TSP #23 - Tutorial on the Design and Characterization of Class-B and AB Amplifiers

In this episode Shahriar continues his investigation of discrete Bipolar amplifier design. The advantages and disadvantages of Class-A amplifiers are explained. The conceptual schematic of a Class-B amplifier is presented which leads to the introduction of Class-AB amplifier circuit to overcome the 'dead-zone' impairment of a push-pull Class-B design. To further improve the Class-AB amplifier and lower its input impedance, a final Class-A followed by Class-AB amplifier is presented The component parameters are calculated and the schematics is explained in detail. All circuits are then implemented on a breadboard and tested both in the time domain and frequency domain. All schematics can be downloaded from The Signal Path website.
Please note that in the schematic diagram the diode D1 is backwards! In the measured circuit the diode is placed correctly which is why the measurement results are correct.
The Signal Path Blog
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[@Thesignalpath]

*Please note that in the schematic diagram the diode D1 is backwards! In the measured circuit the diode is placed correctly which is why the measurement results are correct.*

[@ve2zzz]

Also, in the X-Y scope, Y is input and X, output.

[@7137271]

I thought that looked funny, and kept trying to figure out how that was going to work.

[@andr27]

Video posted 8y ago and only 10 months ago errata? :)

[@SJayanth]

The reason for no output at 5v p-p input I think is because the current through the first stage of transistors went too high and the gate voltages for the next stage of transistors fell below 0.6, thus, there was no output. Anyway, we started to build amplifiers right? but, neither of the 2 designs could output a voltage higher than the input, then, how can we call these as amplifiers? Please, reply quickly. And thanks for the video.

[@torkalovolodymyr5097]

@@SJayanth It amplifies current. Before it you need add stages which amplify voltage - in common emitter configuration.

[@WhiskeyRichard.]

You have no idea how long I've been looking for some kind of tutorial on amplifiers that doesn't just mathematically develop the small signal model. Thank you kindly, and to boot, it's super straightforward and clearly explained.

[@WhiskeyRichard.]

Why doesn't this guy prof Oh wait stupid question

[@jonothankaplan]

@@WhiskeyRichard. hi

[@bloguetronica]

How come have I never seen this! This is a great "must watch" video and, despite its age, should be recommended by KZbin.

[@jonramsey6348]

Yea totally..... finding this video on KZbin is like finding a diamond in a pile of shattered glass

[@neeleshkumar8406]

Same here ,

[@Hexor1211]

This video and TSP #15 are the best intro into audio amplifier design I've seen. Many college electronics Subjects fail to explain it so simple.

[@graemelaubach3106]

You are a better teacher than all of my profs combined, you make concepts so easy to understand and more importantly you make it fun because you're just an awesome dude. I wish you the best in these trying times.

[@Thesignalpath]

Thank you!

[@omniyambot9876]

wholesome comment lol agree 0:09

[@Thesignalpath]

I appreciate your enthusiasm. I am trying to cover a wide audience. Don't worry, there are tutorials on DSB and SSB mixers and complex constellations for wireless transmission which will target a more advanced audience.

[@BobSolimeno]

WOW! What a great tutorial - very clearly explained, spot-on comparisons of the different amplifier designs, and beautiful hands-on lab experiments to demonstrate relative performance. And what a neat and organized lab you have!! Thank you so much!! Can't wait to dive in the the many, many other videos you have published.

[@Thesignalpath]

This is a conceptual schematic diagram. Both diodes D1 and D2 require a bias current to maintain their junction voltage. But, perhaps more importantly for you to consider, my channel has received about 1300 comments so far. You are the first to use obscene language. My channel is a place to exchange information politely and I would like to keep it that way.

[@AltayBrusan]

Thanks for your sharing. I am really disappointed with those that do not consider the value of these videos for the others. thanks and please don't loss your motivation. Your channel is valuable.

[@ernestb.2377]

Very nice short tutorial. 👍 I also see that here, 11 years ago, you have significantly less equipment compared to now 😁 Keep up the good work!

[@thebrakshow7415]

Great tutorial! your D1 is oriented incorrectly and signal needs to be applied to each base of the transistors and not to the middle of the diode network. Adding some input caps to kill any dc is a good practice. Also, consider using a Vbe multiplier for the a-ab amplifier. It saves you a transistor plus you can use a potentiometer to dial in the biasing current.

I just wanted to write this as well. In Class AB amplifier diode orientation is wrong.

[@simoncheung2427]

i can’t believe after i watch a whole video and looks everything make sense

[@m1geo]

Just came to write this! 😂

[@VLWhite88]

The answer of the puzzle: You have clipping, your input signal exceeds the supply voltage

[@andymouse]

Correct ! no prize now though Lol ! 10 years later !

[@Thesignalpath]

Class-D is a good option. I may do a tutorial on that later.

[@generatorlabs]

Very good video. The tutorial was not overwhelming. A perfect mix of theory and practical hands on visual elements to glue it all together. Thanks for your work.

[@MBAgeeJr68]

Thank you for this clear tutorial and refresher for me. I am jealous of your collection of test equipment!

[@relaxationTherapy001]

awesome video, shahriar...keep up the good work.. answer to the quiz is "with increasing input voltage u r saturating the transistors gradually that's y output is getting clipped off. after all you'll never get output higher than the driving supply voltage.."

[@MarysKiril]

About that puzzle... it seems that the higher the input level, the closer the emitter potential of Q1 gets to V+, so less current is available to go into the base of Q3 and so the output levels off, falling behind the rise in input potential. Love your videos by the way, the topics, the format, the explanations, everything. Thanks a lot for making them.

[@andymouse]

And 10 years later...The signal is clipping and your correct ! unfortunately you have no prize !

[@MrAtomicLlama]

I can't commend you enough for putting these videos together. They're tremendously helpful for somebody like myself - trained as a scientist many years ago - rediscovering the joys of electronics. Thanks, and best wishes!

[@DarkInsanePyro]

Just want to state my appreciation to you developing these videos. It is a great way to learn (or review in this case) electronics material. Keep it up!

[@bassk94]

just learned more in 39 minute video then in 16 weeks of advanced electronics taught by some old fart with no clue Awesome video Thank you

[@sudharsansuresh170]

The way you taught this is just very simple and straight forward, I am mech engineer looking to build my own class ab amp for gaming in my home! Learnt many things, Ty!

[@wombatau]

That was the best and clearest explanation of class B amplification I have ever seen

[@serviceshoptoday]

I see some very good comments below. Early in the video the dead spot is caused voltage consumption called (Diode Drop). For the "Question" I agree with the comments bellow. Supply voltage, headroom, saturation, and gain. Very good information. Very well made and thought out. Feel like part of the class. Thank you!

[@jimshaw4424]

I really loved this tutorial. You have a great way of explaining electronic circuits. Keep up the good work.

[@dataviews118]

An outstanding and comprehensive tutorial on amplifiers. Thank you so much.

[@johnadams9041]

Thank you man for the video! This is the best explanation what I ever see. I am not professional and I sow so many videos trying to understand the bias and the class of amplifiers.. Thanks to you know finally understand a lot!. Thank you ! You just have big talent to be a good teacher! God bless you !

[@raulvalenzuela4887]

I was just curious about A and AB amplifiers because I play electric guitar and I found this video. It was still fun to see the explanation and the experiment. Thanks.

[@CmAektP2jxrGAb7]

You deserve more subscriber friend, and I wonder what kind of person are those who disliked this video, its one of the best tutorial explained & demonstrated in details! Thank you for share your video and knowledge.

[@DanielFries]

Thank you very much! Very clear, I'm going to be digging around your videos all night now.

[@FFcossag]

What a fantastic video! The way it's put together makes it very easy to follow and a joy to watch.

[@antoniocastrocarballal4051]

Many thanks for the explanation, Shahriar, absolutely clarifying!!!

[@vispi1944]

Thank for the excellent video presentation. First explaining the theory of various classes of amplifiers and demonstrating their workings on the oscilloscope gave a very good idea of their positive and negative characteristics. Please make a video on how to design a working amplifier, explaining how to calculate and choose parameters of various components. With such knowledge, as yours, you can write books for hobbyists and professionals. We need more videos like this.

[@einball]

He talks about turning off the bias voltage when the input equals zero. -- Great video! I really liked it! Made me happy about the next few months of studying :)

[@peep39]

Excellent resource for someone graduating from class A understanding into class B (like me). Thanks for posting this

[@UpcycleElectronics]

Wow, you answered so many questions I had about the interaction of input impedance, the buffer, and preamplifier stage of an amplifier design. Thank you. (liked/subscribed) -Jake

[@rcleveacp]

Another excellent video! Once again, I appreciate all of your hard work. Very high quality instruction. I believe the answer to your quiz question is the distortion is caused because you are using 2.5 volt supplies. The distortion is caused because there is not enough voltage to drive the transistors through the complete cycle. The transistors are clipping the input signal when they are saturated.

[@realchristopher4334]

This is a very old video! But a very good old one! Thank to God of Science that my 'luck' is good! This is the best ever 'tutorial' on amplifiers! Shahriar's A + AB design actually works and works frighteningly well! I mean, I simulate it. I have nearly zero hope that simulators (I use Multisim) are accurate, let alone precise, but when I match those measured and simulated numbers they are frighteningly similar! I say this because (simulated) component data are far from realistic. I do not mean the intricacies such as temperature, vibration, and so forth. I actually mean basic characteristics such as saturation, breakdown, and so on. Most op-amp, for instance, have zero limitation, you can feed them kilo volts input and kilovolts of power supply and the the output signal still looks normal! Same goes for diodes. I now wonder how to actually make it an amplifier, which component to change. The circuit is not exactly an amplifier but a current/voltage follower. There is no amplification.

[@n1gdawg]

I really wanted to hear some audio through each! Great video as always.

[@CoolDudeClem]

Good video and well explained. My guess for the part near the end of the video is the input signal becoming to strong driving the transistors into saturation, but I could be wrong. Right now I'm actually listening to this through a valve amplifier I made!

[@permitivitym4318]

holy cow with all those equipment's. you have more stuff than what I have in my bench at work

[@permitivitym4318]

that's a $10,000 scope

[@LolanBadeauxMakes]

yes he has an awesome bench setup. i so jelly =)

[@alicekichlu215]

I AGREE!

[@Hanz_theengine]

Brilliant, absolutely loved it! Much better than my teacher...

[@eduardorubin7661]

You just saved my thesis. Thank you!!

[@SJayanth]

The reason for no output at 5v p-p input I think is because the current through the first stage of transistors went too high and the gate voltages for the next stage of transistors fell below 0.6, thus, there was no output. Anyway, we started to build amplifiers right? but, neither of the 2 designs could output a voltage higher than the input, then, how can we call these as amplifiers? Please, reply quickly. And thanks for the video.

[@MinnoW12]

Im liking the video just by reading the title and knowing the level of detail you go in to!

[@s.r.m.m8789]

چون تقویت کننده شما در اون ناحیه به اشباع میرفت.تشکر بابت مطالب اموزندتون

[@andymouse]

!0 years later and I answered some of your students to say they were correct Lol ! still pure gold....cheers.

[@simonyoungglostog]

I love how you think. It's very helpful for me to hear you think out loud. I have a question about using a PNP with a NPN to match a voltage drop? I hope that makes sense..

[@LolanBadeauxMakes]

you have wonderful resources and equipment. taking the time to explain each class and showing the effects on the oscope are priceless time savers for us learning. thankyou so much

[@SamWalsh1]

Thank you so much for these videos, helping me through my University course. I didn't even know what a spectrum analyzer did before watching this. Now I want one!

[@chupascz]

You are awesome teacher! Thousand times better than all my high school el. teachers. Not to mention I have to translate from english. Your 'lucky bamboo' is larger than the last video.

[@4833504F]

hats off for giving such a good and long explanation in a single take!

[@Thesignalpath]

The spectrum analyzer is AC coupled at its input. Furthermore, the output load of the amplifier is 10Ohms. Therefore, adding another 50Ohm in parallel has minimal impact.

[@lonewanderer01]

So is it that you've reached saturation in the transistor?

[@ve2zzz]

Saturatio will be reached if the input signal (peak) reaches the NPN' collector's voltage + 0.6 volt (an PNP collector -Vcc + -0.6V). In the deadzone, both transistors are in cutoff region.

[@x86cowboy]

These are great videos by the way. Honestly, one of the best channels on KZbin. Please keep making these great videos!

[@zaroaster8348]

I have recently discovered your tutorial in the internet they are all very informative thank you for taking time to explain complex electronic concept all so very nicely I am very thankful. My answer to your quiz question at the end of the video about the non-linearity region of the signal is; I think the bandwidth limitation of the transistors in the circuit.

[@raviduprasanna6401]

i don't know how to appreciate to your time and effort to do this tutorial for us

This was a great video! Are you planning on making videos about other amplifier types as well? That would be great!

[@mbabaeevideos]

Thanks Shahriar. But you've placed the top diode D1 in the first AB amplifier schematics in revere polarity.

[@francomarianardini681]

very nice tutorial! congrats for the clarity. question: what screwdrivers are you using? (the ones on the bench)

[@weststarr2046]

Another awesome shot my friend, specially when it comes to measuring. Good stuff around there... THANKS a lot for sharing your knowledge and the way you explain it. BLESS 🎩🙏🏾...!!!

[@musicmakelightning]

I'm guessing you were a great TA as a PhD candidate...wish you were around when I was an undergrad (though, you probably weren't yet born) ;). You teach with great clarity and enthusiasm and remind me why I was so excited to become an EE...which is really uplifting in my, ahem, older years.

[@jtb8631]

great video! very informative, no BS, visual representations are good, well explained and to the point. A+

[@horaciohherasq6526]

Do you always need a dual power supply to power an AB amplifier? I'm a newby at building these amps and your lessons are pure gold.

[@rossrogers2979]

Thank you for taking the time to make this exceptional tutorial.

[@jimm2099]

Shahriar, Very good videos, I wish I had instruction like this in high school "Electronics I" instead of electron hole theory in the early 80's. Not many high schoolers back then were designing semiconductors. I like you very practical theory and demonstration techniques in all your videos, especially the analog circuits Thanks very much!!!

[@nazanwar6469]

Thank you, my friend, Shahriar you saved a lot of time for me

[@stever128]

Great presentation. The answers to the question are listed below. Keep these wonderful videos coming.

[@disneyjoe7]

Your spectrum analyzer video where the XY isn't linear is due to the amp output clipping. This is due to the fact the power supply is 2.5v +/- and the output is 5v p-p.

[@rickysambora8321]

Nice equipment there. May i know the drill stand that you got there (or something similar, I guess yours it's no longer stocked)? i have a similar drill too like yours.

[@davidcottrell1308]

Very thorough and to the point...thanks!

[@johnadams9041]

Hi sir ! I have a simple question about the hFE of the final transistors of an NSA amplifier operating in the parallel SEPP circuit . Particulary I am talking about Pioneer A9 ,which is working in parallel SEPP with a high frequency DC servo bias. So the question is should the end power transistor have the same hFE? Likes the the class letter and likes a digit number to prevent the distortion?

[@jamescoughlin7259]

I am building an audio amplifier with a transformer coupled output. Would the Class A amplifiers before the AB be necessary (for a high input impedance) if the output was transformer coupled? Fantastic video!

[@todkapuz]

I remember this from when I took my amateur radio tests.... 15-20 years ago.... but never have actually seen it real-life. Very cool.

[@ClaudioArrau]

Why did you use power transistors Q1 and Q2 at the A+AB schematic? Do they need to be "power" bjts, or a simple smaller signal bjt with bigger hfe could still work ?

[@Thesignalpath]

Yes, they come with the scope as part of the probe package.

[@paulrajmarichamy4309]

very good explanation.. your lab well organized,

[@213pool]

Insightful and inspiring. Thanks for sharing

[@Thesignalpath]

Glad you enjoyed it!

[@lukesnake123]

this video is absolutely fantastic. brilliant, really.

[@Thesignalpath]

The spectrum analyzer can have up to 30VDC at its input without damage. That is because it is AC coupled. Furthermore, it can see up to 30dBm of input power which I could not generate.

[@zohirbailiche7322]

Dude.... The diode you were pointing at at 11:17 is in wrong direction... Doen't it?🤔

[@saviourslumber]

Great work! Comprehensive and easy to understand! Subscribed.

[@martinda7446]

Just a minor point, the distortion is called 'crossover distortion' and the history of manufacturers to overcome this and other areas of harmonic distortion is fascinating with the THD wars and giving us 0.0001% thd and below. 0.1% is considered ideal and was given us first by H J leak in his ''point one'' amp series in around 1948 (from memory)

[@TheLawnWanderer]

+mart fart push-pull ultra linear valve amplifiers are no joke. The circuit family is called Williamson amplifiers.

[@martinda7446]

TheLawnWanderer Leak predated the Williamson and I dont beleive it was ultra linear design I think williamson used ''overall'' negative feedback as a first and envisaged a triode output not pentode/tetrode. Blumlein invented ultralinear in the 30s along with many other great discoveries.

[@TheLawnWanderer]

mart fart point one wasn't UL, but triode connected instead. The later ones were UL though, really no point to not use UL. Pretty much all modern amplifiers in that style are UL with the option to switch to triode with a simple switch.

[@yxk081]

CHEERS MATE GREAT ONE. Significantly helpful. If you could have some thing similar on class D, would be great.

[@Standing-Wave]

Absolute pleasure learning from you!! Great job!!!!!!!!

[@Thesignalpath]

You are right. This is a simplified diagram. I have added a note the video to clarify that.

[@Thesignalpath]

I am not certain if I undersand. Can you clarify?

[@wa4aos]

Reading your comments and with my concerns, it's confusing to keep al these class amps separate. . Would you consider doing a video demonstrating each class of amp A, B C D etc? I don't think anyone has done this yet on YT.

[@gominosensei2008]

Upper diode in initial schematic of AB amp seems reversed. Am i mistaking? also, initial explanation of XY domain is axis-reversed. puzzle phenomenon is due to clipping, because you get close to the supplly voltage, and the transistors have SOME voltage drop/residual resistance. To counteract that, you can increase supply voltage, or use transistors with less loss.

[@alterratz665]

I like the probe attachments you are using. Do they come with the oscilloscope/probe or can you buy them separately?

[@burakg7246]

Hello, Mr. Path. Many thx for all the valuable content you provide! I've got a question: All the push-pull examples you show, are designed with a rail-to-rail power supply. How would a design look when a single supply line is used? Would it still have a symmetrical design regarding the npn-pnp sections? And which point would be the source signal fed? Again at a symmetry-point?

[@Thesignalpath]

In one of my earlier videos (spark generator), I show how you can create a mid-rail voltage for a single supply operation of opamps.

[@burakg7246]

@@Thesignalpath Thanks for the response, I'll check it out.

[@glenwoofit]

Excellent video. Was the input getting saturated so it has no more range Hense flat topping the peaks of the output?

[@castimann]

Congratulations, your explanation is brilliant. Thank you for sharing!

[@Landotter1]

VERY nice presentation. I would to have liked to see a Vbe multiplier, I call it a bias transistor and how to determine where to start biasing from. if you build a circuit and just put a pot in, how would someone determine where to start as not to blow up the outputs. Another useful thing would be to determine how much bias per output device as with high power amps. also, where to determine bias current and where to look. I have an amp where they have you put a certin resistor in place of the rail fuse and then look at the voltage drop to get current. What they don't say is if you add a few more transistors to the output, what, then would be the bias current or voltage. What would be a good rule of thumb to use to get the proper bias voltage/current so you don't blow it up. High power amps are real tricky like that.

[@0x00FABADA]

No lab is complete without a Guinness! ;-) That lack of output change in the extremes of the input signal sure looks like clipping! Thanks for an interesting video.

[@Lowracerman]

Very educational. college was fun. I could have learned more if we had enough equipment for everyone in the class to experiment with.

[@RobertGallop]

Love this kind of video! Educational and practical, a perfect learning tool. And it's been answers, but your input is exceeding the rails on the output, so you are clipping, in freq domain you would see flats at the top and bottom of the waveform. At least that's my guess.

[@octavmandru9219]

Clipping due to the 2.5V limit of the power supply. Love your videos, don't care how old they are

[@timdeignan900]

Forgive me for my ignorance, but what is the point of an amplifier with a gain

[@shamalperera4875]

Verylike .......what is sir "dc control "power amplifier ??

[@pedrovictor8666]

There's one thing I can't understand. In class B operation, one diode will have its collector-emitter junction reverse bisased, correct? It has to handle this reverse voltage. I can't seem to find what spec I need to look for in the datasheet so I know it won't break down

[@andymouse]

This is pure gold...cheers.