Basic transistor circuits, where to learn - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: April 23, 2021, 08:32:13 pm
content: Hi,I've recently seen an interesting circuit (in the attachment) fromthis topic in this forum.Although I tried studying transistors using tAoE (the only EE book I have tbf), the capacitor between the two emitters makes the circuit obscure to me.So my question is twofold:why does this circuit oscillate?Is there any book/reference/other that can help me (via exercises and study) understand better transistors and how to build circuits with them?Thanks.

author: ucanel
author_rank: Regular Contributor
timestamp: April 23, 2021, 08:49:45 pm
content: When i was asking the same questions years agoi found "talking electronics ... transistor circuits" very helpfull.Technically or mathematically not so detailed explainations butexamples are easy to apply and have good enough theory of operations.It is a very good starting point:http://talkingelectronics.com/projects/200TrCcts/200TrCcts.html?epik=dj0yJnU9ZGd1SVpwTUlzd0JKcWNaZWxDaUxyaGJkR2R3aXZaRnMmcD0wJm49VHpMb3QzVHZYRmJZbGZLa0NqRHdEUSZ0PUFBQUFBR0FWbDU0For details there is always Schaum's Series books.

author: Moriambar
author_rank: Unknown
timestamp: April 23, 2021, 09:09:04 pm
content: Quote from: ucanel on April 23, 2021, 08:49:45 pmThanks for your reply. I skimmed through the website. It really has a lot of examples, but it seems to me (at a first glance) not explaining anything. I mean, I could replicate the circuit but I fear I would be unable to learn much… I'll look it in more detail in the future.By the way what's the book series you mention?Thanks again

author: MikeK
author_rank: Super Contributor
timestamp: April 23, 2021, 09:13:47 pm
content: You can simulate that circuit in LTSpice, which everybody should have.

author: bd139
author_rank: Super Contributor
timestamp: April 23, 2021, 09:21:40 pm
content: https://learningtheartofelectronics.com/

author: Moriambar
author_rank: Unknown
timestamp: April 23, 2021, 09:45:20 pm
content: Quote from: MikeK on April 23, 2021, 09:13:47 pmIt's not about simulating but more about understanding why it oscillates

author: Moriambar
author_rank: Unknown
timestamp: April 23, 2021, 09:47:19 pm
content: Quote from: bd139 on April 23, 2021, 09:21:40 pmI have the book. I admit not having followed it thoroughly until now, but I think it covers more of the "normal" circuits.I tried skimming through it just a couple of hours ago, but I'm clueless about the posted circuit.Nevertheless I'll try and follow it very carefully

author: rstofer
author_rank: Super Contributor
timestamp: April 23, 2021, 10:04:55 pm
content: Maybe this brief description of an emitter coupled astable multivibrator will get you startedhttps://www.daenotes.com/electronics/digital-electronics/astable-multivibrators-working-construction-typesOr Google for 'emitter coupled astable multivibrator'

author: ucanel
author_rank: Regular Contributor
timestamp: April 23, 2021, 10:22:20 pm
content: Quote from: Moriambar on April 23, 2021, 09:09:04 pmSchaum's Outline of Electronic Devices and CircuitsThis book does not directly cover your question butit describes various transistor bias etc. situationsin an enginering or scientific point of wiev.So if you practice 200 pages of transistor explainations of ityou may conmect the remaing dots oryou may simply search the internet as suggested above forquick and dirty answers

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 06:24:46 am
content: Quote from: rstofer on April 23, 2021, 10:04:55 pmThanks, that was my previous course of action actually.Unfortunately the circuit is not explained well enough for someone as stupid as I am, since it outlines where the transistors should sit (in which region that is) and how to bias the network but not why this happens and how…

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 06:27:02 am
content: Quote from: ucanel on April 23, 2021, 10:22:20 pmThanks, I'll look into the book anyway; unfortunately I can't find anything useful online, that is: the circuit is presented many times but nowhere it explains what it happens, why it happens, etc…cheers

author: magic
author_rank: Super Contributor
timestamp: April 24, 2021, 09:19:57 am
content: Honestly, I don't understand their points eitherBut the circuit seems simple. Q1 is a common base amplifier - voltage appearing at its emitter modulates its collector current, causing same polarity, amplified voltage to appear at the collector. Q2 is an emitter follower which buffers Q1 collector voltage. The capacitor provides positive feedback - any random change in Q1 emitter voltage is amplified by Q1, buffered by Q2 and applied back to Q1 emitter, accelerating the process. This causes Q1 to quickly turn off or saturate, a state which lasts until the capacitor (dis)charges and then presumably everything repeats in the opposite direction.

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 02:24:48 pm
content: Quote from: magic on April 24, 2021, 09:19:57 amIf that's the toplogy I actually haven't understood a thing before. I'll try and use your explanation to see what I am missing.Thanks

author: rstofer
author_rank: Super Contributor
timestamp: April 24, 2021, 02:37:27 pm
content: The idea of using LTspice to simulate the circuit was mentioned above. It would be worth trying this before this thread dies so that others can help you get the model working. You can probe various parts of the circuit and display it on a 'scope'.I don't know if the Falstad circuit simulator will handle this or not. I have never used it to model anything. I do enjoy the animation and it's worth the effort to try.Google 'Falstad circuit simulation'There's a video:https://youtu.be/TTZnl7ZjFhA

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 02:39:35 pm
content: Quote from: rstofer on April 24, 2021, 02:37:27 pmWell I actually built up the circuit and probed it, but that did not help at all: it does not tell me anything useful, ie: I'm missing something more basic here.

author: rstofer
author_rank: Super Contributor
timestamp: April 24, 2021, 02:40:57 pm
content: w2aew has some videos on transistor circuits. Here's one that discusses the class of amplifiers.https://youtu.be/c6cmkm3UPUIThere are other videos in the series.

author: bd139
author_rank: Super Contributor
timestamp: April 24, 2021, 02:41:06 pm
content: It's easier to visualise these things as extremes by removing the capacitor to set a DC operating point and then thinking about what happens when you add it back again. The capacitor can be thought of as requiring current to be shoved into it to change the voltage across it and that takes time. Applying that, when you initially put it back in the circuit then the voltage across it is zero, and that takes time. Think about what happens to the circuit when momentarily both emitters are at the same voltage.Before you do this, run through the book I suggested earlier. You're missing some of the basic things about more empirical definitions of what capacitors and transistors are and how they work.This stuff is fairly hard to get a mental model of and absolutely no one seems to know how to explain it properly. At university I could design a transistor circuit fine but I really didn't know how it worked. TAoE taught me that and the mental abstraction required for intuitive understanding. Now I can usually eyeball the circuit.

author: rstofer
author_rank: Super Contributor
timestamp: April 24, 2021, 02:56:41 pm
content: Quote from: Moriambar on April 24, 2021, 02:39:35 pmI haven't tried it but I imagine a 4 channel scope would be real handy. I would probe the collector and emitter of both transistors. Not everybody has a 4 channel scope so LTspice is the next best thing. What you are looking for is the dance around the capacitor. The relationship between the emitter voltages of both transistors. The feedback capacitor is what makes the circuit work.The Falstad animation might show that feedback action quite well.The emitters drive this thing. Knowing how the change in the emitter of Q1 changes the collector of Q1 and how that changes the base of Q2 which impacts the emitter of Q2 and back to the emitter of Q1.Maybe this description will help:https://cadernodelaboratorio.com.br/en/astable-multivibrator-with-a-single-capacitor/

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 03:55:31 pm
content: Quote from: rstofer on April 24, 2021, 02:56:41 pmUnfortunately seeing things does not help me at all understanding why they do.BUTthe link was very helpful and I found the point I don't understand, it's point 4 in the recap, ie, when the second transistor is switched onQuoteWhy does the voltage on the emitter increase?Thanks

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 03:58:58 pm
content: Quote from: bd139 on April 24, 2021, 02:41:06 pmI'll probably have to review all of my AoE prep. A year or so ago I started tinkering with BJTs, designed a couple of simple circuits and got pretty much a good hang on them. Or so I thought. I fear I still know nothing.

author: rstofer
author_rank: Super Contributor
timestamp: April 24, 2021, 04:43:14 pm
content: Quote from: Moriambar on April 24, 2021, 03:55:31 pmAs TR2 turns on, more current flows through the emitter resistor raising the emitter voltage. This increase in emitter voltage on TR2 is coupled to the emitter of TR1 causing it to conduct less (by raising the emitter voltage given a fixed base voltage) which raises the collector voltage which increases the base drive on TR2 causing it to conduct more which raises the emitter voltage of TR2 and so on. Eventually, the capacitor is charged and the voltage on the emitter of TR1 drops causing TR1 to conduct lowering the base voltage on TR2 causing it to conduct less causing less emitter current and the capacitor voltage drops pulling the emitter of TR1 even lower.I'm not sure that my explanation is easy to follow. Just remember, it is the voltage across the capacitor and the way in which it changes that makes this thing work.

author: rstofer
author_rank: Super Contributor
timestamp: April 24, 2021, 04:57:04 pm
content: Some circuits are just a bit complicated and nobody wants to write a description unless they are darn sure they know how it works. I sure didn't. I had an idea but not the complete explanation. The linked paper does a good job and all I did above is pretty much expand on what they said.Feedback circuits in general and oscillator circuits in particular can be pretty hard to explain.As mentioned above, you can make a first cut at it by first assuming initial conditions when the capacitor is replaced by a short circuit (discharged). Figure out what both transistors are doing. Then remove the short circuit (capacitor charged) and see how things change. This gives you both ends of the simulation and you can pretty much guess how things change between the two end points. You can mentally start a new cycle by replacing the short circuit (discharged).

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 05:53:18 pm
content: Quote from: rstofer on April 24, 2021, 04:43:14 pmThanks, yes all is ok now. Very very helpful. And thank you for the patience too.

author: dietert1
author_rank: Super Contributor
timestamp: April 24, 2021, 06:21:20 pm
content: Now you can try whether it oscillates if you leave out the emitter follower and connect the capacitor directly between emitter and collector. Probably it does not oscillate.Answer:Common base = in phase voltage gainCommon collector = in phase current gainRegards, Dieter

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 06:43:05 pm
content: Quote from: dietert1 on April 24, 2021, 06:21:20 pmPerhaps I will.I'm so disappointed by the "common base" circuit to be left out of the AoE… I'll have to study it better (well, perhaps I'll have to restart from the basics, but still!)

author: bd139
author_rank: Super Contributor
timestamp: April 24, 2021, 06:45:46 pm
content: I think it's because it's not really used that much these days. Typically where you'd traditionally have a common base amplifier someone stuffs an off the shelf high bandwidth opamp in there instead with a low impedance input.In this oscillator it'd be an IC too these days. Stuff an LTC1799 in - job done!

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 06:49:23 pm
content: Quote from: bd139 on April 24, 2021, 06:45:46 pmyes but I think these circuits are still very valuable both for knowledge around trannys, and because if I need something easy it's more likely I have a 2n3904 in my "jellybean parts bin" than an ltc1799 or other and sometimes it's good enough

author: bd139
author_rank: Super Contributor
timestamp: April 24, 2021, 06:54:07 pm
content: 100% agree which is why I do a lot of discrete design still (not professionally!)

author: Moriambar
author_rank: Unknown
timestamp: April 24, 2021, 07:20:43 pm
content: Quote from: bd139 on April 24, 2021, 06:54:07 pmthat's wonderful! Make a book about your discrete designs! I'll buy it

author: bd139
author_rank: Super Contributor
timestamp: April 24, 2021, 07:24:45 pm
content: It’s on the bucket list to write a book on that actually. One day I’ll get the time. The day job which is almost entirely unrelated gets in the way too much.

author: Moriambar
author_rank: Unknown
timestamp: April 25, 2021, 10:33:58 am
content: Quote from: dietert1 on April 24, 2021, 06:21:20 pmIt does not oscillate. I don't understand why actually, but I've given up.