedback in these comments, I must quote Edvin Howard Armstrong, " It ain't ignorance that causes all the trouble. It's all the things people know that ain't so". 😉

[@Kathy_Loves_Physics]

One of the best lines in history

[@MarcosElMalo2]

I’m almost willing to bet that Pupil’s first job was at a Greek diner.

[@Trp44]

Brava

[@njlauren]

What you left out is that de forest tried to claim he created the regenerative circuit. Armstrong didn't have the money to file for a patent patent his circuit, he only had a lab notebook that was notarized to prove when he did it . De Forest likely had heard of armstrongs work, and basically filed his own patent for it. It later went to court and initially the courts ruled in de forests behalf, it was a mess,but there is no doubt Armstrong invented it.

[@catchannel4421]

How the FEEDBACK was discovered ?

[@allanrichardson9081]

Q: Why did the radio operator keep on climbing the antenna? A: it had an “Arm-strong rotator!”

[@ronjones4069]

The Major (Armstrong) has always been a hero of mine. I have his picture in my ham shack, next to Maxwell and Faraday. The true holy trinity. And Sarnoff was Satan.

[@richwood186]

Most people know Armstrong as Edwin Armstrong. His middle name is Howard.

[@Kathy_Loves_Physics]

It’s true that his middle name was Howard but his niece called him uncle Howard, and he was referred to by his middle name in a well researched biography called “Empire of air” and he signed all his patents with his initials so I figured that he went by his middle name.

[@Justwantahover]

He invented "thumbs up" but not "thumbs down".

[@marcusdirk]

Underhill reminds me of someone, but I can't think who. And it's not Frodo Baggins!

[@tanneraerospace7301]

WOW

[@pedroreyes640]

Hmmm. I don’t think there is a positive feedback here. It will be a positive feedback if the plate current or voltage is injected to the input grid. If that happens, there could be an oscillations and will distort the sound output. If the resistance of the wire from the filament to the negative terminal of the battery of the plate circuit is high, you could even have a negative feedback, for a lesser gain but wider bandwidth, unless you by-passed it with a capacitor to have more gain or larger amplification. But still, I admire the series of presentations…good work!

[@wouldntyouliketoknow9891]

I don't think you can say positive feedback is an invention. Maybe intentional positive feedback as a design element/technique in a particular circuit might be an invention. But positive feedback occurs in all kinds of naturally occurring phenomenon. Its a mathematical concept.

[@njlauren]

The feedback circuit's full name is the regenerative feedback circuit. This isn't feedback like you have on a microphone. It refers to ,'feeding back' part of the output into the input. This causes signal amplification bc the signals are phased in such a way as the amplitude ( volume) increases.

[@larryteslaspacexboringlawr739]

undergrad for the win

[@jagmarc]

Positive feedback? I thought it was called Regeneration. ?

[@0MoTheG]

I don't understand why the positive feedback did not cause saturation, clipping, oscillation. I am an EE and have never seen positive feedback in an amplifier, we specifically avoid it.

[@Vincent_Sullivan]

Also an EE here. The secret sauce is in how MUCH positive feedback you apply. Just a tiny bit can give a big boost in performance if you are limited in the number and gain of active devices. You are correct that too much positive feedback will result in the problems you mention. Think of a public address system. When a sound is fed into the microphone (assuming the microphone is not isolated from the loudspeakers) some of the sound from the loudspeakers will reach the microphone "in phase" depending on the frequency, path length, room characteristics, etc. If the "in phase" sound is low enough in amplitude at the microphone it will not cause oscillation even though it is in phase (positive) feedback. The critical factor is "loop gain". A PA system with microphone and speakers isolated from one another has a loop gain of zero at all frequencies. As you decrease the isolation between the mic and speakers (or increase the gain of the PA amplifier) the loop gain at some frequency where the feedback to the microphone is in phase will increase - but the system will be stable IF the loop gain is significantly less than 1.0. As you approach a loop gain of 1.0 the system will become less stable. It won't break into unprovoked oscillation but it will start to "ring" like a bell. A transient sound into the microphone will produce a damped tail of sound from the speakers at the frequency where the maximum amount of in phase sound is applied to the microphone. If you continue to decrease the isolation between the mic and speakers (or increase the amplifier gain) the loop gain will eventually exceed 1.0 and the fun will begin with all of the problems you mention. About 40 years ago I was working as an EE for the Canadian National Research Council in Ottawa mainly designing and building electronic equipment for the scientists to use in their research. The building I was in (100 Sussex Dr. in Ottawa) had a beautiful auditorium. I was tagged to look after the sound system in the auditorium as I was the "electronics guy". The amplifier was very interesting. As I recall it was a commercial product manufactured in Britain but I can't remember the brand name now. It had a switch to turn on "feedback suppression" and it worked by shifting the frequency of the sound signal being fed into the amplifier by a few cycles. Any sound from the speakers that arrived back at the microphone in phase couldn't build up to audible feedback because after a few round trips of the loop it would slide off the frequency where the feedback was positive and the loop gain was more than 1.0. The scheme actually worked quite well. If you opened the gain wide with the feedback suppression switch turned on it simply would not under any circumstances build up into a flat out saturated clipping oscillation. It would sort of oscillate with a warbling sound but it was fairly quiet and nowhere near saturating the output stage. In a way it was a bit like Armstrong's super-regenerative circuit where the loop gain exceeds 1.0 but the resulting oscillation is quenched at an ultrasonic rate. I hope the above helps explain how there can be positive feedback in an amplifier without catastrophic results. These days active devices are readily available, inexpensive, have high gain, and low power usage so there isn't much incentive to derive maximum performance from the minimum number of active devices in today's designs. This was not the case in the early days of electronics! For another interesting method of getting maximum performance from the minimum number of devices check out the "Reflex" radio receiver circuit. Most people have never heard of it, but it was another very clever idea.

[@0MoTheG]

@@Vincent_Sullivan There was a technique called neutralization, also using positive FB to increase gain at higher frequency to avoid roll-off. They used the inverted output, added some more delay such that it was in phase again. Not sure what that did to the medium frequencies, maybe it worked out because their phase wasn't delayed enough.

[@0MoTheG]

​@@Vincent_Sullivan I have done the math now and found that the loop amplification can be infinite if the amplifier has an amplification >1.0 If the gain is 1.1x and the FB is +0.9 the loop gain is 11x. Increase the FB just a bit and the loop gain explodes to infinity. I found that this is how cheap 50MHz BJT FM amps work. They simply feed an insane current into the base.

[@ronjon7942]

Ugh. De Forest is like a bad penny showing up in your videos!

[@echodelta9]

Sad what happened 40 years later to the day of him discovering the regenerative effect, he had to regenerate himself. RIP. Sarnoff was the devil.

[@jrb_sland5066]

Hi Kathy - your diagram shown starting at ~ 08:20 does NOT illustrate feedback, either negative or positive, and your voice over isn't a clear explanation, but makes a number of errors. I fear that naive students may not get the point you are trying to make... It would help, for instance, if you explicitly included the battery providing power to the "wing circuit".

[@howardkranz6679]

I adore Kathy Love's romantic RF oscillations so much, I have the urge to plant resonant kisses all over her lovely neck ( her lips becken ) !!! This is definately mutual inductive--coupling impedence--matching, to be sure !!! 💜💜💜

[@rickg8015]

Major Armstrong who was ruined by RCA?

[@ocayaro]

Armstrong committed suicide because of the American business ways embodied by Sarnoff

[@boblewis5558]

With all due respect saying that someone "invented" positive feedback is as dumb as kids who say Sir Isaac Newton invented gravity. Neither statement is true. Positive feedback has pre-existed in nature ALWAYS. Just like gravity. Armstrong discovered A way to USE positive feedback constructively ... Rare for positive feedback, but he didn't "invent" it!

[@squadleader2010]

Um, fun fact, this was actually invented about 3 years after this invention by a black trans woman.

[@Kathy_Loves_Physics]

Who are you talking about? I’d love to learn more