How transistors amplify audio signals in megaphone circuit?

[@danieleden1856]

Another fantastic video, explanation very easy to understand!

[@elewizard]

It's comments like yours that keep me motivated. Thank you!

[@MlokKarel]

Awesome! You're a great teacher, I have learned another thing or two 😀👍

[@elewizard]

Thank you so much for supporting me and of course your donations 🍻

[@MlokKarel]

@@elewizard 😁🍻

[@RexxSchneider]

At 19:20 - no. Any decent capacitor will _completely_ block dc voltages. A smaller value capacitor will not "improve dc blocking". It will, however, allow lower frequency ac signals to pass more easily. There's no reason to guess the value of the capacitor. For audio applications, you just need to pick a coupling capacitor that has an impedance at 20Hz roughly equal to the input impedance of the next stage, which is the lowest frequency of interest for audio. A 100nF capacitor has an impedance of about 80K at 20hz; a 1μF capacitor has an impedance of about 8K at 20Hz; a 10μF capacitor has an impedance of about 0.8K at 20Hz, and so on. Your choice of 100nF would normally be considered too small, unless you deliberately wanted to block bass frequencies. At 14:00 - no. You'll find that 400mV p-p will definitely produce sound from one of those small loudspeakers. But from an ac point of view you've just connected 16Ω across your voltage divider which has an equivalent resistance of around 1K, so you're now loading the signal and reducing it by a factor of about 60. No wonder you can't hear anything from the speaker. Your test only proves that you need to drive a speaker from a _low impedance_ if you want to draw any meaningful conclusions about how much voltage is needed to hear anything. At 17:10 - no. The relationship between Base-Emitter Voltage and Collector current is *not* linear. You're reading off a graph where the axes are *logarithmic*. In fact the collector current increases exponentially with base-emitter voltage. At 19:50 - no. To choose Rb1 and Rb2, you cannot set "one of them as you desire". Those resistors determine the input impedance of the stage, so your example (pick Rb1=1K) would give an input impedance of less than 150Ω. That would significantly load the microphone and you'd lose most of your signal. Using a potentiometer to bias the transistor runs into the problem that you could accidentally have it at the maximum extreme so that you get the full Vcc applied to the base of the transistor. If your PSU has no current limiting, you'll burn out the transistor. Put a resistor of several kΩ from the Vcc to the top of the potentiometer to avoid that problem. At 28:14 - the problem with the emitter follower stage is that the speaker now has a dc voltage of around half of Vcc across it, since the Vbe of the second transistor will only reduce the dc voltage at point C by about 0.7V. We don't want a dc voltage across a speaker, so we need to block the dc with a large capacitor. Generally we'd expect to see 1,000μF which has an impedance at 20Hz of about 8Ω. That means we'd need a low-value resistor from the emitter to ground to bias the transistor. The UTC version of the 2SC945 has a maximum collector current of 150mA and a maximum power dissipation of 750mW (if you can heatsink it to keep the case at 25°C, otherwise more like 250mW without a heatsink). So we shouldn't set the emitter current more than about 75mA for the second stage, giving a quiescent dissipation less than 200mW. That gives the resistor required a value of about 33Ω.

[@MickHealey]

The point of this video is to explain, in simple terms, how an amplifier works. It is intended to teach people starting out in electronics the basic concepts, and encourage people to have a go. Blowing up the odd transistor (by turning the pot fully to the supply rail), wondering what happens if you change the capacitor value, etc is all part of the learning process. The idea is to get a foot in the door. Once someone starts to build a circuit like this, they will either be interested, or not. Those that are interested will ask questions. They will dig a little deeper and discover some of the points you mentioned, then tinker and research it some more. No newcomer to electronics is going to watch a 2+ hour video that tries to address every single point you mention. It would be off-putting. Things such as impedance and frequency response are more advanced topics that come later in a persons learning. This video does a great job of getting someone started. If they are interested, they will soon expand their knowledge. I don’t really know why you watched this video.

[@robertneill3057]

A larger value capacitor will allow lower frequencies to pass. The reactive impedance is always a reciprocal 1 over/divide by 2pifC.

[@RexxSchneider]

@@MickHealey I'm perfectly aware of what the _intention_ of the video was. But as someone who has taught for over 50 years, I'm quite certain of several things: 1. You _never_ lie to your students. You may avoid topics, or only give a part of the information, but when I hear nonsense like a smaller value capacitor will "improve dc blocking", I know full well that the students are being told an untruth. Why? Doesn't the author know any better? Does he actually believe that? 2. Giving an example is a good technique in teaching, but examples have to be realistic. Nobody, but nobody, would use a 1K resistor as the upper bias resistor in a stage that's being driven from an electret microphone. It would have been just as easy to have worked with 100K, so why not do that? Similarly, using a 1μF coupling capacitor would be a practical value. So why suggest 100nF? Even if you don't present the calculation in order to keep the video simple, there's no excuse for not being aware of what a realistic value would be. 3. You don't contradict yourself when teaching. You should not blow on a microphone and then say "never blow on a microphone". Nor should you tell students not to put a dc voltage across a speaker, and then do exactly that in two different places. Does the author not realise that his final circuit puts half the supply voltage across the speaker? Did he have no idea of how much current that will pull through the transistor, nor of the effect of 150mA of quiescent current passing through the speaker? 4. Students have a range of abilities, and many are capable of doing extension work on their own. The best lessons cover a topic at a level that all students can cope with, but provide leads to further material for those that want it. This video neither gave an accurate overview of a basic two-stage amplifier, nor gave any idea of where they could find more information. So, no, this video does not do a "great job" of getting anyone started, and I have no idea why you would choose to pretend that it does.

[@MickHealey]

@@RexxSchneider So, you've taught for over 50 years, and presumably consider yourself an expert on the subject. That being the case, I see no reason for you to have watched this fundamentals video, unless it was for the sole purpose of picking holes in it. I know retirement can be boring, but wouldn't your time be better spent doing something constructive, rather than being critical of other people's work?

[@RexxSchneider]

@@MickHealey That's a pretty thoughtless comment, isn't it? Any decent teacher will always be on the lookout for fresh, quality material, because it's easier to direct a student to a good resource than to have to explain the same thing from scratch. I guess that you didn't bother to read my fourth point, as that should have tipped you off to my reasons for looking at introductory videos. If you don't think that pointing out factual errors in a presentation, and explaining each error is "constrictive criticism", then I suspect that your grasp of the English language is even weaker than your grasp of pedagogy. You should also understand that I feel sorry for students who are mislead by errors, and hope that they would benefit from reading the corrections. Should I assume that you are happy to leave them with mistaken impressions?

[@jbflores01]

Excellent! Excellent! Excellent! I love your very detailed, informative, and easy-to-understand videos! Thank you for your hard work!

[@elewizard]

Thank you for being with me 🍻

[@JoeMcLutz]

One of the best explanations ever on this topic. Thank You. 👍🏻

[@elewizard]

Glad you think so!

[@dancollins1012]

Good video! I like the fact you explain what happens if various values are too high or too low. I'd prefer if your DSO was displayed horizontally instead of down the side of the screen

[@elewizard]

Thanks for the tips! 👍

[@mikeh7704]

As always, very well explained. I like the way you also explain why you choose particular resistor and capacitor values and the effects of changing them.

[@elewizard]

Thank you very much!

[@RCook-iy4xk]

You are fantastic! You explain these concepts better than any video I have ever seen. Thank you.

[@elewizard]

Wow, thank you! 🍻

[@AccuraHonda]

Teşekkürler. You are a real Wizard regrading of explaining things easy and understable. On top sympatic and honestly.

[@elewizard]

Wow, thank you my Turkish friend. I appreciate you taking the time to share your thoughts about the video and my style of teaching. You are so kind 😊

[@surajmandal_567]

I am already loving your videos. The most important things I am loving is that you also tell us why we are using a specific value. Hurray🎉

[@elewizard]

Glad the video was helpful 👍

[@ThomasJagielo-ek8nq]

Great job! Please keep the videos coming! Very well explained.

[@elewizard]

Thanks, will do!

[@MickHealey]

Great video. I think this is set at the right level for people new to electronics, and want to have a go. Keep up the good work.

[@elewizard]

Glad it was helpful!

[@UriEalsa]

Nice teaching, the way you express is admirable.

[@elewizard]

Thank you! 😃

[@t33th4n]

First time I completely understood every aspect of the circuit and the role of every component. Really great learning material!

[@elewizard]

Glad it was helpful!😃

[@kennymanchester]

Outstanding. Now I'm heading to my junk box to find some salvaged microphones and speakers to breadboard on the scope. Now I can't wait for the IC version and the extra goodies that come with using ICs

[@elewizard]

Awesome, test it and please share the result with me 🥂 you can do it

[@amir2010r]

یکی از بهترین ویدیوهایی که توی عمرم دیدم ممنونم ازت ❤❤❤❤❤ لطفا اگه فرصت کردین ویدیوهایی با همین سبک بیشتر بذارین (مثل فرستنده و گیرنده)

[@elewizard]

Thank you amir, will try

[@dimitrioskalfakis]

you should have mentioned, for the beginners, that it is not a good idea passing dc current through a speaker not only for heat dissipation but also to decreased efficiency of superimposed ac signals ans since in the emitter follower shown here it is the speaker that determines the quiescent current of operation it may be quite large depending on its dc resistance.

[@elewizard]

Yeah, that is right, this is not the best audio amplifier in the world!

[@TheEddielomax]

So much knowledge omg 😮

[@elewizard]

Enjoy 🍻

[@amir2010r]

excellent way to teaching electronics to newbies.👌 Thank you so much Ardabilli ghardashim

[@elewizard]

You are most welcome! Keep watching ❤️

[@KarldorisLambley]

interesting to see our man, like myself, doesn't agree with labelling little drawers. fascinating! i have learned so much from this chap.

[@elewizard]

Glad you enjoyed it!

[@KarldorisLambley]

@@elewizard one year ago i didn't understand difference between ac and dc. since then i have learned so much from electroboom medhi, now i have moved on to more in depth stuff, and your videos are perfect for me. thank you so much!

[@elewizard]

@@KarldorisLambley great, mehdi is awesome and your effort is fantastic. Keep learning, I will post more

[@no_360scope]

Sir, thank you so much for the video. You are really making me feel like I am learning because I did not have such a strong grasp of this before!

[@elewizard]

Awesome, keep watching

[@suchethagara2004]

most detailed video on transistor application, includes both theory and practical at same time, awesome video🤯🥳

[@elewizard]

Glad you liked it!🥂

[@yirgalemdesta7869]

I love you explanation very good teaching video. Thanks

[@elewizard]

Thanks and welcome

[@gozattila9575]

Thanks

[@elewizard]

Thank you so much for your donation ❤️❤️❤️

[@msg1956]

Super..! You are simply great..!

[@elewizard]

Thank you so much 😀

[@peterlethbridge7859]

Extremely well explained video. I haven't found anyone else giving this level of detail, which really helps. That said, I had eveything up to the adding the last transistor. I would have thought the voltage as point C would be too great to drive the base of the second transistor, so it would be in saturation mode?

[@elewizard]

You're very welcome! No second transistor is configured in common collector mode and there would be no problem. For more info about that part of the circuit you can watch my other video. There is a link to that video in description

[@hankosaurus]

Very instructional and useful. Thanks!

[@elewizard]

Glad it was helpful! Thank you for watching 😊

[@jboy6944]

Bro I really like and appreciate you. God bless you richly!❤

[@elewizard]

I appreciate that, thank you for watching🍻

[@michaelalex5235]

This video is great, as are your other videos - thank you!

[@elewizard]

Wow, thank you!😃

[@petersdelucaaa2vg306]

Thank you. Another great video with easy to understand explanations. Would it not be wise to put a small 1K resistor on the middle pin of the 50k pot so not to turn the 50k pot to zero resistance and allow full current to pass through the pot to the base emitter junction and burn out the pot or the transistor?

[@elewizard]

Yes, I think it would be wise 👍

[@user-mr3mf8lo7y]

Stunning explanation. Much appreciated.

[@elewizard]

Glad you liked it

[@umesh.kumar.naik278]

Great video.....❤.... Sir... Please make and explain a video on very simple FM transmitter circuit ...❤.... Thank you sir...

[@elewizard]

We will try, it is on my todo list 👍

[@renegaed]

such a good video. you really answered all of my questions as i was following along like why 100nF? and why 50k pot? thank you fro the clear explanations.

[@elewizard]

Glad you enjoyed! 👍

[@AugustineAriola]

Very impressive. I love your video. You made the concept that was hitherto pretty strange to me to become clearer and easy. But then, how do I drive a big loud speaker ?

[@elewizard]

Happy to hear that. High power speakers need high power components and of course an optimal and sophisticated amplifier circuit. This circuit is good to learn basics not to use in real world projects

[@AugustineAriola]

@@elewizard Thanks for your reply, hopefully you design a more power amplifier in the nearest future. I will subscribe and wait the day the video will drop.

[@kennmossman8701]

Valve, MosFet or BJT? Class A, AB, B, or D? Big is what in terms of Watts[RMS] ? Impedance?

[@AugustineAriola]

@@kennmossman8701 Like 40w transistor Amplifier.

[@kennmossman8701]

@@AugustineAriola That is an immediate-advanced project. Look at back issues of Wireless World or Elecktor. Maybe be better to buy a kit

[@anontoakanda5847]

Thank you sir, ❤ from 🇧🇩

[@elewizard]

Most welcome!

[@Po87-555]

You are an amazing man thank you for your effort😊😊😊

[@elewizard]

Thank you so much 🍻

[@AnthonyFrancisJones]

Excellent! Thanks for making this!

[@elewizard]

My pleasure!

[@steves7189]

Great video! Thanks again!

[@elewizard]

My pleasure!

[@tadeaschaloupka6297]

Amazing video! Thank you :)

[@elewizard]

Cheers 🍻 🍻🍻🍻

[@amazagx]

Gooood stufff!!!👍👍👏👏

[@elewizard]

Thanks 👍

[@jyotibasalgude930]

another excellent video

[@elewizard]

Thanks again ☺️

[@styrishrodrigues]

Good video.. Keep going ❤

[@elewizard]

Thank you, I will

[@abdulwaheedouabihi5903]

can you make a video on how inductors work and their functions, also very good video, thank you

[@mikejones-vd3fg]

Yeah that would be a nice one, until then check out this video on inductors, best anology ive seen using a waterwheel, there it makes sense why he keeps pushing after its turned off and how it resists quick changes by sucking up the energy. kzbin.info/www/bejne/i6Omd6lopqh4gqssi=hFG2rsZTgMBWqNcj The capcitor explanation with the elastic membrane is also very nice, there you can see why it doesnt allow DC to pass and AC it does.

[@elewizard]

Great suggestion! I will consider making such a video

[@johnebong7059]

Thank you so much for this 😩

[@elewizard]

Any time!

[@OlumideBabaruti]

you're the best

[@elewizard]

So nice of you ❤️❤️

[@amir2010r]

Hey bro, what's the model of the LCD monitor you have hanging on the wall?

[@elewizard]

I am not sure about that, it's brand is lenovo

[@amir2010r]

@@elewizard does it a way to add HDMI port and power terminal to spare laptop monitor and use it as an external monitor?

[@rw2954]

Can you do the same kind of thing but for RF frequencies? An RF amplifier for say 20Mhz?

[@elewizard]

It worth a try👍

[@mikejones-vd3fg]

Very nice, was easy to follow along, even the biasing part but the 2nd transistor kind of threw me off. Why is the currnet amplified int 2nd transistor and the voltage amplified in the first one? why isnt the current amplified in the first one? oh yeah the 1k resistor. You said there are many disavantages to simply using a low value resistor to allow enough current to flow, but didnt mention any heh, was curiouse to know, anyway, thanks for the great info.

[@elewizard]

Thank you for your feedback 🌷

[@ChandrashekarCN]

💖💖💖💖

[@elewizard]

❤️❤️❤️❤️❤️❤️❤️

[@surajmandal_567]

I am interested in electronics from childhood

[@elewizard]

Just like me🥂

[@fredymredy]

I had no time to watch entire video, but im interested how to make 10, 20, 50W or even more powerful audio amplifier? why some amplifiers have 1-3 transistors and some of them have 5 or more but the W output is same/similar with less components?

[@elewizard]

A dedicated video is required to answer your question

[@123123mike]

Noob question, why the resistor, 2.2k or otherwise? Why not just connect the mic directly to the voltage source (5V)?

[@elewizard]

Because in that case, both sides of the mic are fixed, I mean the voltage on negative or positive leads of the mic can't be changed, so it can't present the voice signal. You have to put it in series with a resistor, then you can get the voice signal between them 👍

[@123123mike]

@@elewizard thanks

[@123123mike]

@@elewizard I think I get it now. A change in resistance in the mic due to sound input results in fluctuations of current through R1, which results in fluctuations in voltage at R1 (aka signal), which then leads to a changing Vout, which then needs to be amplified. Is this correct?

[@3d8d]

It's like voltage divider

[@gkdresden]

A resistive microphone is strange. It should be mentioned, that there is allready an active device inside the microphone, which converts a voltage signal from an electret device into a resistance signal.

[@elewizard]

Yes, I agree 👍

[@persiangulf2282]

دادا ویدیو انگلیسی تا دلت بخاد هست تو یوتیوب شانس ما یه ویدئو خواستیم نگا کنیم درمورد امپلی فایر اونم داره انگلیسی صحبت میکنه 😅😅عجبا

[@elewizard]

😁

[@astalavista5596]

حاجی تو ایرانی یا خارج؟

[@elewizard]

Ardabil ☺️

[@astalavista5596]

@@elewizard yashasin hhh

[@TomCee53]

You simplified your description of the impedance of the speaker by calling it resistance. I know this is intended to be an introduction, but you should at least mention the difference. The dc resistance of an 8 ohm speaker will not be 8 ohms.

[@danielonofre6377]

Bad circuit, no working.

[@elewizard]

It absolutely works, you just need to adjust the potentiometer carefully

[@mafhper]

Thanks

[@elewizard]

Welcome