Class A amp simulation vs reality - why the big difference? - Page 1
Forum section: electronics / beginners

author: dvh
author_rank: Contributor
timestamp: November 08, 2023, 03:18:10 pm
content: Hello folks!( first post!)I've attached an image and a simulation file around a BC547B BJT. The simulation gives me nice and high P2P voltage swings on the collector. DC properties of built circuit (on a breadboard) are very similar, Vbe, Vce, Ve. So I thought I'd be fine, let's hook up some signal and instead of a ~200 gain, I see only 35-ish. Measured the output swing with an oscilloscope.What do you guys think, why could there be such huge differences between simulation and reality?PS: I love the retro feel to the forum, brings back soo many memories

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 08, 2023, 03:33:05 pm
content: 1) You inserted the transistor the transistor C-E pins backward.2) You have your oscilloscope probe set to X10 when you thought it was at X13) You have misread the amplitude of the input signal

author: dvh
author_rank: Contributor
timestamp: November 08, 2023, 05:17:24 pm
content: I double checked all the things you said.I was annoyed at first that none of those things were the cause. Then I realized...I didn't have the kind of resistor for R2 so I had to use multiple ones. I accidentally hooked up the signal line inbetween some of those resistors. Now the PP is much better, believable levels.Next problem I'm facing is that something happened to linearity. The amplified signal doesn't quite look like it should.Why the big distortion, I wonder...

author: iMo
author_rank: Super Contributor
timestamp: November 08, 2023, 05:48:24 pm
content: I think the 20ohm in the emitter is too low. Go with something higher, like 330-1k.And put a resistor in series with the emitter capacitor C2, to regulate the AC gain..

author: jwet
author_rank: Frequent Contributor
timestamp: November 08, 2023, 06:09:08 pm
content: The transistor is not biased properly if I catch what you're trying to do. It looks like you want a gain of 200 and have put 4k/20 in to give it to you. The max gain you can get out of well biased class A amp is Vcc x 20. (see A of E for an explanation). The 20 ohm emitter load even with the high beta of the BC547B is 200, so reflected onto the base will look like 4000 ohms. Your base divider needs be stiffer than this. Let's start over and see where we get. It also looks like you're hoping to get very input Z. Its possible that you can't get all you want out of one class A stage but let's continue and see where we get.1. Set emitter volts at 1v, this will give us pretty good bias stability with temp and will still also for a large collector swing (8v). Set bias current at .5 mA, the BC547 is pretty good at these currents and we'd like to keep resistances on the low side. This can be traded for bandwidth, etc.2. With 1V at emitter and .5 mA, Re would be 2k. 2k times 200 (beta) would be 400K. Base divider should have parallel resistance of << 400K, say 50k for an easy number. Voltage at base will be 1.7v. The divider ratio is 1.7 over 9 or .188. Divider will be r2/(r1+r2) and parallel is 50K. With easy standard values, R2 is 62K and R1 is 270K. This gives the right ratio and par R is 50.4k. This is also input Z approx. Your coupling cap of 100 uF, will take you down to 31 Hz, use 220 if you want to get down to 15 Hz. Note this input Z is only at DC, it will be lower because of the effect of the collector moving faster than the base and adding feedback (later, below).3. Emitter bypass. The small signal r of the emitter re is 26mv/Ie or about 52 ohms. If we completely bypass Re, we will have a residual unbypassed R of 52 ohms.3. Set collector load and maximize collector swing. Vc min should be greater than Vb= (1.7v) don't reverse bias CB, say 2v. The midpoint of 2 and 9 is 5.5v. This will put collector collector at 5.5 with 3.5v of swing. With bias set at .5 mA, Rc would be 7K.4. The DC gain Rc/Re is 7k/2k or 3.5- theoretical only since we're ac coupled. The AC gain is much higher with the bypass of all but 52 ohms of Re, we have 7K/52 = 1345. Miller effect, The BC547 has a Ccb of 3.5 pF. This get multiplied by this high gain and will look like a 469 pF cap hanging on the base. This will drive down AC input impedance- at 1 Khz, you'd have and input Z of 339K (not terrible- in parallel with 50K don't forget), though at 100kHz, it would be 3.4k. This is a low frequency amp.I hope this helps. A "good" simulator- LTSPICE vs Falstad, etc would give you a good analysis of this. A DC simulation even in spice is pretty useless.Your topology is good, you get an amp with about 50K input Z, a bandwidth of audio realistically, a gain over 100 for about .5 mA. out of 1 transistor. Tweaking this to give you all of what you want is the really hard/fun part...

author: iMo
author_rank: Super Contributor
timestamp: November 08, 2023, 06:40:59 pm
content: And the input AC impedance..

author: dvh
author_rank: Contributor
timestamp: November 08, 2023, 07:22:09 pm
content: QuoteCan you point me in the direction of A of E thing, please?Edit:Had to read it a couple of times but I think I get most of it! I'm trying to work in the 50-15k Hz range (I only had signal source of much higher at hand)200 gain was in fact the goal with fairly high Z.QuoteWhat does that mean? How can I further research this reflection effect, what is it called/some pointers would be appreciated?

author: jwet
author_rank: Frequent Contributor
timestamp: November 08, 2023, 11:17:41 pm
content: Art of Electronics, by Horowitz and Hill, nicknamed "A of E" is a great electronics book especially for high performance analog design in real detail. It really covers all topics in a way that I really like. Its on its third edition and costs over a $100. The first edition is from the 1980's and these old discrete basics haven't changed much, you can pick these up for $20 if you're strapped for cash. A good library especially at a college would likely have it. If you have long term interest in electronics, I would just buy it for yourself and chalk it off to education. Read it cover to cover when you get it and keep doing that about once per year for 10 years or until there is a new edition. You'll get something new out of it each time you read it.The reflected business works like this. A transistor has a current gain (ration of Ic or Ie to Ib) of beta (or Hfe, etc). "Beta" for most small transistors are usually around 150, your BC547 can be as much as 400 in some cases, its not a well defined parameter but it turns out it doesn't matter much. If you have an amplifier with a 1K resistor in its emitter circuit, any load that you put in the emitter will get multiplied by beta when seen through the base. The 1k ohm emitter R will look like a base load of beta x Re or about 150k+. There is no magic, the transistor is powering the load and is "amplifying". The "reflected" load looks like 150K. You can see this effect on input impedance in your sims. At very low frequencies, we see the 50K ohm Z, this is just the divider DC parallel resistance. As the frequency gets high towards 100 hz, the emitter load goes to the intrinsic 52 ohm value time beta (350) is about 18 Kohm when "reflected" into the base. You see this impeance (in parallel with the 50K resistance) as about 10K on the graph. You can help this a bit with "bootstraping", I leave this as an exercise but you can probably make input 50K everywhere.Have fun.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 09, 2023, 06:54:21 am
content: The the treatment of theory side of linear transistor amplifiers in the AoE hasn't changed markedly across the editions, early ones are available on the net as pdf downloads for free.

author: dvh
author_rank: Contributor
timestamp: November 09, 2023, 06:55:48 am
content: Right! Thank youI'm not sure about the 10 years, but I do really enjoy it so far. It's a hobby for me and I enjoy exploring stuff for myself. The investment of an oscilloscope was THE things that sped learning up a LOT. Though sometimes I find that it is the hard way to go. The book would be an excellent complement to that

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2023, 07:14:14 am
content: What jwet has explained to you - the input impedance Zin looks aprox like - see below.Z_C3 = 1 / (2*pi*freq*C3) in ohmsZ_C4 = 1 / (2*pi*freq*C4) in ohmsR1=270k, R2=62k, R3=2k, beta=200, I_emitter=500uA

author: dvh
author_rank: Contributor
timestamp: November 09, 2023, 09:06:58 am
content: I think I need to go back to learning a bit. I recognize, what you did @iMo, but I'm not experienced in rearranging the circuit to calculate equivalent resistance, etc. I need sum time, I guess.

author: iMo
author_rank: Super Contributor
timestamp: November 09, 2023, 10:00:24 am
content: You have to start using the LTspice for playing with circuits before wiring them in hardware - most notably the DC analysis, AC analysis, transient analysis and Zin or Zout as shown above. It is easy..

author: jwet
author_rank: Frequent Contributor
timestamp: November 09, 2023, 02:06:25 pm
content: Its a lot to swallow in one bite. Keep hacking, it will come.

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 09, 2023, 02:37:31 pm
content: I don't understand why there is so much praise for A of E when at the same time there is no mention that Youtube has tons of information available free about what this type of Class A Linear Amplifier is more commonly known as.It is just called a BJT "common-emitter amplifier". Besides, free old .pdf editions of A and E can be downloaded for free at many websites.Seems to me like someone is trying to boost sales of an obsolete bookwhere a great source of free alternative information is readily available on Youtube.A common sales technique: Mention a problem, praise and recommend buying the absolutely best cure, but be sure to fail to mention solutions to a problem that are commonly available and free.Youtube might often offer better training than A of E. You can easily find insight into more circuits than A of E, in fact, much more than anyone would need to completely understand a wealth of common electronic circuits.Just Google or search on Youtube to access a free and easy way to learn what you want to learn about!If you then have questions, you can always find someone to answer them here on this board.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 09, 2023, 03:23:17 pm
content: Quote from: dvh on November 09, 2023, 09:06:58 amTry the first chapter in the book "Solid state design for the radio amateur - ARRL 1986", has the merit of being very brief, yet to give understanding of 90+% of how it all works.https://archive.org/details/SolidStateDesignForTheRadioAmateur1986/page/n9/mode/2upThe BJT (Bipolar Junction Transistor) only takes less than 5 pages to cover most of the essentials, and calculates all with close to zero math.

author: jwet
author_rank: Frequent Contributor
timestamp: November 10, 2023, 06:58:02 am
content: There are a lot of ways to learn and there are a lot of great resources. I'm a huge Fan of A of E for several decades now. I've run several engineering departments in large companies and gave my new EE grads a copy of A of E on entry. Its hardly obsolete, its in its third edition and an a supplement was released in 2020. I actually didn't know you could get it online, I prefer real books, just old fashioned I guess. It covers things in such exquisite simplicity but digs into a very deep level- this is rare. It has extensive lists of modern devices like transistor, FET's, Op-Amps, etc that are a great resource. It will take you from zero to a high level of expertise and answer almost all your questions. Most treatments of discrete transistor design involve a lot of hand waving, bad approximations, leaving gaping holes in true understanding and answer one type of question that the author is familiar with. Try to find decent information on how to design for optimum noise figure, minimum voltage noise or current noise, designing HV circuits, techniques like cascodes, diff pairs, pole splitting in complex amplifiers, stability, thermal design, modern devices, modern methods, packaging and construction, power sensistive circuits, etc, etc. I've met both authors when they toured my department about 15 years ago, they are brilliant and have about 80 years of electronics experience between the two of them. I have an autographed copy of their second edition which I just happened to have out of on my desk when they came through. A of E is just my advice, I try to help beginners and others and offer my advice.I'm also a fan of Wes Hayward too and have met him too as RoGeorge recommends. I never met Doug DeMaw but he's a solid engineer and amazingly prolific Ham author. I think the treatment of basic transistor theory in Expermental Methods..., RF Design for Radio Amateur and Introduction to RF Design (all with input from Wes Hayward) all jump right into a lot of algebra immediately, get into models like hybrid Pi kind quickly without any motivation. By page 9 of RF design- the ARRL Edition, they present a model of a transistor with 15 parasitic components in a chapter on low frequency design. By page 15, there are integrals to explain large signal models- I just prefer A of E's approach. Hayward is coming from a degreed senior RF engineer IC designer direction and gets into the weeds all over his books and I wonder why I'm reading it. He is one of the better Ham authors, there are guys like Joe Carr, that just copy data sheets and write drivel.You Tube is a whole 'nother story but its generally very superficial and usually just wrong. It can be entertaining.Look, have fun. Develop your own resources and provide your own advice. When I look at this thread, compare what I provided the poster compared to what others provided. He actually sent me a PM, said I was spot on and thanked me.Give A of E a try. I think what triggered me on this topic- SuzyQ is your bolded near accusation that I was pushing a book for some ulterior motive. Grow up.

author: donlisms
author_rank: Frequent Contributor
timestamp: November 10, 2023, 06:11:22 pm
content: I like Dr Christina's BJT videos on Mateo Aboy's YouTube channel. They helped me a lot when ramping up with transistors. Simple and methodical. (Sometimes a little out of order, though.)

author: jwet
author_rank: Frequent Contributor
timestamp: November 11, 2023, 12:23:18 am
content: I'm sorry, I looked at one titled "Class A Stage: Common-Emitter" by her and its just plain wrong. It completely ignores any biasing. She also has no emitter degeneration, this will be extremely non linear, its gain will vary widely over signal swings. This is really just not correct. She has oversimplified things to get the idea of a load line and a transfer curve across but in the end, its very poor.Here's the link I looked at- very misleading advice.I read another about biasing NPN's by her-Ok finally at lesson 18 at the end, she says introduces gm. She never really explain ebers moll but does talk about "a small signal model". She does this with Hybrid Pi and a dependent current sources which is a complicated way to think about it and not intuitive. Lesson 19, she reviews everything but goes back to all the Ic/Ib stuff as in the first stuff. Don't get me wrong, this is how I learned transistors 40 years ago but it took me 10 years of self study and A of E to really appreciate how these circuits work.She never wants to bring up the real crux of how transitors work, they're aren't current amplifiers fundamentally, they're expoential transconductor that generate current based on voltage in a non linear way. This is "the diode equation", ebers-moll, whatever you want to call it. You can't apply transistors to any depth without understaning this.I'm glad you like it. The pace is good, she's a very pleasant person but the content is below high school level for what I've seen.

author: donlisms
author_rank: Frequent Contributor
timestamp: November 11, 2023, 03:40:28 am
content: I'm sorry you didn't like them, but I would first point out that the suggestion wasn't directed to you. It's pretty clear you already know everything. The Dr. isn't trying to present the whole thing in one session; she's a teacher, and she knows teaching -- that students have to have time to think about things as their brains absorb the material. Don't present a whole pile of peripheral issues -- stick the main point, and make sure it's clearly expressed.All of the issues you mentioned were missing are actually in the other videos -- you can't judge a whole series by just watching one and saying "it's not all in there!" That's just not realistic. She covers all the biasing techniques and advantages and disadvantages of each, as well as how to choose values. She covers all the configurations. She covers choosing the collector and emitter resistors, and bypassing. She covers input and output impedance in a way that's quite understandable and useful. She explains the math for all of it. She has a PhD, for goodness sake, as well as experience teaching lots of different people. Let's assume she actually knows more things that she is able to put in each single targeted video.Yes, it's about starting from the beginning; and yes, it would take some patience to go through them all. It's not realistic to think you can present, or learn, all there is to know about BJT amplifiers in one half-hour session. I think you're way off the mark, here, and it looks like you're attempting to discourage the original poster from taking advantage of a potentially valuable resource because of... well, it seems like some sort of personal quirk, actually.

author: jwet
author_rank: Frequent Contributor
timestamp: November 11, 2023, 02:48:05 pm
content: Learning style is a personal thing. No offense to anyone. I guess A of E's approach is a good match to my style. PS- don't be too wowed by PhD's, I've known techs that can run circles around them outside of their narrow field.Have fun.

author: dvh
author_rank: Contributor
timestamp: November 12, 2023, 06:23:39 pm
content: This thread hits some really good chords with me.I've been trying to read specialized material and watch YT videos. I learned a crapton, without any prior electronics knowledge. I got to a point where I could design a working, albeit crappy amp.I think videos and tutorials take you only that far. I remember having a hard time just searching for names of stuff. How do you call that thing that I just measured? A good example is Schmitt trigger. Who the heck is that dude and how would I guess that this is called like so?So I take a bit of time, read some material. A of E and that radio amateur link.Let's see if I come out with a much better understanding of the recommendations here. AC analysis I have trouble with and thus impedance calculations too.

author: jwet
author_rank: Frequent Contributor
timestamp: November 13, 2023, 01:52:58 pm
content: dvh- you seem to have a pretty good handle on things especially spice. I try to find a very authoritative voice and stick with them. Dave is certainly one. My other hobby is machine shop hacking around which I know much less about. I have to be more careful because I can't judge good from bad there. I have found several great YouTubers in that Field (Joe Pie, Tom Lipton, AVE, Sherline among others). Keep learning and enjoy.

author: dvh
author_rank: Contributor
timestamp: December 10, 2023, 11:28:07 am
content: Uhm, who is Dave?I thought I'd drop a bit of an updateJust in case anyone is interested.I am mostly done with Chapter 2 of AoE. I'm certain, I would have not been able to chew through this without prior knowledge, it is not a beginners book. Now, I'm fairly confident designing different amps. Practice shows that I have a pretty good handle on it by now and I now know more about the jwet's comments about Miller effect and early effect, impedances, AC analysis, equivalent circuits and who knows how many different areas were touched in that chapter.Next to my job, holidays, moving it took me quite some time to get here but was worth the effort. I estimate, it will take about a year to get to the end of the 1200 pagesThanks for the help!

author: watchmaker
author_rank: Unknown
timestamp: December 11, 2023, 01:47:43 pm
content: It really depends upon how you define "learning electronics". I was originally a high school Voc Ed (Now competency based education) Electronics graduate taught in the days of tubes via the USN (NEETS) curriculum. Had to take a career detour and make money in order to do what is really my nature (make broken things work).The people here will help you accomplish your goals as long as you are serious. But your goals are YOUR goals. Now is a good time to think that through.I recently reignited my desire in electronics. Started with NEETS, found it lacking. Purchased Art of Electronics which was useless to me (far too zen like). It IS a good source for refreshers and ideas once you understand things.A lot depends on whether you want to build established ckts, or if you want to understand them and modify them with confidence. This is analogous to what I experienced in my career as a watchmaker. Many wanted to "make em tick" and only learned how to use some tools and at best replace parts. Very few took paths in which they learned the physics and the theories behind precision time keeping.This is not a judgment, but the extent of your foundations of knowledge determines (in the real sense) what you can achieve in an intentional manner with full confidence that you will hit your goal.Guess and golly vs. knowing precisely what to do. This is only slightly overstated. As observed in this thread, in EE theory is a first approximation. But in my experience, theory helps to understand the discrepancies.I created a self study program for myself that emulates the first couple years of EE or a 2 year tech course. I posted some of the sources in the sticky at the top of this forum. I just purchased additional texts that suggested by contributors that cover the topic I just completed (DC Ckt analysis) because I wanted more practice at reducing networks. This is fundamental for figuring out how to match an output to an input, as well as how to develop needed voltages.I also make use of the Heathkit Educational Series. While this series avoids complex situations, it can help when a different source seems confusing.Many texts are available for download or loan at Internet Archive.I purchased an Analog Discovery 2 which provides virtually all the instrumentation and sources you need for breadboarding ckts for labs (and labs are essential). While I have the resources to have built a pretty complete "lab" based around 20 year old (ebay) Fluke and HP DMMs, power supplies and a Siglent scope, The AD2 and a laptop is all you really need to learn a LOT of theory. It is what my son in law used in in his EE program. And it seems graduates do not sell theirs; virtually none on eBay.I am using the Diligent AD series of courses and labs (materials can be used online or downloaded).AMD and National Instruments also have a very good collection of courses and labs that make use of the AD2 or something similar.Even if this is not directly helpful to you, it may be useful to others.And BTW, I am back to relearning middle school algebra, trig and matrix algebra. It all depends on how deep of an understanding you want to accomplish.

author: jonpaul
author_rank: Super Contributor
timestamp: December 11, 2023, 05:37:26 pm
content: Bonjour a tous:For my recreation of a 1942 WII top secret speech encryptor, with a dynamiv mic preamp, we had 2. stages of tube/valve/ampoule triodes, 6SL7.We just built it from the 1953 RCA Vacuum Tube manual.With a VOM, scope and a few resistance boxes, we adjustd the plate and cathode reisitors to optimize. Works great!Lots of fun,NO SIMULATOR NEEDED!Jon