#273

This video provides some basic design tips and shortcuts for the Common Emitter, Class A, Amplifier. The considerations for selection of the DC Bias point are presented, as well as some tips on how to set/compute the gain for both degenerated and high-gain configurations. Below you will find links for related videos and for a PDF copy of the notes used in this video:
#185: BJT Bias and Beta Dependence: • #185: Back to Basics: ...
#113: Bias point and Class of operation: • #113: Basics of Transi...
#114: Common Emitter/Base/Collector configurations: • #114: Tutorial: Common...
#67: Common Emitter Gain and Frequency Response: • #67: Basics of Common ...
A much more detailed treatment of a single BJT amplifier is presented by TheSignalPath here:
• TSP #15 - Tutorial on ...
Video Notes:
www.qsl.net/w2aew/youtube/com...

[@tomsmith3045]

I went to school with a guy who could explain a weeks worth of classroom lecture in math in 10 minutes over a beer. Clear, concise, and simple. You're like that guy. Great video!

[@micomrkaic]

The best explainer of electronics on KZbin. Bar none.

[@Daklon17]

I've been 3 days trying to understand how the different values for the resistors are choosen and how can I calculate them to have a defined gain. You explained it very clearly in this video, and with the other videos that you pointed I think I finally fully understand it. So thank you very much

[@jacobfaseler5311]

Just about every other tutorial gets lost in the math. I appreciate your approach from the angle of design philosophy : especially in how you clarify the decision tree of a typical design. Chef’s kiss.

[@cetola]

I regularly recommend your videos as high quality electronics tutorials. Those videos you mentioned really helped me through my post-bac work in microelectronics. Thanks for all your hard work and attention to detail. Much appreciated.

[@eatdrinkwineguy]

OMG! So many over-complicated videos out there on this subject. I have been trying to find a simplified explanation for all of this and this is the first I have seen that is clear and concise in both its explanations and examples. As well as it doesn't make any assumptions about what you should or shouldn't know. Just the right amount of math too. Thank you so much.

[@TheRadioShop]

A lot of times I read similar notes like this in a book and find myself having to go back and read again and again. Your tutorials and demonstration process makes this so much better to understand. Very well thought out video approach. Thanks for sharing. Your knowledge is amazing.

[@w2aew]

This one took me about two weeks of off-and-on thinking, postulating, scribbling, experimenting, planning, etc. in order to just come up with a plan for presenting in a logical, easy to understand way.

[@PapasDino]

Folks often don't appreciate how much effort goes into what we used to call "platform time" that when I was an instructor at Ft Leavenworth (no, not on the prison side!) I found that you put in at least 3 hours of prep time for every hour teaching, sometimes even more; I imagine it can be even more than that when you're shooting a video...at least your questions here don't come in real time! Thanks for all the hard work Alan, it's really appreciated. 73 - Dino KL0S

[@w2aew]

I usually figure that each minute of video translates to 10-30 minutes or more of total invested time.

[@idpromnut]

It shows you have both the knowledge, care and ability to present. Thanks for doing videos like this, they are very useful!

[@nlimchua]

@@w2aew A lasting legacy of invaluable knowledge shared to all, saved for posterity. A simple 'Thank you' just doesn't cut it, but... Thanks (very much) anyway!

[@davidmaruska1641]

Its been early 80's since I worked with these formulas, Great brush up!

[@johndczerwinski]

Another great video! I've been researching information on BJT common-emitter amplifiers for over 2 weeks using several books. This is, by far, the most comprehensive series of videos on the subject. Thanks for taking the time to put this together

[@williamogilvie6909]

Very thorough presentation on CE amplifiers. Definitely the best on utube, and probably better than most university lectures on the subject.

[@johnnicolaou7166]

It is nice to finally find someone who explains electronics in a way I understand. Thanks for your hard work. Please keep it up.

[@amanuelnigatu4621]

That what we call teaching!!! 4 or 5 classes with just 20 minutes.thank you sir

[@danielstimpson7792]

You are such a good teacher Alan. I just love the videos like this, they make the subject so understandable and enjoyable. If I had the time I could immerse myself in transistors. Many thanks for all your hard work putting these video together.

[@pirateman1966]

This explains so much! So many light bulbs went off in my head. Thank you. You are a great teacher.

[@ColossusEternum]

Bravo sir, I have been driving myself crazy with math, Ive been attempting to learn the hybrid and R(pi) models. But all these videos leave out simpler insights like the bias string to base current ratio. You are doing a great service to humanity with these videos, and I hope you keep making them

[@silasmarner7586]

Well demonstrated, well explained. Nice video!

[@gordonwedman3179]

Thanks once again for a well presented video. I think your videos are the best electronics videos on YT when you calculate the ratio of useful content over length. If you did a one hour video it would be like a full semester at a University.

[@MrEkg98]

I really appreciate your videos. I have not fully watched all of them on bjt biasing but #113 answered a lot of questions I had previously. I been trying to understand these topics for a few years and biasing outside of a simple single resistor base bias is confusing. Hopefully this makes more sense to me. Thank you.

[@unklebidii]

Great video... I always go back and re-watch those linked videos. This video answers some of the questions I've had. Thanks. Passive/active components always fun to learn about.

[@els1f]

This is the best video on real deal transistor setup I can imagine! 👍

[@microdrone]

I'm getting back to basics for an ongoing project, destined for modularity and miniaturization, the potentially tiny bipolar transistor circuitry is really important for these applications. W2AEW is by far one of the most useful and enjoyable sources for this really important subject matter, thanks dude!! I'm a long time fan , and I am really thankful for your sense of detail and nice way of explaining things. Thanks to you I am a proud owner of 2 Tek 4 channel scopes, an old crt from ebay :) and a TBS1064. For what I'm doing they are more than sufficient, the latter being my basis for accurate measurements, and quick and easy to look at comparisons, thanks to the trace colors. Especially coupled with your expert advice and great quality video productions, I have lots of 'fill in the gaps' for my general knowledge in relevant areas. Cheers!!

[@troutrou0]

Hi Alan, I discovered your channel recently, 3 months or so. I watched each and every one of your old videos one by one, I am up to date now. I have never commented before (I do give a thumb up to every one of them, though, obviously), but I thought I would this time, for a change ! ;-) I mean, as you said it took you quite some time to come up with this video, but the result is excellent and I do, like many others it seems, appreciate it immensely. So I hope that the recognition you get and deserve, will keep you motivated and that you will keep doing great stuff like this whenever you find the time. Every second of your videos is pleasing to look at and listen to. I love every aspect of your videos. The technical content, the way you present it, your lovely hand drawn notes and schematics, your experiments, quick calculations to demonstrate the point... you act and talk like a proper engineer that you are, as well as a man of passion, a pleasing and efficient blend. I think that's why many electronics teachers here in France just aren't good... most of the time they actually don' t have ANY experience in professional electronics design (and not much passion in them either). They just study EE then go straight to teaching, zero "real world" professional experience ! :-/ I also studied EE in the UK as well, at Uni for 3 years (did a B.Eng there, 20 years ago), and the teachers also looked clueless and uninspiring, couldn't sense any trace of passion in them either. Was pretty depressing... Anyway, enough rambling. Just wanted to comment at least once, to express my true appreciation for your contribution to EE education. Keep up the excellent work ! :-) The number of subscribers, 75K to date, is a good metric I think. This alone should convince you that your are doing something good and valuable here. Sure, EEVBlog is much bigger, nearly 500K subscribers, but I think this is largely due to, as I remember him confessing once, the fact that he is more (overall I mean) of an entertainer and show man, than a teacher. Your videos however target more those interested in the educational value that an experienced design engineer can bring. This obviously is a narrower audience. In this respect, I think 75K is a really strong number which I hope you appreciate as well. Thanks again for the time and effort you put in all your videos :-) Vincent Trouilliez, from across the pond.

[@w2aew]

Wow, you've watched every video?? That's a lot of time invested, and I certainly appreciate that! I hope you've seen that the production quality of the videos has improved since the early days of my channel. Thank you for your very kind comments. I received my engineering degree in 1985, and I remember that many of the professors were as you described. I did have a few that had a real passion, but many did not. Most of my videos take a lot of thought and planning - typically 10x or (much) longer than the video duration. Others are a bit more impromptu (like my video on the Z-Match tuner that I did while on vacation). Even these often require many "takes" until I'm happy with the content and flow. I'm certainly not the "entertainer" that Dave Jones is, so I definitely understand the differences in the size of the following. Of course, he also has a blog and a very popular forum - I have none of those, just no time to do that type of thing. Thanks again for your very nice comments - knowing that the content I put together is truly appreciated and *used* by my viewers is very rewarding and inspirational. Thanks again for being such a loyal viewer! Alan

[@troutrou0]

Yep, every one of them, watch I did ! LOL I don't have a TV anymore (gave up on this thing 20 years ago). These days I replace it with YT videos. So instead of watching silly and brain damaging stuff on TV, I instead spend it on watching selected/hand picked quality content on YT. I find it much more interesting and useful than TV... I also like TheSignalPath and watched all his videos, then once I had finished TSP (and subscribed of course) I looked at your channel because YT suggested it in the "Related Channels" list on TheSignalPath page. I am glad I clicked ! :-) Then I listened to your interview with Dave on TheAmpHours radio show to know more about the man. Yes the quality of the videos did improve ! I just re-watched your video #2 for a side by side comparison ("using the delayed time base on your Tek 485"), to refresh my memory, and the improvements are obvious... The camera is on a stand now, makes a world of a difference. Picture is neater/sharper too, and the lighting is much better/brighter. It's indeed very well sorted out in your newer videos :-) The technical quality of your videos is now great and I don't see/feel any particular area that would need improvement. Now we just concentrate 100% on what you say and what you do on the bench, which is great. We can focus on learning rather than bitching about such or such technical problem in the filming. Anyway, back to the lab, my old Tek 317 is waiting for my assistance.... yeah like you I have this "disease" of "collecting" various Tek scopes (mainly) as I go. They have this little something that keeps me coming back for more no matter how reasonable and rational I try to be... Regards, Vince

[@airmann90]

I'm working through some textbooks as a hobbyist and while I understand all the equations and rules etc, I find myself not following it as well when I actually go build something. You really made it a lot easier to find a starting point for me and I'm going to go enjoy building some amps for a bit now. Thank you for the awesome videos!

[@JamesAnderson-bz4cg]

There are good teachers and bad teachers. You are one of the very best. I've been watching several of your videos. The knowledge that you hold and then make relatively easy to understand (certainly easier than other tutors) is first rate. I actually find myself understanding what you are saying because you teach it in such a great way. Theory and practical to show your workings is especially great instead of pure theory which can be mind boggling. I'm a visual learner predominantly and with you showing the electrons wobbling around on a scope is brilliant. With excellent short cut methods that only a real engineer can pass onto a student is invaluable as a way of referencing quickly and easily to verify that the build or design is going in the right way without over complicating things too much. Its bite sized snippets of information like that, that make it more manageable to remember. As others have said and especially at my age (50) I find myself going back to reference books several time to read the theoretical text, which is very important, but very time consuming. Your time and huge efforts are very much appreciated. Your other videos pick up on much more theoretical detail which is also superb. Please keep up the excellent work and thank you.

[@w2aew]

I appreciate your kind words. I do put a lot of effort into these videos to convey good, concise, practical information. Even with all this effort, some people are still not satisfied (there are 12 thumbs down on this video), and I'm not sure why...

[@rfburns5601]

Another excellent video. I save all of your circuit design videos and notes to the "Circuit Design" folder.

[@allthegearnoidea6752]

Boom. Nice to see little re my favourite transistor parameter. Thanks for sharing this helped blow the dust off.

[@omarel-ghezawi6466]

A good blend of theory and testing. Excellent. Well done.

[@OIE82]

VERY good video. A simple, detailed explanation that didn't skip over important points and just assume I knew what you were talking about. I have watched a bunch of these videos and you are the best instructor I have found. You got a sub.

[@tonybell1597]

Excellent tutorial Alan, precise, concise and delivered superbly, thank you.....

[@declanallan885]

awesome video dude, need i say your explanations are clearer then those given by some of my lecturers at my university.

[@w2aew]

Thank you - please pass along my videos to your fellow students, *and* your university lecturers!

[@TKomoski]

Great to see you back doing the theory to practical. Always reminds me of college, and one teacher I had was Chinese. When he was doing transistor theory his accent was so bad we made him write it down. Image Vbe in a Chinese accent (no disrespect intended) and your acronyms just brought back old memories. Cheers and glad to see you back in the class.

[@AliensInc.]

It's been along time since I did some calculations of this so I so when I did this a couple of weeks ago I had totally forgotten these shortcuts. Thanks for reminding me :)

[@bfx8185]

Great! I really like your videos about this basics. Its simple and straightforward. Excellent!

[@Shawn_White]

WOW I've been trying to figure this circuit out for years thanks!

[@mortenlund1418]

This is one of the very best videos I have seen on this subject. Thanks so much.

[@carldavis8228]

I must really thank you. You are one of the few who bothers with the math in a demonstration. I have re-watched almost every one of your videos, most more than 3 times. However, I want you to understand, the math portion of your tutorials is what makes them great for me.

[@w2aew]

Thanks - although I do try to minimize the complex math, and always intersperse with practical demonstrations and measurements.

[@robertcalkjr.8325]

Math is our friend. Many mistakes could be avoided by using a little math first.

[@art58wong]

Bravo. Reminds me of my electronics class in engineering but much better explained. Thank you for being considerate enough to share your practical knowledge.

[@parleyk4503]

Hi Allan I just wanted to say thank you for all the thought that goes into these videos. They are by far the most simple and informative videos on this subject. Im 15 and I’m learning electronics purely for the fun of it but even at such a young age I am still able to clearly understand these videos because of your simple and straight forward way of teaching. Thanks again.

[@hyperdimensionallight4931]

Thank you w2aew! This is very clear and concise. Very helpful.

[@eeltauy]

Sir, your videos are amazing! I learn more from you than I do from my own teachers in the university! Couldn't thank you enough. please keep us posted and thank you.

[@HellaFunnyShorts]

Your tutorials are ridiculously awesome

[@AmRadPodcast]

Great tips! This info is something lost in a lot of textbooks.

[@robertcalkjr.8325]

I have found that the main thing lost in most books that I have seen is "grounding". Even the new "The Art of Electronics - Third Edition" only has a few pages about grounding. No wonder so many scopes get blown.

[@AmRadPodcast]

Robert Calk Jr. Yes. Also ground loops in pcb layout. It's always explained as black magic or an art form. Follow these simple rules and it may work out in the end. (Not always)

[@curly767]

The Current Source A PiL fan?

@@robertcalkjr.8325 One bachelor's degree later and I, too, only learned about scope grounding after unintentionally running current through the scope probes' ground leads. Thankfully those were milliamperes.

[@tevo_za]

honestly, thank you so much. superbly high quality.

[@krisjk999]

Thank you very much!! This video helped me a lot in designing one of my circuits. Thanks a lot

[@brownmonkeybananayellow]

Love these bjt videos!

[@vonzigle]

Great video, thanks! 😃

[@BryanByTheSea]

Excellent tutorial and video. Tthanks so much and keep them coming.

[@DerExperiminator]

Great and neat explanation. Thank You. I really like Your explanation-videos of analog circuits.

[@mohimadvani6425]

Very good explanations and shortcuts, loved your video.

[@francomarianardini681]

great video! excellent teaching capabilities!

[@m1geo]

Excellent. Thanks again, Alan!

[@StevenLilley]

Awesome video, thank you.

[@rolfts5762]

As always very nice&highQ video-lectures, thank so very much. (..friday evening here in scandinavia/norway, and coffee + videoes like this one is top-notch. Thanks again for sharing your quality knowledge to all of us Alan.)

[@ScireMi]

Great video, very clear explanation.

[@kungfumaster8171]

Up to your usual awesome work. Great video.

[@amirb715]

Another great video as always from an exceptional teacher. Can you please do a video on common base amplifier and its main features and also a video on tuned RF amplifiers. There are not many tutorials out there on these curcuits.Thank you.

[@robertcalkjr.8325]

Nice lesson. Thanks, Alan.

[@davidkierzkowski]

Excellent handwriting

[@jeanious2009]

NICE!!! MORE AMPLIFIER TIPS and TUTORIALS PLEASE!!!

[@EarlWallaceNYC]

Very helpful. Thanks

[@willhelmx8388]

Very nice video Alan, never thought about these shortcuts really - feels like one is back in school :-):-) well done 73´s / Andy

[@tonychristoph1063]

very good and usefull vidéo thank you

[@jarrellestes1793]

I needed u about 60yr ago. THANK YOU! (NOT TOO OLD)

[@PrincePloppy]

Just subbed... Your a great tutor!

[@willashland4597]

Amazing video, thank you sir

[@JasonLeaman]

Thanks Good video, Class A is my Favorite !

[@RexxSchneider]

It's important to emphasise how much extra distortion is introduced for larger signals if you bypass the emitter resistor. With a bypass capacitor in place, the gain is directly proportional to the collector current, so if the output swings close to the positive rail, the gain drops to almost nothing. If the output swings low and doubles the voltage across the collector resistor, then it doubles the instantaneous gain. That's a recipe for really nasty distortion. The formula for gain is actually Av = - Rc / (Re+re) where re is the incremental emitter resistance = 25mV/Ic at 17°C. To reduce the effect of transistor variations and to reduce the distortion caused by the non-linear re, it makes sense to set Re at least ten times re. But the voltage across Re is Ie.Re = 10.Ie.re = 10 x Ie x 25mV/Ic. Since Ic=Ie for all practical purposes, that shows that the voltage across the emitter resistor should be at least 250mV. Making it higher reduces the gain, so a practical upper limit on the gain is achieved by designing for 250mV across Re. Another tip is that because you want to bias the collector around half-way between Vcc and Vb, the voltage across the collector resistor will not be too far different from Vcc/2 (assuming Ve=0,25V and Vbe=0.6V). That means that the practical upper limit for the gain without bypassing the emitter is approximately -Rc/Re = - Ic.Rc / Ic.Re = - Vcc/2 / 250mV = Vcc / 500mV. So for a 12 volt supply, you won't get more than x24 gain unless you bypass the emitter resistor and then you have to keep signals below about 10mV to avoid the non-linear distortion. A final tip: with an emitter bypass capacitor, the gain becomes Av = - Rc / (Re||Xc + re) where Re||Xc is the impedance of the parallel combination of Re and the reactance of the bypass capacitor. If we want the gain to be flat with frequency down to low frequencies, then we need the reactance Xc to be smaller than re (note: *not* just smaller than Re). For the circuit shown, with a collector current of 0.42mA, the resistance re is 25mV/0.42mA = 60R. A 47μF capacitor has a reactance of 60R at 57Hz, so that is the -3dB frequency. If you want to have the roll-off occur below 20Hz, you need around 150μF for the bypass capacitor.

[@jasoneyes01]

I keep revisiting this lesson, have a curious amplifier mind now. Any other important lessons on amplifier design, matching components, testing... would be much appreciated. If possible would love to see a lesson on DC servo for an amplifier, theory and operation...

[@berndb.5097]

you do my life easier :-) thumbs up for that educational video..... mny tnx 73

[@bigbread9000000]

I love these discrete component video's, I am trying to build a class ab audio amp using discrete components(chip amp to easy)

[@Bianchi77]

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

[@Bursadesain]

nice video, thanks

[@simonyoungglostog]

Great, Thanks.

[@mortenrolsing7137]

Fantstic video as always, thanks. Idea for a future video: Calculating input and output impedance of 2, 1 transistor amplifiers, after each other, and calculation of the size of the dc blocking cap between the 2 stages, especially the last of the two, i have searched youtube and i have found none that have made a, for me, usable video on the subject, maby you could make that video. Greatings from Denmark Morten

[@w2aew]

Thank you. I did answer the question about calculating the input and output impedances of a common emitter amplifier in one of my comment replies below. I agree that a video on how to select the interstage capacitors would be useful. I can add that to my looooooong list of topics...

[@W1RMD]

Thank you!

[@neilsonblair9388]

Great video Neilson Shetland islands

[@ornithopterindia]

👍Thank you sir.

[@anthonycalia1317]

You are good, really good. I do hope you teach professionally as you are a natural.

[@w2aew]

Not really. I am a Field Applications Engineer for Tektronix. I do some teaching/tutoring in this role, but I am not a teacher per se.

[@viswesh713]

Sir, thanks for sharing.

[@TKomoski]

Happy Holidays & Best Wishes for the New Year. CHEERS 73

[@StrongFives]

wonderful video and thank you for using a camera tripod. Video #67 had me running for the Dramamine having watched it so many times, getting up and walking around like a sailor new to port. I tried recreating the circuit in #67 but kept getting VC and VE values 13-15% off in my calculations vs. measured (VC calc = 2.29V, measured = 1.946V). I'm learning so this might be common knowledge but it appears while an emitter-bias is somewhat independent of beta variation, the formulas are significantly sensitive to VBE. I was using the 0.7V approximation for silicon transistors and getting incorrect values for a 2sc945 transistor. The 2sc945 datasheet has a VBE min = 0.55V, typ = 0.62V, and max = 0.65V. Using the typical 0.62V VBE in the equations resulted in the calculated values matching the measured. Thank you for showing the trick for AC signal to the oscilloscope. Not sure if you used the same in video #67 but when I tried there was so much noise in the low freq I couldn't keep the same attn. that was used on the HF and then I was no longer comparing apples to apples.

[@octavmandru9219]

I am only afraid YT is not really the best way to show your amazing knowledge. I often see the subscription to your channel going down in the list, based on the fact that you don't release videos often enough. Quality content takes long to be produced! The education you give us must to be protected for future generations, and I am not joking here. Ideas anybody?

[@w2aew]

I wish I had the time to produce videos more often, but my *real job* and family life take first priority. I hope that the videos I produce will be preserved for generations - that would be a nice legacy to leave behind.

[@JoseOlin]

What a wonderful explanation! Any video in this series where you show the design process given a desired voltage gain Av for a low gain amplifier?

[@w2aew]

The Signal Path has an excellent one: kzbin.info/www/bejne/j2OofaqCp7-le68

[@warplanner8852]

Alan, as always a magnificent effort. It goes in my library of w2aew videos on theory. (Now that I have retired, I build and test as you do and verify which makes me understand better.) My only question is who in hell gave you two dislikes? 71/72 Bill, k6whp dit dit

[@w2aew]

Thank you, glad you liked it. I don't know who gave the thumbs-down, but I always get a few.

[@theoryandapplication7197]

thank you

[@mosiotv]

Thanks a bunch. Very helpful as always. Can you please do a follow-up demo on those direct coupled multi stages? They are really confusing. Thanks again 👍🏻

[@w2aew]

I can add that to my long list...

[@mosiotv]

That's great! 💗 thank you so much.

[@andygranis8780]

Excellent Tutorial, Thanks! /SM0IHR

[@CarlosdaCunhaeSilva]

Excellent tutorial, thanks for sharing. I would like to recreate your test setup but I don't have 10x attenuator. Can you share some information about it? Maybe I can try to build my own one.

[@w2aew]

I did a video on attenuator design: kzbin.info/www/bejne/d2bKeJiMbKich5I

[@electronicssciencebysherry5050]

Thank u please make 4 us amplifier design techniques for beginners

[@area46241]

I love this topic. Biasing is a hard thing to wrap the brain around. This year I finally pulled off putting several amps together some worked well and other needed tweaking. How would you set up a transistor as a digital switch for high current like a 3055 on pwm?

[@w2aew]

When doing switching, you generally want to switch the transistor between OFF and Saturation. For OFF condition, make sure that Vbe is near 0V. For the ON (saturated condition), make sure that Vbe is fully forward biased, with a base current ~10% of the expected collector load current.

[@Soupie62]

Most [Class A Common Emitter] amps have a transformer on the output for impedance matching and removing DC bias. A follow up on transformer design & build would be great. Building transformers is rarely documented, and is at risk of becoming a lost art.

[@w2aew]

I'll have to add that to my looooooong list of future topic considerations.

[@patrickohara9901]

Another wonderful video - well done! You explain that by adding a bypass cap can cause distortion but provides higher gain. You described the higher gain, but you did not address the distortion issue. At what point (cap value or res value) might one expect distortion to become an issue?

[@w2aew]

The distortion is due to the fact that the transconductance (gm) is a function of the collector current. In the bypass case, the gain is -gm*RC, so if the transconductance changes from the highest to lowest level of the signal, the signal will be distorted. So, the more that re (1/gm) dominates the emitter resistance, the more distortion you can have.

[@PelDaddy]

Thanks for sharing. If you have a moment, can you please comment on the criteria used for selection of the bypass capacitor size? Is the goal just to have a cutoff frequency well below the signal frequency? Thanks.

[@w2aew]

The goal is to have the capacitive reactance be much smaller than 1/gm at the signal frequency. 1/gm is also known as re, so the gain can alternatively be written as -RC/re. If there is any emitter resistance, then the gain is -RC/(re+RE). If the bypass cap is not much less than re, then it will reduce the gain the same way that an addition emitter resistor (degeneration) would.

[@AstroSam66]

Quite cool! I would like to know more about the value of the bypass cap. I think that this cap is pretty much influencing the frequency response of that amplifier, right? Edit: Sorry, i just realized that you already did this in video #67. So thx :-)

[@fernandohood5542]

Excellent tutorial. Should you go with two low gain stages e.g 10 by 10 or one with 100?

[@w2aew]

It depends. It's sometimes easier to setup biasing for low gain stages, and you'll be able to get more BW out of cascaded low-gain stages.

[@deanneumann8594]

great explanation Alan, thank you. How did you calculate the values needed for the decoupling cap and bypass cap? Those are pretty large components... what would be the effect of using much smaller caps?

[@w2aew]

Nothing too critical. I just wanted to be sure that the impedance of the emitter bypass cap was much less than re (1/gm) at the test frequency so that the gain approximation was valid. And I wanted to be sure the input cap's impedance was much less than the input impedance of the circuit so that there wasn't any significant voltage drop at the test frequency.

[@charlieb.4273]

Another fantastic video. Thank you. Can you tell me how to calculate the input and output impedance? If you covered it in another video please direct me. I know how to measure it, but how do you design a circuit with a specific impedance in mind?

[@w2aew]

Output impedance is simply the collector resistor, RC (or R3 as shown in this video). The input impedance is slightly more involved. It is the parallel combination of R1, R2 and the input impedance looking into the base. The input looking into the base in the "emitter bypassed" case is something called r_pi, which is simply r_e * Beta. From the video, r_e is 1/gm, or v_t/Ic. In the case with emitter degeneration (no bypass), the input impedance is r_pi + (Beta+1)*RE.

[@R2AUK]

I would like to clarify that r_pi + (Beta+1)*Re is an input impedance (!) looking into the base, not an input impedance of the amplifier. The overall input impedance is R1 || R2 || [ r_pi + (Beta+1)*Re ] which can be approximated as R1 || R2 || Beta*(r_e + Re), since Beta is unknown and >> 1.

[@JoaoSilva-bc7jc]

hi , first i would say, its great to see your videos, thanks. I would ask if you can make a video with a common emitter class A push pull analise. It woul be so great . Thanks.

[@kaustubhponkshe6186]

Can you please make a video for design of single tuned amplifier ?

[@Giovanni2862]

Hi! Could you give me some more indication to be able to buy a "10x attenuator" and "50 ohm terminator"?