transistors (usage in current designs) - Page 1
Forum section: electronics / beginners

author: charles
author_rank: Contributor
timestamp: July 18, 2014, 02:55:23 pm
content: Hello,I have recently repaired/been repairing an old 1967 hp power supply; as a result, I have been teaching myself about transistor circuits. This made me wonder, are transistor circuits like these used (directly) in modern electronics anymore?For example, this power supply has a whole host of transistor circuitry to make differential amplifiers, presumably this could be replicated with a single op-amp. Or as another example, the input uses multiple zeners and transistors to create a reference voltage, which could presumably be replaced by a linear regulator ic.Since I am new to electronics, nearly any experience is valuable experience and I enjoyed learning that little bit of transistor circuitry I needed to repair the power supply. However, I can't shake the impression that I am essentially teaching myself the 'assembly language' of the electronics world. In other words, it is perhaps useful to learn some of these circuits but most modern electronics uses the 'higher level' language of integrated circuits. Is this an accurate impression? Thus, as a follow up, how valuable is learning all about the intricacies of transistor circuits? Should I dive all the way in, or wade around the edges?

author: bwat
author_rank: Frequent Contributor
timestamp: July 18, 2014, 06:50:52 pm
content: The book I'm working my way through to relearn electronics still covers transistors (how to build different kinds of amplifiers with BJT and FET) before the opamp. I remember it being the same order back when I was taught this at school. The book I'm using is probably at the level of an introductory course for undergraduates. I think the curriculum covered is pretty standard.For the sort of chips you mention I bought the TI Analogue Systems Lab Kit and it covers opamps analogue multipliers DAC etc. It's a practical course that assumes you know basic electronics including transistors as one of the projects needs some I think. The board is describedhereand you can download the manual. It seems to be an undergraduate level course.So all in all it seems that modern undergraduate courses will require learning about and using transistors. Now if that means you need it for practical work, I'll leave that to others to answer.One more thing. The biggest difference between amateurs and professionals is that the professionals have done the ground work and learned the basics. Just think of all the masterchef wannabes who are cooking at home with expensive ingredients all the trainee chefs aren't allowed to touch. I don't think the experience you've gained will be useless.

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2014, 07:34:17 pm
content: Quote from: charles on July 18, 2014, 02:55:23 pmDiscrete solutions are still used when they they provide some advantage over the integrated solution and that could be price or performance.QuoteRegulators and references are one place where an integrated solution is going to provide better performance at equivalent or lower cost anyway so operational amplifiers replaced differential pairs and references replaced zener diodes very quickly after they were introduced. The voltage regulators in power supplies did not require anything that an integrated solution could not provide like bandwidth.The accuracy of a different pair depends on matching so if you were going to make a monolithic differential pair for high accuracy, you might as well make the whole integrated circuit. Integrated references require similar matching which is why bandgap references are not made with discrete transistors although it is possible to make one with a couple of matched pairs but again, the integrated solution is either less expensive or higher performance anyway.Monolithic and discrete matched differential pairs are still made but they carry a premium price for a given performance because of low volumes compared to a fully integrated operational amplifier.QuoteAll of the same rules apply to both but understanding the more abstract integrated solutions requires understanding the building blocks that they are built on. Integrated building blocks are great until you either need one that does something uncommon or you need to understand why the one you are using is misbehaving.

author: w2aew
author_rank: Super Contributor
timestamp: July 18, 2014, 07:41:17 pm
content: David covered many of the same points I was going to cover (thank you David for saving me the typing).You're analogy of transistor level design vs. integrated circuits being similar to assembly vs. a higher level language is somewhat accurate - but I wouldn't say it's universal. There are a lot of circuit functions that are better handled with discrete components. The reasons for this can be quite varied - performance, cost, efficiency, function, simplicity, etc.In my opinion, there is a lot to be gained by learning fundamentals of transistor level circuitry. Even if you don't work with discrete devices directly, having an understanding how these circuits work can give you insight to things like "why doesn't this op amp output swing to ground?" By looking at the schematic of the op amp's output stage, it might be perfectly obvious.

author: Stephen Durr
author_rank: Contributor
timestamp: July 18, 2014, 11:19:14 pm
content: Quote from: w2aew on July 18, 2014, 07:41:17 pmWell said!As proof of this, I offer an experience I had just today. I was studying a basic BJT based AC voltage amplifier circuit (in a text book) and there was a part of its operation I just couldn't seem to grasp. Determined to understandwhythe circuit worked instead of just taking the book's word for it, I dug in my heels and walked through the circuit analysis step by step asking myself if I truly understood each element. To my surprise (and embarrassment) I discovered a fundamental flaw in my basic understanding of Ohm's law! I'm sure the hour I spent staring at that circuit today will save me countless hours of frustration as I proceed with my studies. I doubt if I would have learned as much from reading the datasheet of the equivalent IC.

author: charles
author_rank: Contributor
timestamp: July 19, 2014, 06:27:10 pm
content: Thank you everyone for your very thorough responses! I have always been nervous to post anything on a public space like a forum and the eevblog is the first place that I really gave it a try. So far, it has been a great experience and I wanted to thank everyone for being so wonderful and helpful.The point about understanding the limitations and function of these integrated circuits is well taken. I also like the chef analogy, although I hope my ability to build electronics isn't tied to my ability to cook or I am going to have some pretty inedible circuits on my hands.As a side note, I also want to thank w2aew for your excellent videos. Your video on common emitter, common collector and common base transistors was one of the first places I went when beginning the process of learning about transistors.Thanks again everyone!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2014, 07:23:13 pm
content: Nope.60s tech was the peak of elegance: just enough transistors to do the job, they were still expensive -- consumer applications like transistor radios and TVs still aggressively minimized parts count, since tubes were still in use (your average B&W or color set from those days used transistors for the low level oscillators and amplifiers, occasionally RF amplifiers, and the rest -- UHF amp/osc, deflection, audio output -- in tubes), while more expensive pro equipment, test instruments, etc. went for more, usually resistive loaded amplifiers with low gain and reasonable bandwidth, but a few stages chained together gets you what you need.What wasn't used so often -- unless absolutely required -- was the monolithic style structures: current mirrors, bandgap references, multipliers, etc. Don't know why current mirrors weren't so common, they're not too finicky in discrete form -- but then, instead of using a few current mirrors and sources and sinks, you probably get more bang for the buck by using all those extra transistors to chain a few gain stages together. They're most useful in circuits with wide operating voltage range, which is typical of ICs (say, 3 to 30V for an LM358, is it?), but an amplifier in a fixed application (say 8-16V supply) can just be tailored to fit.The real quirky stuff, like multipliers, saw use in synthesizers and stuff. Variable gain amps, variable frequency filters, modulators, exponential VCOs (linear-pitch rather than linear-frequency), the works. All insanely tricky to get right, tons of thermistors, matched transistors, thermally lagged transistors, calibration trimpots, and so on. If they had enough quantity to do it monolithically, they would've been doing much better, but by the time that came along, I think, there were digital FM synthesizers with so much more interesting sounds, and perfect pitch.Today, everything is sucked up into an IC. Op-amps, ADC/DAC, microcontrollers (for the first bit of history, a processor on par with a dumb Arduino would've been a million bucks, and ran much slower, a few MHz perhaps; it wasn't until the 70s when microprocessors came along (everything on one or a couple chips), and until the late 70s / early 80s when cheap processors of the same capability finally saw consumer availability), it's all available on chip, and if you can't find some function, it probably doesn't exist! (With only a hint of sarcasm: some people more than others will actually believe it.)So my resounding "No" at the top is pretty much true today, BUT don't ever let that keep you from using discrete transistors.1. Knowing transistors means you're prepared to understand the inner workings of ICs. You can read between the lines of the datasheet and guess some things they're just not quite telling you; or, if it's something they left out entirely, you'll have a good idea of exactly what information is missing, and just the question to ask. And if they don't want to answer that question... run away, because they probably know less than you do, and they designed the freaking chip!2. You regularly need "glue" between subcircuits. Sure, you could buy a 74HC1G04 just to invert that one single clock line that's backwards. Or you could use a transistor. (For digital, you'd probably use the gate anyway, so you aren't wasting space on the resistors needed with the transistor. But there are better examples.) There are some digital level shift applications that you just can't find an IC for, let alone anything analog.3. You can create strange circuits like this,http://seventransistorlabs.com/Images/Discrete_Tube_Supply.pngwhich perform better than ICs in far, far fewer transistors (semantically speaking).4. And along those lines, working from the lowest (implementation) level gives you a better idea of the most basic elements required for the circuit. Although you candraw something out in its entirety, you're better off understanding it in terms of the block level diagram, and merely understanding the stuff inside those blocks. Most of those blocks will be something simple (the oscillators in this example can be any comparator or timer based solution with the same waveform), and there will be just a few blocks that you're better off drawing in more detail, to get a deep understanding of their function (say, the sampler -- pretty much no one makes analog samplers anymore, even though the function is still used in every sampling type ADC).Another example: maybe you have a power switching circuit that you're looking at. Like, a motor controller or something. So you concentrate on the power switching itself, drawing not only the switches doing the work, but the parasitics around them, to optimize everything and keep the switching as clean as possible. Then the control -- you know you need some sort of PWM signal, but beyond that, it's pretty wide open: it could be discrete, assembled from blocks (op-amps and such), or done entirely digitally (in an FPGA, or if you're feeling even more risky, an MCU). Or maybe you want a bit of both, like digital setpoints but analog control, or analog control with digital monitoring. All of this, you'll just be pushing around blocks, not minding the implementation until it's time to do it.It's the same way with good programming, by the way. Say you have some algorithm, that's doing a lot of heavy lifting at the bottom; and it needs to be told what to do: what data to operate on, how to be configured, etc. But the implementation itself, as long as it includes those inputs, is where you're spending 90% of your time, and needs to be implemented first, and optimized the most heavily. The higher level code around it might be complicated and ugly, but even doing it in a high level language doesn't cost you much time, because it's executed much less frequently. You can optimize that as well, but it's not going to save you much, either.Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 19, 2014, 08:10:00 pm
content: Quote from: T3sl4co1l on July 19, 2014, 07:23:13 pmI think I can answer this.In a monolithic design, a current mirror is less expensive than the discrete equivalent because it uses a pair of transistors instead of a transistor and resistor or two. Diffused resistors take up space and have poor tolerance. The discrete equivalent using a transistor and a resistor or two is common in discrete designs. High performance discrete designs did use true current mirrors with emitter degeneration to compensate for mismatched Vbe but that level of performance was not often required.

author: w2aew
author_rank: Super Contributor
timestamp: July 19, 2014, 09:41:05 pm
content: Quote from: charles on July 19, 2014, 06:27:10 pmYou're welcome Charles. I'm glad to know that you've found my videos useful!

author: vk6zgo
author_rank: Super Contributor
timestamp: July 20, 2014, 01:00:25 pm
content: Discrete component stages have a lot of idiosyncracies which can lead you very much astray,if you don't know about them.On another forum I frequent,a chap was asking for help.He had found a circuit for a "wideband amplifier with an upper cut off frequency at 100MHz",somewhere on the 'Net,& simulated it with good results.He built it up with "through hole" components on the sort of PCB you would build an Audio stage on.When he tested it,he was horrified to find it was 3dB down at around 5MHz.Stray capacitance & inductance have a massive effect in discrete component amplifiers.