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!
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.
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.
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.
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 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!
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.
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.
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!!
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
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.
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.
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...
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.
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.
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!
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.
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.
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.
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 :)
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.
Bravo. Reminds me of my electronics class in engineering but much better explained. Thank you for being considerate enough to share your practical knowledge.
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
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...
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.
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.
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...
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
Fantastic video Alan! I would love for you to explain how you set up your scope. Is the attenuator really necessary for low voltage and low frequency work? Just curious!
I used the attenuator (as shown at 2:18) for two main reasons: 1) To provide a 50 ohm termination for the generator, so that the applied voltage wouldn't vary with the frequency dependent input impedance of the amplifier, and the main reason is: 2) To make a larger signal available at the scope for input signal monitoring. If we just probed the signal at the amplifier input, it would be quite small, and the signal-to-noise ratio would be poor, resulting in a noisy calculation of gain in the Math waveform. By measuring a larger signal, and then simply scaling it's value with the attenuation value, the scope sees a larger signal with better SNR.
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.)
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?
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.
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.
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 👍🏻
Can you show how to calculate the input capacitor value and output capacitor value because you have to add in the input impedance as Rin and output impedance formula for Rout to apply to get the capacitor value.
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?
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.
When the collector is directly connected to the power supply, the circuit is not a Common Emitter amplifier. This configuration is called a Common Collector amplifier, or Emitter Follower. The output is taken from the emitter. Voltage gain is basically unity, non-inverting. See my video the common emitter, common collector and common base amplifier configurations: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-zXh5gMc6kyU.html
Hi great video, but can you explain why the gain should increase so much simply by putting in the emitter bypass capacitor? Apologies if this is already covered in another video.
In short, it reduces the negative feedback, thus raising the closed loop gain. Page 3 of the notes compares the two cases (not-bypassed and bypassed). Also, Take a look at video #67, it has more details.
w2aew thanks. I’m going to try your examples myself. Interested to see what spice says as well. Looking for a small signal rf amp to boost the output of a ad9851 dds.
Tube Rectifiers have a Drop out voltage called Vdrop on the datasheets. 5AR4 tube the dropping voltage is 17vdc, 5AU4 the dropping voltage is 54vdc, 5Y3 tube the dropping voltage is 50volts. Why does a tube rectifier start to drop the output plate DC voltage when its output current draw is at its maximum? What is going on inside the tube rectifier when its in the Vdrop condition is the cathode and plate electrons less? I'm not understanding why the tube rectifier is causing a dropping voltage on the rectifiers plates output voltage.
The damping factor is the "Ratio" of the load impedance To the amplifiers output impedance. A high damping factor means that the audio sinewave is clean and a low damping factor means that the audio signal or sine waveform is distorted?
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 :-)
Gee. Finally, a good one. Thanks! By the way: I really miss my HP15C. Incidentally; how do you get such clean waveforms? I get so much more noise, even at 20 mV/div (200, with probe at 20x). 73 de VA3MVW
Getting clean waveforms is all about maximizing signal to noise ratio. Use no more BW than needed (turn on BW limiting if you can), use no more attenuation than needed (use 10x instead of 20x if you can, etc.), use averaging if your scope supports it, use high-res sampling if your scope supports it.
The input impedance is the parallel combination of the bias resistors and the input impedance of the base (dominated by beta*emitter_load). Output impedance is basically RC. If your signal is going away when you connect your load, then either: Load impedance is much much lower than RC, or you might be DC coupling your load which upsets the bias conditions.
Input biasing resistor value choice will depend on several design and application factors. Good practice says that the current through these resistors should be at least 10x greater than the base current. The application may have requirements on the input impedance to this circuit - so that might have an influence on the choice of values as well. And of course, the desired operating point determines values and ratio. Ultimately, it is often a compromise between a number of these and other considerations. Similarly, the collector and emitter resistor values will be based on a number of factors for the application - desired gain, desired collector current, desired emitter voltage, desired voltage swing, desired output impedance. Usually, one or two of these design considerations will be important for a given application, and they'll help to drive the choice of the specific values.
Hi! It looks like there is an oscillation in the circuit. It is more pronounced with the 20k collector resistor at 12:32. Or a picked up noise? Is it there? What it colud be?
I don't remember what I used, but likely something like the 2N3904 or other small signal general purpose transistor. Often times, an engineer will stick with popular, high volume, low cost devices unless there is some specific performance parameter that requires a more careful selection - such as requirements for a specific operating current, leakage performance, noise performance, bandwidth requirements, etc. It all depends on what the design performance requirements are.
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?
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.
Can u suggest a book for advanced bjt and fet transistor? I guess it would be pretty nice you make a video suggesting some books for us. And Im hoping we will have a Fet amplifiers teaching too. I studied fets by books, but they all say to use gm from datasheet. But it has no accuracy for gain calculation... or even drain current calculation. for example, source current with fets depends of Vgsoff, but generally it is a interval, not a fixed value, what give us poor acuraccy defining drain current. But sometimes, when we look at some internal ICs circuits, we see FET current sources... how do they know the current??
Hmm, I'm no expert, but regarding your question I believe you're either talking about jfet type sources (usually single) or mosfet mirrors, but I think the FET sources. While these will not have the best defined current value, they work by connecting the gate to the source (n-type), usually through a resistor, such that the fet will saturate and hold constant after a certain Vds has been reached. The rest of the circuit functions as expected based off of this relatively constant current. the wiki article on Jfets has more info + sources
Does that emitter voltage factor in the Beta gain of the transistor? I was expecting you to run into Beta limitations with the real high gain. Would that last circuit have resulted in a gain of 100 if the transistor had a Hfe of 100?
I don't have a video on this, but here's a short explanation. These refer to "small signal" behavior, which is typically when the BJT is in the active/saturation region (flat part of the typical IC vs VCE curves). gm is the small signal transconductance. It is the ratio of how much the collector current changes in response to a small change in base-emitter voltage. The re term is simply 1/gm. It also represents the small-signal output impedance of the emitter terminal. The ro term represents the small signal output impedance of the collector terminal.
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.
Very understanding video,keep making and sharing more... But I have a question that what it took to be so good in electronics? I am pursuing electronics engineering 3rd year ,any suggestions
In my case it has been a combination of factors: engineering degree, ham radio hobby, over 30 years of design and applications experience in industry and nearly 40 years of being an electronics hobbyist.
Great video! I have a circuit with a switched emitter bypass cap and a 500 ohm resistor to ground after the cap. What does that do to the signal and would the main emitter resistor still be bypassed and not factor into anything? Thanks in advance.
At DC and very low frequencies (where the capacitive reactance is high), the main emitter resistor is used to compute the gain (basically RC/RE). At higher frequencies where the capacitive reactance is low, then the 500 ohm resistor appears in parallel to RE, and the gain basically becomes RC/(RE||500).
w2aew thanks for that information. It makes some sense to me. I’m new to electronics and I am just looking for a way to adjust the output of my transistor on the fly. Can I get away with putting a trimmer pot on RC to adjust the output (lower rc/re value)? Its a 2 stage guitar boost pedal and I just want to have ability to control the volume out of stage one, the transistor.
@@Strumbum01 You can use a potentiometer to adjust the volume. It can be done at the input or at the output. But, do NOT use a trimmer pot as a volume control. Trimmers are not rated for frequent use, they are only designed for infrequent "set and forget" applications. They wear out very quickly, often rated only for a few hundred swipes. See this video for detail: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-5d_TTQ2OJtM.html
Transistor amplifier circuits and Op amp amplifier circuits are outputting real power, reactive power or absolute power? because I know how to get the gain which is the ratio of output voltage to input voltage but I don't know how to measure and calculate the output power. How is this done using an oscilloscope to measure the output power of a transistor amplifier circuits and op amp amplifiers circuits?
It depends on the load the amplifier is driving, because the type of power delivered is a function of the phase of the voltage and current. Read all about it here: www.allaboutcircuits.com/textbook/alternating-current/chpt-11/true-reactive-and-apparent-power/
I have a voltage 4v pk to pk and I’m trying take it all the way up to30Vrms and I would usually use op amp buffer but I want to see it how it would work with this. I’m thinking about 3 stages but is there easier way figure out the resistor values or do I just have to work backward and also I’m thinking about using 3904npn
I'm not sure what you are asking... The simplest way to see *what*? If you know you input and output voltages, then you can calculate the required gain. You will also know how much voltage headroom needed on the output side. That is enough to get started.
You're likely not going to do this with an op amp! 30Vrms is equivalent to about 85Vp-p! Thus, with 4Vpp input, you're looking for a gain of about 21 or 21x. You won't be able to use a 2N3904, it's voltage rating is too low. You're going to need a supply voltage of at least 90V or more if you want to amplify this directly. If your signal is purely AC, can you use a step-up transformer instead?
Hi Alan As always this is a another great video :-) Could you please load a video of a simple Video / Audio transmitter circuit and explain the flow of video / audio signal ? I have seen few video transmitters on U-Tube but all of them not properly explained. I also appreciate if you could explain at the same time, why Video input feed in to the Emitter leg of the Transistor kind regards , dinna
I don't have a AV transmitter circuit that I can review, but when I get one, I can do a video. In the meantime, one comment I'll make regarding the video input often gets applied to the emitter - it is possible that they're using a Common-Base style amplifier, which is suitable for low impedance high-speed applications. Check out this video to learn a little more about common-base (and common-emitter and common-collector) amps.
Hi Alan Thanks a lot for the response . Below links show the AV transmitter. That is most common circuit can be found in the U-Tube. I really appreciate if you could do a video on ( Video/Audio transmitter ) Links to the circuit ( This is the only circuit use most of the channels and sites ): www.zen22142.zen.co.uk/Circuits/rf/tv_tx.htm www.eleccircuit.com/diy-the-wireless-video-audio-signal-sender-circuit/ ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-rmGAd_MVaWw.html ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-D6mXAjd32bY.html Kind regards, Dinna
im trying to make a 50Gain amplifier with ouput of 5Vp-p. what should I do? because when I did the calculation. and tried it on multisim its far different from the targeted values. please help
Ok. Can I have some tips for my project? I need a 50Gain output with load of 1k ohms. Im using 12V VCC. Im going to use a 2n3904 transistor. Where should I start and what config do you recommend?
@@gimon2233 if you want 5Vpp, you'll need to have enough voltage across the 1k collector resistor accomodate that swing. So, that means at least 2.5V across it without a signal. Suggest 3.5V to give you some headroom. That means collector current is 3.5mA. Setup bias on Base to be about 4V, this will ensure headroom against saturation when collector is swinging 5Vpp. This puts emitter at about 3.3V, and you know emitter current is about 3.5mA, so total emitter resistance will need to be about 3.3/3.5mA = 940ohms. Since you need a gain of 50, you'll want about 20 ohms unbypassed in emitter. So, from emitter to ground put a 20ohm in series with 910 ohm, and bypass the 910 with a large capacitor. This will be a good starting point.
With a beta of 100, the base current will be 1/100th of the collector current. By making the bias current through the resistor divider equal to 10x the base current, that will make the bias voltage at the middle of the divider (at the base connection) very insensitive to variations in beta (base current).
I'm noticing a trend here in my lessons; the more skilled/intelligent the individual, the simpler their calculator is. (...my calculator was like $200... I'm off to a bad start here...)
