Discussing audio amplifier design -- BJT, discrete
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From: George Herold on 31 Jan 2010 00:57 On Jan 29, 11:01 pm, John Larkin <jjSNIPlar...(a)highTHISlandtechnology.com> wrote: > On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold > > <ggher...(a)gmail.com> wrote: > > >"I'd probably replace the two diodes with > >one of those BJT and a few resistor constructions I can't > >remember the name of (which allows me to adjust the drop.)" > > "Vbe multiplier." > > The classic output stage biasing scheme uses small emitter resistors > and biases the output transistors to idle current using a couple of > junction drops between the bases, or a Vbe multiplier with a pot. Both > are good ways to have a poorly defined idle current and maybe fry > transistors. Two alternates are: > > 1. Use zero bias. Connect the complementary output transistors > base-to-base, emitter-to-emitter. Add a resistor from their bases to > their emitters, namely the output. At low levels, the driver stage > drives the load through this resistor. At high levels, the output > transistors turn on and take over. > > 2. Do the clasic diode or Vbe multiplier bias, but use big emitter > resistors. Parallel the emitter resistors with diodes. > > In both cses, the thing will be absolutely free frfom thermal runaway > issues and won't need adjustments. Bothe need negative feeback to kill > crossover distortion. > > Or... > > 3. Use mosfets > > John Cool thanks John, I tend to only use transistors when I need more poop on the output and always have an opamp in the loop. George H.
From: John Larkin on 31 Jan 2010 10:49 On Sat, 30 Jan 2010 21:57:11 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Jan 29, 11:01�pm, John Larkin ><jjSNIPlar...(a)highTHISlandtechnology.com> wrote: >> On Fri, 29 Jan 2010 10:34:49 -0800 (PST), George Herold >> >> <ggher...(a)gmail.com> wrote: >> >> >"I'd probably replace the two diodes with >> >one of those BJT and a few resistor constructions I can't >> >remember the name of (which allows me to adjust the drop.)" >> >> "Vbe multiplier." >> >> The classic output stage biasing scheme uses small emitter resistors >> and biases the output transistors to idle current using a couple of >> junction drops between the bases, or a Vbe multiplier with a pot. Both >> are good ways to have a poorly defined idle current and maybe fry >> transistors. Two alternates are: >> >> 1. Use zero bias. Connect the complementary output transistors >> base-to-base, emitter-to-emitter. Add a resistor from their bases to >> their emitters, namely the output. At low levels, the driver stage >> drives the load through this resistor. At high levels, the output >> transistors turn on and take over. >> >> 2. Do the clasic diode or Vbe multiplier bias, but use big emitter >> resistors. Parallel the emitter resistors with diodes. >> >> In both cses, the thing will be absolutely free frfom thermal runaway >> issues and won't need adjustments. Bothe need negative feeback to kill >> crossover distortion. >> >> Or... >> >> 3. Use mosfets >> >> John > >Cool thanks John, I tend to only use transistors when I need more >poop on the output and always have an opamp in the loop. Exactly. Opamps make gain and precision cheap, so every power transistor deserves one. John
From: John Larkin on 31 Jan 2010 10:54 On Sat, 30 Jan 2010 21:31:40 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Jan 29, 3:11�pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: >> On Fri, 29 Jan 2010 09:19:31 -0800 (PST), George Herold >> >> <ggher...(a)gmail.com> wrote: >> >Hi Jon, �I'm enjoying your posts. >> >> Thanks. �I feel like I'm way behind some curves, but it's fun >> taking a moment to think about things and it is fantastic >> that anyone else is willing to help talk about things with >> me. �That is priceless. �So the real thanks go to those who >> are sharing their knowledge and experience here. >> >> >What's a pin driver? >> >> Hmm. �I think I first heard the idea when talking about >> testing ICs, to be honest. �But imagine instead a micro with >> software to test some discrete part (could be an IC, too, >> that that's more complex.) �For example, to automatically >> derive some modeling parameters for a BJT. >> >> Take a look at this datasheet, for an example of the features >> one might support: >> >> http://www.analog.com/static/imported-files/Data_Sheets/AD53040.pdf >> >> >I made a nice >> >switchable current source (10nA to 1mA) from a voltage reference, >> >opamp and switchable resistors. �(circuit cribbed from AoE.) >> >> I'd require at least one that can either sink _or_ source to >> the pin. �And that would be only one of the pin driver's >> required features. �I think the datasheet mentioned above >> provides some more. �But that part is expensive and not >> readily available to us hobbyist types and doesn't teach me >> anything about various trade-offs I might want to make or how >> to design it at all, besides. >> >> Jon > >Wow, that's some chip. >Thanks, >George H. But $25 at 100 pieces, and rather slow. HFA1130 is interesting as a pin driver, run open-loop and slamming into its voltage-set limits. John
From: David Eather on 31 Jan 2010 15:38 On Jan 30, 9:06 am, Jon Kirwan <j...(a)infinitefactors.org> wrote: > On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather > > <eat...(a)tpg.com.au> wrote: > >On Jan 28, 12:51 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: > >> On Thu, 28 Jan 2010 11:17:02 +1000, David Eather > > >Sorry Jon, > > >I'm stuck on google groups for a little while - I can't believe people > >actually use it full time or that google could make an interface this > >bad. (I suspect it is very fine for simple threads) anyway... > > Cripes. Google didn't even show the thread when I'd looked, > a day ago or so. And it had been around for at least 24 > hours by then. Used to be the case that google groups would > show the posts within an hour or so. Doesn't seem to be > true, anymore. If not, there is no possibility of having a > discussion very quickly via google. It would greatly > lengthen out the interactions. Maybe that's on purpose, now, > to cause people to find some other solution? > The "Google Delay" is my delay. In a couple of days it will be back to normal > >> <eat...(a)tpg.com.au> wrote: > >> >Jon Kirwan wrote: > >> >> On Wed, 27 Jan 2010 17:31:00 +1000, David Eather > >> >> <eat...(a)tpg.com.au> wrote: > >> >>> <snip> > > >> >>> My particular bias for an amp this size is to go class AB with a split > >> >>> power supply. The majority of quality audio amps follow this topology > >> >>> and this is, I think, I great reason to go down this design path (what > >> >>> you learn is applicable in the most number of situations). I should hunt > >> >>> down a schematics of what I'm seeing in the distance (which can/will > >> >>> change as decisions are made) - some of the justifications will have to > >> >>> wait > > >> >> I'm fine with taking things as they come. > > >> >> As far as the class, I guessed that at 10 watts class-A would > >> >> be too power-hungry and probably not worth its weight but > >> >> that class-AB might be okay. > > >> >> I have to warn you, though, that I'm not focused upon some > >> >> 20ppm THD. I'd like to learn, not design something whose > >> >> distortion (or noise, for that matter) is around a bit on a > >> >> 16-bit DAC or less. I figure winding up close to class-B > >> >> operation in the end. But I'd like to take the walk along > >> >> the way, so to speak. > > >> >10 watts / PPM thd? Mmmm... maybe more like .1 - .05 % are realistic and > >> >a few detours to see what would help or harm that. > > >> Hehe. I'm thinking of some numbers I saw in the area of > >> .002% THD. I hate percentages and immediately convert them. > >> In this case, it is 20e-6 or 20 ppm. Which is darned close > >> to a bit on a 16-bit dac. That's why I wrote that way. I > >> just don't like using % figures. They annoy me just a tiny > >> bit. > > >Sorry. > > Don't be. I was just explaining myself, not complaining > about your usage. > > >> Regarding .1% to .05%, I'm _very_ good with that. Of course, > >> I'm going to have to learn about how to estimate it from > >> theory as well as measure it both via simulation before > >> construction and from actual testing afterwards. More stuff > >> I might _think_ I have a feel for, but I'm sure I will > >> discover I don't as I get more into it. > > >A little experience will get you into the right ballpark when > >estimating what you could expect for distortion. It is basically the > >same "rules" as you would see with op-amps - the more linear it is to > >start with the better. Higher bandwidth stages generally mean you can > >use more negative feedback to eliminate distortion - but the lower the > >final gain the more instability is likely to become a problem. And bad > >circuit layout can increase distortion (and even more so hum and > >noise) easily by a factor of 10. > > >As for how low you need distortion to be one rule of thumb (I forget > >the reference) is to be clearly audible the message must be 20db above > >the background noise and to be inaudible distortion has to be 20db > >below the background noise - which pretty much sets "low" distortion > >for PA and similar uses at 1% or 10000 ppm. For HiFi the "message" has > >a high dynamic range and you (allegedly) want a distortion figure at > >least 20db below that. So a 60 db signal range 0.0001% (or 100PPM). > >The you start getting into all kinds of trouble with power output / > >dynamic range of the amp etc and you relies that it is all a > >compromise anyway. You do the best you can within the restrictions of > >the job description. > > Understood. > > >> But speaking from ignorance, I'm good shooting for the range > >> you mentioned. It was about what I had in mind, in fact, > >> figuring I could always learn as I go. > > >> >>> The first step is to think about the output. The basic equations are > > >> >>> (1).....Vout = sqrt(2*P*R) > > >> >>> With R as 8 ohms for a common speaker and 10 watts that is 12.7 volts - > >> >>> actually +/- 12.7 volts with a split power supply. > > >> >> If you don't mind, I'd like to discuss this more closely. Not > >> >> just have it tossed out. So, P=V*I; or P=Vrms^2/R with AC. > >> >> Using Vpeak=SQRT(2)*Vrms, I get your Vpeak=SQRT(2*P*R) > >> >> equation. Which suggests the +/-12.7V swing. Which further > >> >> suggests, taking Vce drops and any small amounts emitter > >> >> resistor drops into account, something along the lines of +/- > >> >> 14-15V rails? > > >> >> Or should the rails be cut a lot closer to the edge here to > >> >> improve efficiency. What bothers me is saturation as Vce on > >> >> the final output BJTs goes well below 1V each and beta goes > >> >> away, as well, rapidly soaking up remaining drive compliance. > > >> >>> (2).....Imax = sqrt(2*P/R) > > >> >>> This comes out to 1.6 amps. You should probably also consider the case > >> >>> when R speaker = 4 ohms when initially selecting a transistor for the > >> >>> output 2.2 amps - remember this is max output current. The power supply > >> >>> voltage will have to be somewhat higher than Vout to take into account > >> >>> circuit drive requirements, ripple on the power supply and transformer > >> >>> regulation etc. > > >> >> Okay. I missed reading this when writing the above. Rather > >> >> than correct myself, I'll leave my thinking in place. > > >> >> So yes, the rails will need to be a bit higher. Agreed. On > >> >> this subject, I'm curious about the need to _isolate_, just a > >> >> little, the rails used by the input stage vs the output stage > >> >> rails. I'm thinking an RC (or LC for another pole?) for > >> >> isolation. But I honestly don't know if that's helpful, or > >> >> not. > > >> >Mostly not needed, if you use a long tailed pair for the input / error > >> >amplifier, but you might prefer some other arrangement so keep it in > >> >mind if your circuit "motorboats" > > >> Okay. I've _zero_ experience for audio. It just crossed my > >> mind from other cases. I isolate the analog supply from the > >> digital -- sometimes with as many as four caps and three > >> inductor beads. There, it _does_ help. > > >> >>> Are you OK with connecting mains to a transformer? or would you rather > >> >>> use an AC plug pack (10 watts is about the biggest amp a plugpack can be > >> >>> used for)? The "cost" for using an AC plug pack is you will need larger > >> >>> filter capacitors. > > >> >> I'd much prefer to __avoid__ using someone else's "pack" for > >> >> the supply. All discrete parts should be on the table, so to > >> >> speak, in plain view. And I don't imagine _any_ conceptual > >> >> difficulties for this portion of the design. I'm reasonably > >> >> familiar with transformers, rectifiers, ripple calculations, > >> >> and how to consider peak charging currents vs averge load > >> >> currents as they relate to the phase angles available for > >> >> charging the caps. So on this part, I may need less help > >> >> than elsewhere. In other words, I'm somewhat comfortable > >> >> here. > > >> >Ah, then there are questions of what voltage and VA for a transformer.. > >> >So there are questions of usage (music, PA, PA with an emergency alert > >> >siren tied in etc) and rectifier arrangement and capacitor size / > >> >voltage to get your required voltage output at full load. > > >> I figure on working out the design of the amplifier and then > >> going back, once that is determined and hashed out, with the > >> actual required figures for the power supply and design that > >> part as the near-end of the process. Earlier on, I'd expect > >> to have some rough idea of how "bad" it needs to be -- if the > >> initial guesses don't raise alarms, then I wouldn't dig into > >> the power supply design until later on. The amplifier, it > >> seems to me, dictates the parameters. So that comes later, > >> doesn't it? > > >Yes and No. All the published circuits are made by people who want to > >sell transistors, > > A concern I care not the least about. My _real_ preference, > were I to impose it on the design, would be to use ONLY > PN2222A BJTs for all the active devices. One part. That's > it. Why? Because I've got thousands of them. ;) > > Literally. Something like 22,000 of the bastards. I give > them away like popcorn to students at schools. Got them > _very cheaply_. So if I were pushing something, I'd be > pushing a 10W PN2222A design, use signal splitting approach > probably (because it's the only way I think think of, right > now), and distribute the dissipation across lots and lots of > the things. > > What to go there? :) Signal Splitting? Can you sketch out what your thinking? The wikipedia type circuit can use a few n2222 - I count a max of 5. Even if you could use only n2222 it would not be a good idea - making the circuit stable would be more difficult. On the good side the n2222 is a good choice for Q1,Q2 and as active replacements for R5,R6 and one other (optional) we haven't met yet. What makes it a good transistor is the large current gain / bandwidth product and the flat DC current gain over a wide range of viable bias currents. Both contribute to low distortion. http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3 graph) compared to say 2n3904 http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF where the flat portion of the DC gain curve is over a very limited range. > > >not audio systems, power supplies or transformers. > > Got it. > > >As a result the power supply is often assumed to be regulated, which > >is not true in this case, or the power supply is treated in a very > >perfunctory manner that is not at all compatible with good design. > > >In this case you have the voltage you need for the 10 watts, plus > >voltage drop for the driver circuitry and output stage , plus ripple > >voltage, plus whatever is required for transformer regulation and > >mains regulation. When you add it all up you might find that a chosen > >transistor/component is actually not at all suitable for the job. Back > >to the drawing board. Change this change that recheck everything again > >etc. > > In this case, though, there is nothing particularly > remarkable about the rails. Taken across the entire span, > even, doesn't exceed the maximum Vce of a great many BJTs. So > no real worry there. But I see some of where problems may > arise. Luckily, at this level I can side-step worrying about > that part and get back to learning about amplifier design, > yes? I come up with a figure of 50 volts rail to rail no load voltage - after picking out a common transformer with 15% regulation. > > >If you do the power supply first you have the figures needed for your > >worst case already. It saves time and makes a better result (no > >tendency to comprimise to save all the calculations already done). > > Well, does this mean we should hack out the power supply > first? I'm perfectly fine with that and can get back to you > with a suggested circuit and parts list if you want to start > there. We could settle that part before going anywhere else > and I'd be happy with that approach, too, because to be > honest I don't imagine it to put a horrible delay into > getting back to amplifier design. So I'm good either way. I'm looking at some of your other posts and I don't think you need a maths lesson from me. If you want to do a power supply great. Its a small one so nothing much too it. If you don't want, I'm OK too. > > >> >>> I should also ask if you have a multi meter, oscilloscope (not necessary > >> >>> but useful)and how is your soldering? But it would be wise to keep this > >> >>> whole thing as a paper exercise before you commit to anything. > > >> >> I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS > >> >> handheld, two oscilloscopes (TEK 2245 with voltmeter option > >> >> and an HP 54645D), three triple-output power supplies with > >> >> two of them GPIB drivable, the usual not-too-expensive signal > >> >> generator, and a fair bunch of other stuff on the shelves. > >> >> Lots of probes, clips, and so on. For soldering, I'm limited > >> >> to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and > >> >> somewhat wider spade tips in the 1.5mm area. I have tubs and > >> >> jars of various types of fluxes, as well, and wire wrap tools > >> >> and wire wrap wire, as well. I also have a room set aside > >> >> for this kind of stuff, when I get time to play. > > >> >OK. Next serious project, I'm coming around to your place! > > >> You come to the west coast of the US and I'll have a room for > >> you! > > >> >Your gear is > >> >better than mine. I had to ask, rather than just assume just in case my > >> >assumptions got you building something you didn't want to, and got you > >> >splattered all over the place from the mains, or suggesting you choose > >> >the miller cap by watching the phase shift of the feedback circuit - I > >> >don't read a lot of the posts so I didn't know what you could do. > > >> To be honest, I can do a few things but I'm really not very > >> practiced. My oscilloscope knowledge is lacking in some > >> areas -- which becomes all too painfully obvious to me when I > >> watch a pro using my equipment. And I'm still learning to > >> solder better. It's one of a few hobbies. > > >> >> Jon > > >> >Have a look at > >> >http://en.wikipedia.org/wiki/Electronic_amplifier > > >> Done. > > >> >The bits on class A might be interesting as it says 25% efficiency and > >> >50% obtainable with inductive output coupling (i.e. with a transformer) > >> >which is what I said, not what blow hard Phil said. > > >> What I first see there is the amplifier sketch at the top of > >> the page > > >I wasn't going to prompt, but it is close to the sort of thing, I > >think, you should be aiming for . As someone has already noted (I > >would attribute you if I wasn't on GG, I'm sorry) it has been drawn up > >for a single supply, rather than a more common (for this size / > >configuration) split supply. > > I had assumed we'd be using a split supply. I think that's very much the preferred way. > > I had assumed a speaker would be hooked up via a cap to the > output, so DC currents into a speaker coil would be removed > from any concern. But I was also holding in the back of my > mind the idea of tweaking out DC bias via the speaker and > removing the coupling cap as an experiment to try. And if > so, I'd pretty much want the ground as a "third rail." Exactly right! There are two common ways to reduce/remove any offset from the output. Neither is shown on the wikipedia circuit. If you have another split rail circuit it will certainly have one method - both methods involved use the diff amp. > (Playing just a bit upon the Chicago parlance about the once > dangerous rail in their transit system.) > > >(I don't really care too much about arguing about > >> efficiencies right now -- I'm more concerned about learning.) > >> The input stage shown is a voltage-in, current-out bog > >> standard diff-pair. First thing I remember about is that R4 > >> shouldn't be there > > >Correct. Theory says it does nothing. I practice the theory but have > >the occasional heretical belief about that. > > Actually, I think I've read that theory says it is _better_ > to be removed. The reason seemed pretty basic, as it's > easier to get close to a balanced current split; and this, I > gather, lowers 2nd harmonic distortions produced in the pair > -- notable more on the high frequency end I suppose because > gain used for linearizing feedback up there is diminishing > and can't compensate it. > > In other words, it's not neutral. It's considered to be > better if I gathered the details. Then even better, the > current mirror enforces the whole deal and you've got about > the best to be had. > > Of course, mostly just being a reader means I have no idea > which end is up. So I might have all this wrong. No. Thats all correct. I'll show a different circuit latter > > > and better still both R3 and R4 should be > >> replaced with a current mirror. > > >This would provide more differential gain. > > _and_ improve distortion because the currents are forced to > be balanced in the pair, yes? yes. > > >>R5 should be a replaced with > >> a BJT, as well. > > >In the right configuration it would reduce the common mode signal gain > >of things like mains hum and supply ripple (you mentioned power supply > >isolation before). > > Yes, that's how I thought about it. > > >Also, from another (what do you call it branch? thread?) you were > >discussing boot-strapping R6. This is not done so much as amplifiers > >get bigger but a BJT configured in the same way as the replacement for > >R5 is very common. I'm leaving the details to you - perhaps there is a > >way to reduce component count without affecting performance. (I am > >hoping this is what you wanted "nutting it out for yourself") > > Yes! I don't want things handed on a platter. But I also > don't want to have to rediscover all of the ideas by making > all of the mistakes, either. This is the kind of "pointer" > towards something that I like a lot. It gives me a place to > think about something, but leaves me some reason to have to > do so and that helps me own it better. > > One general truth about learning is that you don't present > someone with a problem so out of their depth that they have > no chance at it. Doing that means they fail, they feel like > a failure, and it causes a student to just want to go away. > They lose motivation, usually, in cases like that. On the > other hand, providing no difficulty at all merely means > repetition of what they already know and they grow bored from > that, too. Finding the sweet spot where a student is faced > with interesting problems that are not already known, but > perhaps within reach of grasping at with some effort, is the > key. Then it can be fun, educational, and motivate. > > That's what you just did for me. > > >I assume the input impedance of that example > >> is basically the parallel resistance of R1 and R2, but if we > >Yes. > > Okay. There is the parallel resistance of R5 x Beta Q1 as well, but this is normally so high it won't affect the result. And if R5 is replaced with an active device it can become essentially infinite. > > >> use split supplies I'd imagine replacing the two of them with > >> a single resistor to the center-ground point. > >Yes, but you should probably think of a whole passive network to > >filter out low and high frequency - (think what happens if you amp is > >operated near a source of RF) > > Well, every trace picks up like little antennae. All kinds > of trace voltages appearing here and there. Not good. > > So. Can you make an audio amplifier that can withstand a > microwave oven environment and deliver good performance while > irradiated with 1kW banging around in there? ;) If you can do that the military wants you to EMP harden all there electronics. The input is a little different because some user always want to stick a bloody gret big long wire onto it. > > >> There's no > >> miller cap on Q3, > > >Depending on transistors layout etc it might not be needed, but more > >often it is the size that is the question. > > I was thinking it helped locally linearize the VAS section > and that such would be "good" most anywhere. But I am just > taking things without having worked through them on my own. > So... It sets the bandwidth of the VAS stage so you can use negative feedback without the whole thing turning into smoke. Do you know of control theory / bode diagrams. There is a minuscule amount needed for this app. > > >> I'd probably replace the two diodes with > >> one of those BJT and a few resistor constructions I can't > >> remember the name of (which allows me to adjust the drop.) > > >Vbe multiplier... > > Okay. Thanks. > > >> The feedback ... well, I need to think about that a little > >> more. There's no degen resistors in the emitters of Q4 and > >> Q5. > > >Why would/should you use them? > > I'm still thinking about that. In general, I was thinking > about them because of the "little re" that is kT/q based in > each BJT, and varies on Ie. Since Ie is varying around, I > was thinking about something fixed there to overwhelm it and > "make it knowable" for the design, I suppose. Maybe that's > all wet, given your query. I'll toss the idea off the side, > for now. > Try working through the thermal stabilization. Just make a stab at the transistor junction temperatures - it will be pretty hot (unless you can afford mega bucks for heatsinking) > >> Um.. okay, I need to sit down and think. Mind is spinning, > >> but I've not set a finger to paper yet and there is lots to > >> think about in that one. I could be way, way off base. > > >Not at all. > > Thanks for that. I'm just glad to be able to talk to someone > about any of this, at all. So please accept my thanks for > the moments you are offering. > > >Is there a way you could post a schematic of where your thinking is > >and what you would like to discuss - there is no need for a complete > >circuit. > > Yes. I can use ASCII here, for example. But before I go off > into the wild blue with this, do you want to focus on the > power supply first? Or just jump in on the amplifier? I don't mind. Earlier I put a stab at a no load worst case voltage, you can use that if you want to. Until you get to output stage power dissipation that is all you need. > > Jon
From: Jon Kirwan on 1 Feb 2010 05:23
On Sun, 31 Jan 2010 12:38:49 -0800 (PST), David Eather <eather(a)tpg.com.au> wrote: >On Jan 30, 9:06�am, Jon Kirwan <j...(a)infinitefactors.org> wrote: >> On Fri, 29 Jan 2010 13:49:16 -0800 (PST), David Eather ><snip> > >> >Yes and No. All the published circuits are made by people who want to >> >sell transistors, >> >> A concern I care not the least about. �My _real_ preference, >> were I to impose it on the design, would be to use ONLY >> PN2222A BJTs for all the active devices. �One part. �That's >> it. �Why? �Because I've got thousands of them. �;) >> >> Literally. �Something like 22,000 of the bastards. �I give >> them away like popcorn to students at schools. �Got them >> _very cheaply_. �So if I were pushing something, I'd be >> pushing a 10W PN2222A design, use signal splitting approach >> probably (because it's the only way I think think of, right >> now), and distribute the dissipation across lots and lots of >> the things. >> >> What to go there? �:) > >Signal Splitting? Can you sketch out what your thinking? Yeah, I think so. Something like this: >: | | >: \ | >: / R2 | >: \ | >: / | >: | | >: | |/c Q2 >: +---------| >: | |>e >: | | >: |/c Q3 | >: -------| +----- >: |>e | >: | | >: | |/c Q1 >: +---------| >: | |>e >: | | >: \ | >: / R1 | >: \ | >: / | >: | | The "signal splitter" here is Q3. It's also providing gain, too, though. The emitter and collector move in opposite directions and the signal "splits" at Q3. (The emitter follows the base, the collector inverts the base.) If I read with any understanding about these things, properly biasing Q3 is a pain, the Q3 gain varies with the load itself as well as its bias, and compensation issues are complicated a bit. >The wikipedia type circuit can use a few n2222 - I count a max of 5. >Even if you could use only n2222 it would not be a good idea - making >the circuit stable would be more difficult. Yes, ignorant as I am still of the details, I think that's very true. The splitter has significant signal voltage on its input and I've read that pole-splitting methods for improving stability are harder to apply here. >On the good side the n2222 >is a good choice for Q1,Q2 and as active replacements for R5,R6 and >one other (optional) we haven't met yet. What makes it a good >transistor is the large current gain / bandwidth product and the flat >DC current gain over a wide range of viable bias currents. Both >contribute to low distortion. > >http://www.onsemi.com/pub_link/Collateral/P2N2222A-D.PDF (page 3 >graph) > >compared to say 2n3904 >http://www.onsemi.com/pub_link/Collateral/2N3903-D.PDF > >where the flat portion of the DC gain curve is over a very limited >range. Interesting point to consider. Something that had slipped by me, so far. >> >not audio systems, power supplies or transformers. >> >> Got it. >> >> >As a result the power supply is often assumed to be regulated, which >> >is not true in this case, or the power supply is treated in a very >> >perfunctory manner that is not at all compatible with good design. >> >> >In this case you have the voltage you need for the 10 watts, plus >> >voltage drop for the driver circuitry and output stage , plus ripple >> >voltage, plus whatever is required for transformer regulation and >> >mains regulation. When you add it all up you might find that a chosen >> >transistor/component is actually not at all suitable for the job. Back >> >to the drawing board. Change this change that recheck everything again >> >etc. >> >> In this case, though, there is nothing particularly >> remarkable about the rails. �Taken across the entire span, >> even, doesn't exceed the maximum Vce of a great many BJTs. So >> no real worry there. �But I see some of where problems may >> arise. �Luckily, at this level I can side-step worrying about >> that part and get back to learning about amplifier design, >> yes? > >I come up with a figure of 50 volts rail to rail no load voltage - >after picking out a common transformer with 15% regulation. Okay. This is going to force me to sit down with paper and work through. I was stupidly imagining +/-18V max, or 36V rail to rail. I haven't considered the details of the output section yet, driving a load from rails that run up and down on capacitors that charge and discharge at 1A-level currents into the load, and perhaps I need to spend some more time there before moving on. There are so many ways to cut this. Start at the input and that's one focus that may work okay. Start at the output stage and that provides important power supply information, though. So maybe I should start at that end? >> >If you do the power supply first you have the figures needed for your >> >worst case already. It saves time and makes a better result (no >> >tendency to comprimise to save all the calculations already done). >> >> Well, does this mean we should hack out the power supply >> first? �I'm perfectly fine with that and can get back to you >> with a suggested circuit and parts list if you want to start >> there. �We could settle that part before going anywhere else >> and I'd be happy with that approach, too, because to be >> honest I don't imagine it to put a horrible delay into >> getting back to amplifier design. �So I'm good either way. > >I'm looking at some of your other posts and I don't think you need a >maths lesson from me. If you want to do a power supply great. Its a >small one so nothing much too it. If you don't want, I'm OK too. I still haven't been down the path on my own, yet. So I don't have strong opinions about this. It's like going to Disneyland for the first time. Which land should I go to, first? Later, after being there a few times, I may look at the flow of people and decide that "Adventureland" is the best first start. But first time out? Who knows? I'm open to guidance. Everything is new. >> >> >>> I should also ask if you have a multi meter, oscilloscope (not necessary >> >> >>> but useful)and how is your soldering? But it would be wise to keep this >> >> >>> whole thing as a paper exercise before you commit to anything. >> >> >> >> I have a 6 1/2 digit HP multimeter, a Tek DMM916 true RMS >> >> >> handheld, two oscilloscopes (TEK 2245 with voltmeter option >> >> >> and an HP 54645D), three triple-output power supplies with >> >> >> two of them GPIB drivable, the usual not-too-expensive signal >> >> >> generator, and a fair bunch of other stuff on the shelves. >> >> >> Lots of probes, clips, and so on. �For soldering, I'm limited >> >> >> to a Weller WTCPT and some 0.4mm round, 0.8mm spade, and >> >> >> somewhat wider spade tips in the 1.5mm area. �I have tubs and >> >> >> jars of various types of fluxes, as well, and wire wrap tools >> >> >> and wire wrap wire, as well. �I also have a room set aside >> >> >> for this kind of stuff, when I get time to play. >> >> >> >OK. Next serious project, I'm coming around to your place! >> >> >> You come to the west coast of the US and I'll have a room for >> >> you! >> >> >> >Your gear is >> >> >better than mine. I had to ask, rather than just assume just in case my >> >> >assumptions got you building something you didn't want to, and got you >> >> >splattered all over the place from the mains, or suggesting you choose >> >> >the miller cap by watching the phase shift of the feedback circuit - I >> >> >don't read a lot of the posts so I didn't know what you could do. >> >> >> To be honest, I can do a few things but I'm really not very >> >> practiced. �My oscilloscope knowledge is lacking in some >> >> areas -- which becomes all too painfully obvious to me when I >> >> watch a pro using my equipment. �And I'm still learning to >> >> solder better. �It's one of a few hobbies. >> >> >> >> Jon >> >> >> >Have a look at >> >> >http://en.wikipedia.org/wiki/Electronic_amplifier >> >> >> Done. >> >> >> >The bits on class A might be interesting as it says 25% efficiency and >> >> >50% obtainable with inductive output coupling (i.e. with a transformer) >> >> >which is what I said, not what blow hard Phil said. >> >> >> What I first see there is the amplifier sketch at the top of >> >> the page >> >> >I wasn't going to prompt, but it is close to the sort of thing, I >> >think, you should be aiming for . As someone has already noted (I >> >would attribute you if I wasn't on GG, I'm sorry) it has been drawn up >> >for a single supply, rather than a more common (for this size / >> >configuration) split supply. >> >> I had assumed we'd be using a split supply. >I think that's very much the preferred way. I feel more comfortable assuming it, too. >> I had assumed a speaker would be hooked up via a cap to the >> output, so DC currents into a speaker coil would be removed >> from any concern. �But I was also holding in the back of my >> mind the idea of tweaking out DC bias via the speaker and >> removing the coupling cap as an experiment to try. �And if >> so, I'd pretty much want the ground as a "third rail." > >Exactly right! There are two common ways to reduce/remove any offset >from the output. Neither is shown on the wikipedia circuit. If you >have another split rail circuit it will certainly have one method - >both methods involved use the diff amp. Thanks. >> (Playing just a bit upon the Chicago parlance about the once >> dangerous rail in their transit system.) >> >> >(I don't really care too much about arguing about >> >> efficiencies right now -- I'm more concerned about learning.) >> >> The input stage shown is a voltage-in, current-out bog >> >> standard diff-pair. �First thing I remember about is that R4 >> >> shouldn't be there >> >> >Correct. Theory says it does nothing. I practice the theory but have >> >the occasional heretical belief about that. >> >> Actually, I think I've read that theory says it is _better_ >> to be removed. �The reason seemed pretty basic, as it's >> easier to get close to a balanced current split; and this, I >> gather, lowers 2nd harmonic distortions produced in the pair >> -- notable more on the high frequency end I suppose because >> gain used for linearizing feedback up there is diminishing >> and can't compensate it. >> >> In other words, it's not neutral. �It's considered to be >> better if I gathered the details. �Then even better, the >> current mirror enforces the whole deal and you've got about >> the best to be had. >> >> Of course, mostly just being a reader means I have no idea >> which end is up. �So I might have all this wrong. > >No. Thats all correct. I'll show a different circuit latter Okay. I'll enjoy the moment when it happens. >> > and better still both R3 and R4 should be >> >> replaced with a current mirror. � >> >> >This would provide more differential gain. >> >> _and_ improve distortion because the currents are forced to >> be balanced in the pair, yes? > >yes. Okay. So I am picking up details not too poorly, so far. >> >>R5 should be a replaced with >> >> a BJT, as well. � >> >> >In the right configuration it would reduce the common mode signal gain >> >of things like mains hum and supply ripple (you mentioned power supply >> >isolation before). >> >> Yes, that's how I thought about it. > > >> >> >Also, from another (what do you call it branch? thread?) you were >> >discussing boot-strapping R6. This is not done so much as amplifiers >> >get bigger but a BJT configured in the same way as the replacement for >> >R5 is very common. I'm leaving the details to you - perhaps there is a >> >way to reduce component count without affecting performance. (I am >> >hoping this is what you wanted "nutting it out for yourself") >> >> Yes! �I don't want things handed on a platter. �But I also >> don't want to have to rediscover all of the ideas by making >> all of the mistakes, either. �This is the kind of "pointer" >> towards something that I like a lot. �It gives me a place to >> think about something, but leaves me some reason to have to >> do so and that helps me own it better. >> >> One general truth about learning is that you don't present >> someone with a problem so out of their depth that they have >> no chance at it. �Doing that means they fail, they feel like >> a failure, and it causes a student to just want to go away. >> They lose motivation, usually, in cases like that. �On the >> other hand, providing no difficulty at all merely means >> repetition of what they already know and they grow bored from >> that, too. �Finding the sweet spot where a student is faced >> with interesting problems that are not already known, but >> perhaps within reach of grasping at with some effort, is the >> key. �Then it can be fun, educational, and motivate. >> >> That's what you just did for me. >> >> >I assume the input impedance of that example >> >> is basically the parallel resistance of R1 and R2, but if we >> >Yes. >> >> Okay. > >There is the parallel resistance of R5 x Beta Q1 as well, but this is >normally so high it won't affect the result. And if R5 is replaced >with an active device it can become essentially infinite. Okay. I've got that detail from other discussions, too. So yes, understood. Also, I mentioned replacing R5, I think. In replacing R5 with active parts, I'm thinking of two BJTs in a usual form that seems to work pretty well over supply variations. >> >> use split supplies I'd imagine replacing the two of them with >> >> a single resistor to the center-ground point. � >> >Yes, but you should probably think of a whole passive network to >> >filter out low and high frequency - (think what happens if you amp is >> >operated near a source of RF) >> >> Well, every trace picks up like little antennae. �All kinds >> of trace voltages appearing here and there. �Not good. >> >> So. �Can you make an audio amplifier that can withstand a >> microwave oven environment and deliver good performance while >> irradiated with 1kW banging around in there? �;) > >If you can do that the military wants you to EMP harden all there >electronics. The input is a little different because some user always >want to stick a bloody gret big long wire onto it. :) I actually _do_ work on low-mass, direct-contact temperature measuring devices designed to work within a microwave environment. (But no electronics or metals inside.) But you brought up the microwave environment, so I hope you don't mind the teasing about it. >> >> There's no >> >> miller cap on Q3, >> >> >Depending on transistors layout etc it might not be needed, but more >> >often it is the size that is the question. >> >> I was thinking it helped locally linearize the VAS section >> and that such would be "good" most anywhere. �But I am just >> taking things without having worked through them on my own. >> So... > >It sets the bandwidth of the VAS stage so you can use negative >feedback without the whole thing turning into smoke. Do you know of >control theory / bode diagrams. There is a minuscule amount needed >for this app. I am familiar with _some_ closed loop control theory, sufficient to get me by with PID controls (using _and_ writing code for them.) Bode diagrams are something I have not used, though I've seen them. My math is adequate, I suspect. But I will have to read up on them, I suppose. For Laplace analysis, I'm familiar with complex numbers, poles and zeros, partial fraction extractions, and so on. Just inexperienced in the "short cuts" that many use to get (and think about) answers. >> >> I'd probably replace the two diodes with >> >> one of those BJT and a few resistor constructions I can't >> >> remember the name of (which allows me to adjust the drop.) >> >> >Vbe multiplier... >> >> Okay. �Thanks. >> >> >> The feedback ... well, I need to think about that a little >> >> more. �There's no degen resistors in the emitters of Q4 and >> >> Q5. >> >> >Why would/should you use them? >> >> I'm still thinking about that. �In general, I was thinking >> about them because of the "little re" that is kT/q based in >> each BJT, and varies on Ie. �Since Ie is varying around, I >> was thinking about something fixed there to overwhelm it and >> "make it knowable" for the design, I suppose. �Maybe that's >> all wet, given your query. I'll toss the idea off the side, >> for now. > >Try working through the thermal stabilization. Just make a stab at the >transistor junction temperatures - it will be pretty hot (unless you >can afford mega bucks for heatsinking) I need to understand the output configuration a little better before I do that. Including thinking more closely about swinging one end of an output cap around so that 1Amp rms can pass through it at 20Hz. I = C dv/dt, but V=V0*sin(w*t), so I=C*w*V0*cos(w*t). Assuming max current at the max slew rate for a sine at phase angle zero, the w*t is some 2*PI*N thing, so cos(w*t) goes to 1. That makes I=w*C*V0. But w=2*pi*20, or about 126 or so. So I=126*C*V0. So with I=1A, C=1/(126*V0). With V0=15V, I get about 530uF for the output cap. That's an amp peak only at the right phase, too. It'll be less elsewhere. To make that an amp rms, the cap would need to be still bigger. Peak current via the cap will take place right about the time when the two BJTs's emitters are at their midpoint. One of the BJTs will be supplying that. Not only that, but also depending upon class mode of operation, supplying current to the other one as well. How much is important to figuring out the wattage. I need to sit down with paper, I suspect. But if you want to provide some suggested thinking process here, I'd also be very open to that, as well. I'll take a shot at it either way, but it helps to see your thinking, too. If you can afford the moment for me. >> >> Um.. okay, I need to sit down and think. �Mind is spinning, >> >> but I've not set a finger to paper yet and there is lots to >> >> think about in that one. �I could be way, way off base. >> >> >Not at all. >> >> Thanks for that. �I'm just glad to be able to talk to someone >> about any of this, at all. �So please accept my thanks for >> the moments you are offering. >> >> >Is there a way you could post a schematic of where your thinking is >> >and what you would like to discuss - there is no need for a complete >> >circuit. >> >> Yes. �I can use ASCII here, for example. �But before I go off >> into the wild blue with this, do you want to focus on the >> power supply first? �Or just jump in on the amplifier? > >I don't mind. Earlier I put a stab at a no load worst case voltage, >you can use that if you want to. Until you get to output stage power >dissipation that is all you need. Maybe I'd like to focus on understanding different output pair configurations, first. I frankly don't like the "haul the output pair around with a collector on one side and a resistor on the other with a rubber diode in between to keep them biased up" approach. It's smacks of heavy-handedness and I simply don't like the way it looks to me. Everything tells me this works, but it is indelicate at the very least. However, it is crucial that I understand it in detail before deciding what I really think about it. For example, I might want to replace the resistor with a current source. But without apprehending the output stage more fully, its time domain behavior over a single cycle for example, I'm not comfortable with hacking it here and there, ignorantly. Jon |