Are amplifier circuits like this still used/built?  - Page 1
Forum section: electronics / beginners

author: bonzer
author_rank: Regular Contributor
timestamp: February 22, 2019, 10:38:57 am
content: Hello everyone! I think this isn't an amplifier that has any class (like class A,B etc), it's just a circuit. To be honest it even might be useless, only for studying purpose, I don't know. But anyway if it wasn't it feels like all what we studied recently about analog at university is kinda obsolete. I'd like to understand what is the trend today? For example is a circuit like what I attached still used and built in any case of amplifier design or they use only mosfets today or there's even some digital implication (like in the case of digital filters).This is an amplifier with constant gain (25 dB from what I remember) that has a working band of 250Hz-400kHz. What would this circuit look like in the majority of today's cases for the most of purposes?

author: dzseki
author_rank: Frequent Contributor
timestamp: February 22, 2019, 11:19:30 am
content: Every amplifier has a class, here Q1 is the gain element, and is operating in class A.The trick of this circuit is that it takes a ground referenced input and has two outputs with inverted phase, in other words it has symmetrical output. An engineer today probably would use two single opamps or one differential output opamp for this purpose, but depending on the application the posted circuitt can work just as well.

author: Berni
author_rank: Super Contributor
timestamp: February 22, 2019, 11:25:25 am
content: This is a class A design. But its not only a amplifier but also a single ended to differential converter.What makes you think that BJT transistors are obsolete? They are still used in all sorts of places. Its just that for high power applications they are usually not the best choice.But yes most analog stuff is now done using Opamps because they are easier to use, cheap and can offer some very impressive performance if you are prepared to pay enough for a fancy kind of opamp. But there are still cases where analog transistor circuitry is the better choice. Perhaps the opamp that can do it costs too much or uses too much power, or maybe there is no opamp out there with the right specs (Like perhaps you need +/- 100V output swing).Its not useless knowledge. If you work on analog electronics enough it eventually comes in useful for a tricky situation where a chip doesn't do the job. Its just another useful tool in your circuit design toolbox. Tho its only really useful if you understand how these analog transistor circuits actually work rather than copy pasting the design.

author: bonzer
author_rank: Regular Contributor
timestamp: February 22, 2019, 11:52:02 am
content: thanks for your answers. I didn't know high power op.amps exist and are used in these cases.Sorry if I go offtopic but talking about op. amps, observing them from the outside what would be a good way to rappresent them as real? A part from input resistance and output resistance like in this image, aren't there other parassite capacitances to be considered? I see the inputs go to internal bjts. It is usually necessary to consider bjt's capacitance in high frequency otherwise it's getting hard to get the desired bandwith or cutoff frequency. I don't know if here it works the same as the transistors are internal so these capacitances could have very low value (I suppose) that could have time constants that activate even further than the open loop gain of the op-amps, so there might be no reason to consider them.Besides - maybe I said some stupid stuff, if that's the case than sorry for that I'm new into this, I'm not an expert.

author: dzseki
author_rank: Frequent Contributor
timestamp: February 22, 2019, 12:17:58 pm
content: I wouldn't call the picture as a "real opamp" it is rather a less ideal opampOf course more and more complex models are possible to make of a particular opamp, but in most cases (especially when learning the basics) this is absolutely not neccesary.As for high frequency performance you really don't have to know how the internal parasitics of a particular opamp looks like, because you can't do much about that anyway.The physical layout of the circuit (as on PCB) is often critical, and the datasheets are often very clear on this. For high bandwidth usualy the problem is not getting up to the frequency but to keep the system stable. The system stability is usualy most affected around the inverting pin of an opamp, so it is a good general idea to avoid stray capacitances as much as possible there.

author: Berni
author_rank: Super Contributor
timestamp: February 22, 2019, 12:29:26 pm
content: You can check the datasheet of the opamp to get the various non ideal parasitics like input resistance, input capacitance, input bias current, input offset, output resistance, open loop gain etc.Knowing how to chose the right opamp for the job is a significant skill for an analog engineer. Sometimes you might combine multiple opamps to get the characteristics you need. Or even add some transistors around a opamp to help it meet a specific requirement.The whole thing is becoming a bit of a lost art with new engineers due to so many things being done with dedicated chips, done digital or its all done in a MCU. Knowing how to design digital things is indeed important (especially in this day and age) but the world is still analog in nature so there will always be a need for some analog circuitry here or there.

author: jackthomson41
author_rank: Regular Contributor
timestamp: February 22, 2019, 01:55:52 pm
content: Quote from: Berni on February 22, 2019, 12:29:26 pmWe can't deny a little importance of op amps butmicrocontrollerskind of revolutionized the electronics, so I don't think its that important now to learn basic things. Electronics is getting modern so you should dive with it and not stuck with basics.

author: dmills
author_rank: Super Contributor
timestamp: February 22, 2019, 02:31:46 pm
content: Quote from: jackthomson41 on February 22, 2019, 01:55:52 pmAnd yet elsewhere on this very forum there is a guy having a problem with an LDO regulator because he failed to recognise a resonant circuit due to parasitic inductances and using too 'good' an input cap could case a transient over voltage on hot plug!Micros are cool and all, and pretty much everything I find myself designing has at least one, because it is often the cheapest way to do the almost trivial, but then you need to control real loads and deal with real sensors and real signals, and all of a sudden understanding the fundamentals matters because it turns out that at a high enough power or fast enough edge, "digital" usually isn't.Clock signals driving data converters for example are (contrary to appearances) NOT digital in nature, switching power supplies are basically RF design these days, and the SI issues for say DDR4 are all right out of the fundamentals.You will not really understand how to design any of these things without at least a first course in electromagnetism, and really you want at least Maxwells eqns, which means enough maths to deal with line and surface integrals.I have a rail switching drain modulator on the bench that is based on a whole pile of bipolar transistors, and absolutely I am using everything I know about those devices, a micro would not help in this part of the system.Also, someone has to DESIGN those cool new chips.Regards, Dan.

author: dzseki
author_rank: Frequent Contributor
timestamp: February 23, 2019, 12:10:14 pm
content: Quote from: jackthomson41 on February 22, 2019, 01:55:52 pmUpon this post I am very tempted to build this circuit using "modern philosophy", let's see: I need an AD converter running at 1Msps, and two DA converters running at the same speed and an MCU, can you advise me the price of a suitable MCU?Using just two transistors is so old school...

author: Nerull
author_rank: Frequent Contributor
timestamp: February 23, 2019, 03:56:23 pm
content: Quote from: jackthomson41 on February 22, 2019, 01:55:52 pmThere comes a time when you want to use a micro to actually do something, and then you need to connect it to those other components you didn't learn want to about.Also I really want to see you build an amplifier with an arduino.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 23, 2019, 04:11:37 pm
content: There is a group of people who extract satisfaction form building extremely simple Class A amplifiers.For example the attached picture fromhttp://www.firstwatt.com/pdf/art_delite.pdfA good forum for circuits like this and other diy audio is...https://www.diyaudio.com/

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: February 23, 2019, 08:43:23 pm
content: As regards the mosfet amp, mosfets are not very linear when used like this, the gain rising rapidly with drain current. A BJT has the disadvantage of needing current drive, but current out is nearly proportional to current in.Speaking generally, there are far too many examples of digital circuits being used when analog circuits would actually work better. Pushbutton volume controls are one such. Absolute pain, especially when you hit that YT video with over-loud sound.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2019, 09:26:58 pm
content: The only time I see disrete buffered shunt feedback circuits like that is where wideband performance is required with lower distortion (better linearity) than a transconductance amplifier will provide. The configuration shown is often seen in integrated amplifiers and audio amplifiers as the voltage gain stage with a Miller integrator replacing the shunt feedback so it is very common.

author: GregDunn
author_rank: Frequent Contributor
timestamp: February 23, 2019, 10:39:27 pm
content: Quote from: dzseki on February 23, 2019, 12:10:14 pmAlso, good luck troubleshooting anything (especially the 'vintage' test gear so popular on this site) without that basic knowledge of transistors.