#356

Inspired by a friend's question from the book "The Art of Electronics", here is a back-to-basics video on the BJT (bipolar junction transistor) Emitter-Follower, or Common-Collector amplifier. Some useful video links for background on some of the topics presented are:
Bipolar C-E amplifier biasing:
• #113: Basics of Transi...
Comparing Common Emitter, Common Base and Common Collector amps:
• #114: Tutorial: Common...
Common Emitter design hints and tricks:
• #273: Common Emitter A...
Vbe Multiplier - and push-pull amp biasing:
• #198: Basics of a Vbe ...
Notes from this video:
www.qsl.net/w2a...

[@ClaudioGabrielCastillo-uq4hj]

Eternal life to this man, if you were on the Titanic the entire crew would agree to give you a boat just for you.

[@erikderuiter7475]

Sir, your video's are outstanding, I know of no one who can explain difficult subjects (for me they are) so well. Your drawings are so helpful! Thank you for taking the time making these video's...

[@PeterGagen]

45 years since I did this at collage. So nice to be refreashed. I now understand better than I did back then, Thanks.

[@utoothheartyeight]

When I was teaching at a local university I did a session "proving" that an emitter follower could have a gain greater than one. Use an audio frequency that can be measured by your standard VOM of the day (20,000 ohm/volt). Measure the base signal, then move the meter prob to measure the emitter signal. Voila, the emitter is greater than the base. This is an "illusion of measurement" When you read the HIGH impedance base, you load it more than you would when you move the probe and read the LOW impedance emitter. This is a well-known phenomenon in the industry but it's a wonderment to a new student.

[@w2aew]

Teaches a valuable lesson of the effects of making a measurement...

[@omniyambot9876]

is it the oscilloscope that consumes current?

[@MD-sj2dn]

Great video Alan, I’d have gone the EE route if you were one of my early teachers. Really enjoying finally understanding a lot of details my young mind didn’t get, now years later making sense through your videos.

[@mykedindeal]

Me too👍

[@tav9755]

"Basic" turns into "Simple" (kindof) once a good teacher such as you explains it. Thanks for sharing and the good paper- and camerawork

[@whippoorwill1124]

Another superb video, Alan - thanks! I think it'll become one of your classics. That question about clipping, from The Art of Electronics, has caused a lot of head-scratching. Much better to see it solved as you've done here, by equipping people with the background, than by addressing just the question in isolation.

[@fourfortyroadrunner6701]

I'm 75 and enjoy some of your videos. Always informative. I was first licensed amateur about 65 in hi school, Navy ET-R from 68-74 (maintained GCA RADAR and TACAN) and have been in and out of electronics since, but I'm way way behind on modern stuff.

[@johnkyingilisi4395]

The way you explained in/out impedances was so delicious I could almost eat it. Well done. With love and respect from Shiraz.

[@AcctistaZ]

I’ve been watching you since my 2nd year of electronics engineering and now as a young professional with 2 years of experience I still watch your videos to remind myself what I’ve learned. Analog is my passion but unfortunately high speed digital and power is my day-to-day. Thank you for doing this for all of us. Ipsa scientia potestas est

[@pa4tim]

When I started I had troubles understanding this because on a scope you see the base voltage versus emitter voltage (or in other cases the collector voltage). The theory also talks about a voltage follower if they talk about a follower. While it is a current driven component. Building things to experiment did not clear up things. For me all became clear when my mentor asked during my first serious repair (old scope) 'what is the voltage over that resistor' . Why ? I asked, 'so you know the current' he told me and looking at current gave me so much more insight and information. The problem it is that you need expensive probes if there are no resistors. Measuring current is often not easy. When I got a Tek 6042 current probe I started to look at the current much more often. Nowadays I repair "unrepairable" electronics for a living and it makes things so much more easy if I look at current. I have a nice collection of rather special current probes for that (for most flux gate and hall based, to I can use them to 'see' current in traces ). Using them is often not easy but it can tels a lot. For instance, if you measure 0 volt this can be a short with some current or just no voltage because there is no current. Looking at the current can make repairing a lot more easy. I am talking by the probes that can measure current in traces. The normal "around a wire" probe needs more work. What I sometimes did was lifting a pin and adding a piece of wire in series to connect the probe or cut a trace and solder a wire over it to connect the 6042 probe. Btw, a thermal camera can also be a great way to see current, because everything that conducts has resistance and warms up. It is very cool to watch traces and components on a cold PCB during start up. You see the smallest differences in temp. In a Fluke scope meter I could see the power asic come to life first and also the problem, one of the pins being short. And this before things start to smoke :-)

[@fmashockie]

Trying to work through the 'Art of Electronics' to better understand the theory. I've learned most of what I know about electronics through repair, but that isn't enough of its own. Your videos always help me to better understand concepts addressed in the 'Art of Electronics' that for some reason I feel they didn't make clear enough to me. Thank you so much!

[@HM-Projects]

Man, missed these back to basics videos. Thanks!

[@change_your_oil_regularly4287]

The hand drawn diagrams are clear and well done. They really help clarify what you are talking about and make it much easier to reproduce for older simple minded low time hobbyists like myself 👍

[@qzorn4440]

The Emitter-Follower is a simple circuit for running low impedance instruments from a weak signal. Thank you very much.

[@emailuser3869]

Alan, deep gratitude for your hard work to teach we knuckleheads. I was stumped re: how to get the signal output from my LimeSDR to drive a pallet MRFX1K80H until I stumbled upon your material and a few other generous folks. Hope you have a good field day. 73's W6MGV.

[@sebastian_harnisch]

With every video I'm impressed that you always maintain such a precise language and use the proper terminology and it's still easy to follow (probably rather because of that). Hats off! I'm an EE myself (power systems though, not that experienced with analog circuits) and would love to see a video (series) of a slightly more complex analog circuit - e. g. a hifi amplifier or a higher voltage cascode amplifier - and follow your thoughts from start to end: How do you analyze the problem (starting with the requirements), how do you come up with a circuit (what are the methods to come up with a good design), the math, (maybe a simulation?), and finally a prototype/measurements/refinements. I know it's a bit much to ask, but I think I might not be the only one that would really appreciate something like this. Again, thank you for sharing your knowledge!

[@shabeesatsangi]

amazing solved my lots of questions, thanks again you are the best teacher i have ever seen. Thanks from bottom of my heart.

[@Infinitesap]

Probably your best video till now. Really appreciate your in depth explanations. Especially I finally understood your definition of impedance. Great with the math and circuit explanations. Thanks a lot! :) I hope you continue soon.

[@t1d100]

Oohhh, sooo goood! Thank you, Alan. I will have to watch it, several times, to extract all the goodness. Cool!

[@raygosper1656]

Another very good video Alan, one aspect of circuit design that I see is left hanging is the calculation for the coupling and de-coupling capacitor values. I feel that would be of benefit to many involved in the practical side of circuit design.

[@w2aew]

Good suggestion - I'll add that to my long list of potential future topics. Short version: Coupling capacitors are generally chosen based on the impedance of the circuits they're coupling together, such that the capacitive reactance is substantially lower (4x lower or more) than the circuit impedance at the lowest frequency of operation.

[@W1RMD]

I really enjoyed this. I've been into ham radio for almost 30 years, and I've only REALLY started learning electronics for the last 2 years. Thanks for this and 73! I hope to hear you on field day. It's time to dust off the old cw key!

[@w2aew]

I hope to get on during Field Day, but not sure yet. I'll be at a family reunion in upstate NY, but might be able to find time to setup a rig for some operating...

[@W1RMD]

@@w2aew have fun!

[@nickwood4367]

Thanks so much Alan. Another brilliant video. I love the mixture of schematics and theory coupled with practical demonstrations. It really helps to see the results on the scope and then to understand why I'm seeing what I'm seeing. Thanks again. 73 Nick M0NTV

[@TheCondoInRedondo]

A more tangible way of looking at the input impedance is to consider that the base current must "share" Re with the collector current. I.e., flip it from a question of impedance to a question of conductance. That emitter resistor becomes (Beta + 1) times less effective a conductor of the base current because Re is also conducting the collector current through the same bottleneck. If the emitter resistor is thought of as a toll booth capable of processing N electrons per picosecond and the base current on-ramp is funnelling base current electron traffic into the toll plaza where those base electrons are blending-in with Beta lanes of collector electrons, that lane of base electrons will be processed at a rate of N/(Beta+1) electrons per picosecond. To the backed-up base electrons waiting for a chance to enter the toll plaza, they perceive the toll booth as being (Beta+1) times slower... which amounts to being perceived as a higher impedance by a factor of (Beta+1)

[@srikarvemuri7859]

Great Video Alan, the explanation was silky smooth

[@X-OR_]

Your Illustrations look like it straight out of the Engineer’s Notebook by Forrest Mims. Also, Great Video !

[@w2aew]

That's quite a compliment - I learned a LOT from the Engineer's Notebook series in the late 70s....

[@X-OR_]

​@@w2aew So did I ....

[@chrislamb4723]

This is outstanding! Please consider making a series of this for all the other transitor configurations.

[@w2aew]

I do have some videos on other configurations - check the links in the video description.

[@jotnarymir1393]

Just started going through douglas self small signal book and one of the first chapters is and emitter folower. He goes through changes to improve performace like adding a current source instead of Re. Good read if anyone wants more info on the emitter follower. Thatnks for the video

[@reedreamer9518]

I haven't got to The Art of Electronics yet, I'm still studying "Electronic Devices" by Floyd. Maybe when I'm finished with Floyd I'll move on to A of E. Nonetheless, this video is right up my ally. ; )

[@mtkoslowski]

A of E is pretty much essential to your continuing electronics education. Make sure you include it.

[@craigdupree1687]

@@mtkoslowski Meh. The book is so-so. But the lab manual especially the newest version with the gold cover is well worth the purchase.

[@kneecaps2000]

A of E is a great reference not a read front to back.

[@martinmartinmartin2996]

In order to simplify calculation the emitter current in 5:09/21:33 as approximated as Ie ~ β*Ib where as the the actual Ie =(β+1)*Ib , but even that does not accoumt for leakage current Ie]bo The point I a trying to make is that junction transistors are complicated so calculations such as shown in < #356 Back to Basics > are REQUIRED to get reasonable answers . A power transistor such as 2N3055 has βmin = 5@ Ic=10 amp ; in that case the Ie Ie =(β+1)*Ib is REQUIRED !

[@nomen_omen]

Amazing I could understand more than during my studies at university. Thanks a lot!

[@OctavMandru]

Damn Alan, I forgot how high the level is on your lectures on such "simple" topics. I need to learn more. I now realized how much I missed the series

[@airmann90]

My favorite type of your videos, back to basics. Love it. Thanks

[@Retinatronics]

Thank you Alan for the video!

[@andrerieck3801]

good to see a new "Back to Basics" video ... thank's a lot for taking the time to do these videos!

[@tonyd1149]

Thank you so much for making this video and posting it. (I love it that you use an oscilloscope to show the cause & effect). God[Bible] Bless.

[@frankdogui7195]

Excellent explanation!

[@ThePaulbilek]

Excellent explanations. I could follow it. And enforce my understanding in simplistic way. Kudos

[@rickwise9910]

Wonderful, as always!

[@riccardopapi5992]

This is an amazing explanation and video. But I was convinced that at higher frequencies rather than the Beta we should be considering HFE. I would really love to see from you an explanation on what happens at frequencies higher than just a few Kilohertz.

[@ronilif]

Fantastic! Many thanks... :)

[@Tech2C]

Brilliant

[@amanuelnigatu4621]

thank you for taking the time making this video!!!thank you sir

[@krzysztofsoja5301]

Thank You for sharing knowledge and experience :)

[@nightly_builds]

Quality content! Thank you

[@overengineeredinoz7683]

Great video. Explained it so well. Thanks.

[@glenmartin2437]

Thank you. This video helps me understand what I did not quite get before. Wish I had gotten a EE after my bacculaureate in chemistry. Again thank you. N0QFT

[@darrylgodfrey9604]

An excellent explanation - thanks.

[@TheMechatronicEngineer]

I love these kind of videos. You can teach very well.

[@eldervieira85]

Love this subject, thanks!

[@tohopes]

i sort-of understand. i was looking forward to hearing the little speaker noises! you dashed my hopes!

[@MrWasim100100]

back to basics videos are awesome you explain very clearly ... thank you Alan

[@aaronr.9644]

really cool

[@CalebePriester]

The basic stuff is the most important stuff because it's the foundation of everything. If you don't know the basics you won't get far.

[@marmurua]

Every time so much enjoying your videos! Thank you!

[@21thTek]

Great , a masterpiece !! 💫

[@elkrutarth]

at 20:22 transistor is in cut-off region, now it has just became a simple resistor divider network where -10volt droup equally through both series resistors Re and Rl 5v drop through Re and 5v drop through Rl (simply, just erase/ignor the transistor and positive supply we just end up with -10volt and two series resistors Re and Rl)

[@randygraff2825]

I love your videos.Very informative .

[@waynegram8907]

If you didn't have the datasheets for the Transistor or FET and didn't know the rated wattage or dissipation wattage of the part number device, what can you do to find the rated wattage and dissipation wattage? When biasing a transistor or FET you adjust the grid/gate voltage until the output waveform is clean ( not clipping ) and symmetrical, but the common problem is you can have a clean not clipping symmetrical waveform but when you compute the dissipation wattage is either below or above 70%. Example, the transistor or FET wattage is 10watts but the dissipation wattage you want it at 70%. So 70% of 10 watts is 7watts. The idle bias current should be adjusted so that the dissipation wattage is at 7 watts ( not at 10 watts ). The dissipation wattage formula is = idle bias current X Collector/Drain Voltage. The common problem is that when adjust the idle bias current until you get a clean no clipping symmetrical waveform is that the dissipation wattage is way below or way above 70%. To fix this problem you would need to do what?

[@dukeibzusa]

Another way to understand the clipping is doing a thevening at the emitter circuit, then your thevenin voltage will be -5V, as long as voltage at the base is more than that ( plus 0.7) the base emitter diode will conduct and follow the input, when the voltage at the base is less, base emitter diode will be open and the output will be only the thevenin voltage

[@yytvo212]

Alpha bench demo and breadboard work. First time make clear the input and output impedance for transistor circuit.

[@malcolmhodgson7540]

This was just perfect. Thumb and bell!

[@byronwatkins2565]

A very good job!

[@RixtronixLAB]

Nice video shot, keep it up, thank you for sharing :)

[@markhall3323]

Very well explained and presented thanks

[@yoksel99]

Great way to follow up to the more advanced stuff: explain why emitter follower is known for its tendency to self oscillate in certain conditions.

[@w2aew]

Well, that certainly takes it beyond "basics". The more important question, is how to prevent it... ...in that case, ensuring that at least two of the three terminals of the BJT have some small series resistance connected (like a 100 ohm in series with the base, for example), is usually sufficient to prevent self-oscillation.

[@yoksel99]

@@w2aew I thought you already had a theory video about Colpitts oscillator. Which is essentially what emitter follower turns into when it oscillates. But then I found that video - it was rather about building it.

[@objection_your_honor]

May I request you do a video series analyzing the schematic circuit of any battery operated AM/FM transistor radio from the 70's? Before everything went the way of ICs. If someone else has made such a video, please leave a comment.

[@w2aew]

Can you point me towards a schematic for a good example?

[@objection_your_honor]

@@w2aew I have no idea. Maybe HeathKit?

[@leiferickson3183]

Nice! Impedance matched! I will tell you the initial plot was a bit of a head scratcher.

[@manub6]

Great video, thanks a lot. :-P

[@ornithopterindia]

👍Thank you sir.

[@saviourainebyoona1170]

this video is too outstanding ..... and for that i have hit the like button 5 times.... thanks bro👍👍watched this from Uganda

[@jogreeen]

these videos are awesome.

[@simonyoungglostog]

Great explanation.

[@BrendaEM]

Could you please, do a Back-toBasic video on DC-Coupled amplifiers. There's scarcely any information out there about them, except for valueless, untested schematics. I would imagine that they could be driven from a Op-Amp, with a trimmer to set the zero crossing. I had thought about TIP120/125 complimentary pair. Perhaps MOSFETs would work, but the may have a smaller linear region, and I am not sure which are a good pair. It would seem that there would need to be a push-pull, kind of thing, as a DC coupled amp cannot be capacitively coupled. Perhaps I will do some experimentation. If I could get it going with a LM358, then perhaps I would risk my TI OPA amp--all for a function generator AMP.

[@mike-PlatinumAudio]

Great video :)

[@chuckcrizer]

The Art of Electronics is TERRIBLE for learning about basic electronics. Older texts from the US Navy are EXCELLENT for explaining how these circuits work.

[@absurdengineering]

TAoE is a text for physicists and other physics-enjoying folk who want to make some electronic instrumentation. It is not meant as a beginner’s textbook.

[@aaronartale]

I've never been very fond of that book. I found older textbooks by Malvino to be much more useful and illuminating

[@romancharak3675]

Wonderful stuff. Thanks, Alan!

[@JurekPrzezdziecki]

Great as always. Could you please make a video about "frequency dependent negative resistor"?

[@cnccontroller]

The oscilloscope probe as a pointer - original!

[@caprisundad]

Fantastic content. Please keep it up.

[@ralphburroughs9218]

Very well done, and it is easy to follow your topic.

[@maksberlec4832]

Very good. Thank you!

[@viswesh713]

Nice video, thanks. Every time I see that scope. I feel like I won't be able to get one. need to upgrade my TBS1072

[@6alecapristrudel]

Now the question becomes: How do we stop thermal runaway in the push-pull pair if we want to drive heavy loads with beefy transistors? Emitter resistors only help so much if you want high current before starting to sacrifice output voltage, but you need them. More parallel less beefy transistors?

[@anthonya7990]

Alan, Your back to basics series are great and I enjoying them even now(at 58). Wonder if an old vintage Germanium, like AF116, AC127, 128, etc, say 6 or 8 transistor Radio were completely convered to Silicon transitors such as the BF and the BC series, how would be to re bias the whole stuff and get it to work even at the 1.5 Volt , 1 cell. as some of those old pockert radio did. Or would we be forced ot use only 3 v ( 2 cells) ? . None have done it on you tube & sure there arre a lot of the Radio boys ( they too as vintage as the radios !!) itching to do it just for fun & learning.

[@w2aew]

Hmmm - good question. I'd have to study the schematic of one of those example Germanium sets to be sure, but I would imagine if it can be done, the main thing you may lose is headroom for the signal levels.... My gut says it is probably possible, but would need further study to be sure. The audio output stage will probably be the thing that will make it or break it - if it is a push-pull design... then you run out of biasing/signal headroom with the 2-3x larger Vbe.

[@anthonya7990]

@@w2aew , Alan, I understand, as you say it would be doable, for the moment allowing for loss of signal headroom, but the really interesting thing as you pointed out, the final amp & more, biasing, so to operate it at 1.5 Volts (1 Cell). Alan also, just a thought, any hope of modding the Driver/ Op Transformer Winding to at least succeed. I am sure that you already hav quite lot of ckts in your mind. Just to suggest, I had the such as the 1Cell in mind, like the vintage 6 trans, 3/2 Diode !! design, Expo 70, also sold as Emperor AS2, Ajax Dandy, Seacom, etc. NPN trans 4 qty and The Audio Final are both PNP trans. The 2 bias Diodes, perhaps can be replaced with one Silicon. The Ckt, is on Radio Museum site ( I can email ckt pdf if you need).

[@w2aew]

@@anthonya7990 email is (channel name) at a.r.r.l dot net (remove the periods from arrl).

[@bitlong4669]

Impedance always messes me up. I get the circuit going, then attach 8ohm speaker and the wave forms collapse, and I hardly get any power through them while the transistor becomes a miniature space heater. I get that class A is like 20% efficient, so for 1Watt speaker I expect the circuit to burn through 5 watts. I get more like 1% efficiency tho :( . I’ll get it one day :)

[@MrNetmask]

Can you please make a video on Feedback Amplifiers ? PLeaSE :)))) Or High Frequency analysis of bjt/jfet amplifiers :D Great video as always! Regards, Blaz

[@larrygrant8818]

Perhaps you have covered it in another video but I have never seen a description of possible damage to split power supply op-amps (+-15v for example) if both power rails do not track on power up or power down. What if an old mechanical switch doesn't connect one or the other rail and only one side of the power supply is applied to the op-amp? Any damage possibilities? What if the two power rails are achieved in different ways (different type of regulators maybe) and do not track evenly on power up or down? Any damage possibilities in that?

[@w2aew]

I've never observed any problem with split supply sequencing.

[@Shawn_White]

Where can I get those wireless scope probes pictured at 1:30

[@sumarouno]

good teacher

[@elijaheisenman6903]

thank you sir

[@stnwrd]

I am looking for a circuit diagram to receive a 27.185 Mhz signal.

[@Drforbin941]

great video thanxs

[@mickblackmore290]

Hello; Alan you have one of the best channels on KZbin. I have a question. you keep referencing ground at the bottom of your screen, is this just offset? I have tried to copy a lot of breadboard circuits but the ground and getting the sinewave to move around the ground is killing me. Do I not know how to use my scope? Is it the trigger coupling AC/DC set wrong, or channel coupling wrong or does my scope just not do this? Rigol MSO5272 / MSO5354. I really hope you can answer this. note I have watched at least 75% of your videos so far. As everyone has already stated, you are a most EXCELLENT! instructor.

[@w2aew]

The Vertical Position control will move the reference point (usually ground) on the scope up or down on the screen. Vertical Offset on the scope changes where the reference point is. Neither of these changes the *actual* signal. In order to see the signal and where it is with respect to ground, be sure to use DC coupling on the scope.

[@mickblackmore290]

@@w2aew Thank you Alan, for your quick response. I figured it out, your answer sent me back to video #238 Vertical Position and Offset explained. Apparently on my Rigol scope offset voltage is always on! I found a setting in the channel setup called EXPAND (center or ground) setting it to center lets me move the sinewave around the offset voltage, that had been set at ground preventing me from doing this before today. I have only had this scope for a couple of months, I'm learning new things all the time. I regret not having the money to get a Tektronix scope that does everything this Rigol MSO5345 does. You and others have so many instructional videos on the scopes. Rigol are far and few in between compared to Tektronix. Thanks again for the education. Wishing I lived next door.

[@user-jp4lr6lv1l]

Hi, I wanted make a circuit with a transistor which is better for video amplifier circuit :emitter follower or collector follower.... . thanks

[@w2aew]

These are very different circuits. An emitter follower has no voltage gain (only current gain). Not sure what you mean by "collector follower".

[@gower1973]

Can you go over other types of transistor configurations?

[@w2aew]

I briefly review the other BJT amplifier configurations in this video: kzbin.info/www/bejne/sInLZpqDmJuer7c

[@sammin5764]

🌟🌹🌟

[@ltlt6117]

Why in most rf common emmiter amplifiers we add an inductor to collector for increase impedance at the collector??why we don't use just a resistor

[@w2aew]

Usually to provide high gain at RF while being able to set collector current at a desired bias condition (where a resistor might cause saturation)

[@ltlt6117]

@@w2aew thanks.

[@Qwezu4028]

thx bro

[@TKomoski]

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