Discussing audio amplifier design -- BJT, discrete
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From: pimpom on 29 Jan 2010 11:30 Jon Kirwan wrote: > On Thu, 28 Jan 2010 19:19:06 +0530, "pimpom" > <pimpom(a)invalid.invalid> wrote: > > >> Q2 needs about +/-50uA peak of base >> current at full drive. At signal frequencies, R2 (plus the >> much >> smaller input impedance of Q1) is effectively in parallel with >> the output. > > R2 is connected from the output to an input, which > effectively doesn't move much after arriving at it's DC bias > point. As you later point out, the _AC_ input impedance is > lowish (near 600 ohms), so the 10k is pretty close to one of > the rails at AC, anyway. Is that a different way of saying > what you just said? Or would you modify it? > That's another way of putting it, yes. >> The output swings by about 4V peak at max power, >> which has 400uA of negative feedback current going back >> through >> R2. The input current requirement goes up by a factor of 9. >> IOW, >> a negative feedback of 19db. This is substantially better than >> nothing and should significantly reduce distortion and improve >> frequency response. > > Okay. This goes past me a little (as if maybe the earlier > point didn't.) I'd like to try and get a handle on it. > > Let's start with the 4V peak swing at max power. > > Since you are discussing AC and converting it 400uA current > via the 10k, I would normally take this to mean 4Vrms AC. > Which in Vp-p terms would be 2*SQRT(2) larger, or 11.3V which > I know is impossible without accounting for the BJTs, given > the 9V supply. So this forces me to think in terms of > something else. But what? Did you mean 4Vpeak, which would > be 8Vp-p? If so, that would be about 2.8Vrms. Yes. It's 4Vp, 8Vp-p and 2.8Vrms. I wanted to give you a mental picture of how much the output voltage can swing. Each output Q has about 4.4V of Vce available, and about 4V before hard saturation is reached (these are all round figure values). That's 4V peak for a sinusoidal wave form. > In that case, > wouldn't a better "understanding" come from then saying that > the negative feedback is closer to 280uA? > Yes, it's 280uA rms. But I was talking in terms of the maximum amplitude of instantaneous change, which is why I used the terms "swing" and "peak". > The next point is on your use of "goes up by a factor of 9." > Can you elaborate more on this topic? Where the 9 comes > from? For volts, not power, I think I can gather the point > that 20*log(9) = 19.085), so I'm not talking about that > conventional formula. I'm asking about the 9, itself, and Without feedback, the input transistor Q1 needs 50uA of AC input signal to drive the output Qs to full power output (still talking in terms of peak to avoid confusion). With NFB, we need an additional 400uA to overcome the current fed back from the output. That's a total of 450uA peak, which is 9 times the original 50uA. Actually, I made an error when I cited the 50uA figure. Q1 is biased at Ic = 7.6mA, Ib = 50uA. But only 5mA peak is needed from Q1's collector to drive the output transistors. Divide that by Q1's hfe of 150 and you get 33uA (peak) of AC signal current needed into the base of Q1. The corrected total needed from the signal source is now 433uA. The gain reduction factor due to NFB is now 13 instead of 9. That's 22db (feedback is usually given in db). > also your thinking along the lines of concluding that it > significantly reduces distortion. The basic principle of NFB is that it reduces THD and extends frequency response by a factor equal to the feedback ratio. So, in our example, if you have 10% THD without feedback, it will drop to 0.77% with the feedback factor of 13. But there are caveats. E.g., phase shifts can cause undesireable effects, especially with large amounts of feedback. I'm afraid a detailed treatment of such things is really outside the scope of this discussion - unless someone else is willing to take it up. > How does one decide how much is enough? For one thing, how much distortion one is willing to put up with. Another factor is input sensitivity, or IOW, how much gain is needed. E.g., to drive the 1W amp to full output, we need 433uA peak (306uA rms) from the signal source into 1k. That's 306mV rms, plus some millivolts at the b-e junction. Say about 0.32V rms total input voltage into about 1k input impedance. To present the basic concepts, I've made several approximations. E.g., I neglected the shunting effect of R2. Besides, the input resistance of Q1 is constant at 600 ohms only for very small signal amplitudes relative to the quiescent dc levels. This dynamic input resistance changes significantly with large signal swings and adds distortion while also complicating precise calculations.
From: George Herold on 29 Jan 2010 12:19 On Jan 27, 2:19 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: > On Wed, 27 Jan 2010 13:23:28 GMT, Bob Masta wrote: > >On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan > ><j...(a)infinitefactors.org> wrote: > > >>I'd like to take a crack at thinking through a design of an > >>audio amplifier made up of discrete BJTs and other discrete > >>parts as an educational process. > > ><snip> > > >When I was first getting interested in power amp > >design (back in the '70s) I started collecting > >schematics for all the power amps I could get my > >hands on, to compare them. I noticed that the > >schematics for simple bipolar op-amp ICs were > >remarkably similar to those for big discrete power > >amps. If you have an old National Linear Databook > >(or don't mind a lot of rooting around on the Web > >for individual datasheets), you might take a look. > > >You can build a pretty decent amp with only a > >handful of transistors. The same basic circuit > >can be used for a wide range of output powers, > >just by changing the power supply voltages and the > >output device ratings. > > >Best regards, > > Thanks, Bob. Audio amplifiers, especially ones delivering > _some_ power, seem to offer such an excellent way to learn. > The basic idea, at a behavioral level, is fairly simple. An > implementation requires some knowledge and thought in the > end. So the destination is arrived at by taking a great path > to walk, with such wonderful vistas to see, I think. Much of > interest is along the way of getting there. > > I may have an old National databook on linear parts > somewhere. I keep a lot, but I also have several thousand > books in my library which covers all of the walls in one of > the rooms. I'm at a point now where to get room for more > books, others must be boxed and stored or simply destroyed > and pulped. So it's a _maybe_. > > One of the nice things (to me) about this kind of a path, > too, is that what I learn can be used for lots of things. An > audio amplifier is, in effect, not that much different from > an op amp. There is the usual basic idea of open loop gain > and closed loop gain with negative feedback, phase margins, > problems to solve over a frequency range spanning many > decades, and so on. > > A completely separate project I'd like to play with, which > this learning will help prepare me for, is designing a pin > driver. I'd like to sink or source a programmable current > spanning decades from perhaps 100nA to perhaps 100uA while > reading the voltage at the node, as well as being able to > program a low impedance voltages spanning from -15V to +15V > there and read the current, or read a voltage at the same > node while presenting a fairly high impedence to it. I > imagine what I learn here will aid me there. And I'd like to > do this at some speed, as well. I may then start with a BJT > tester, for example, making up only three of these to start > and tying them into a micro for playing. Expanding that for > other purposes, later. It would be fun. > > Jon- Hide quoted text - > > - Show quoted text - Hi Jon, I'm enjoying your posts. What's a pin driver? I made a nice switchable current source (10nA to 1mA) from a voltage reference, opamp and switchable resistors. (circuit cribbed from AoE.) George H.
From: George Herold on 29 Jan 2010 13:34 On Jan 27, 9:51 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: > On Thu, 28 Jan 2010 11:17:02 +1000, David Eather > > > > > > <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. > > 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. > > 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? > > > > > > >>> 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 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 and better still both R3 and R4 should be > replaced with a current mirror. R5 should be a replaced with > a BJT, as well. I assume the input impedance of that example > is basically the parallel resistance of R1 and R2, but if we > use split supplies I'd imagine replacing the two of them with > a single resistor to the center-ground point. There's no > miller cap on Q3, 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.) > The feedback ... well, I need to think about that a little > more. There's no degen resistors in the emitters of Q4 and > Q5. > > 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. > > Jon- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text -- Hide quoted text - > > - Show quoted text - "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.)" First Jon I know less about amplifier design than you do... That said, I would be careful about replacing the diodes in the push-pull stage. Way back in college I had a Sony stero amp that I had to fix. It came with a nice circuit diagram. I seem to recall that the bias diodes in the push pull stage were thermally attached to the same heat sink that held the output transistors. As the output transistors warm up their Vbe drop decreases. You want the bias diodes to track this change. Or else the whole thing could 'run-away' on you. ... degenerative emmiter resistors (as you suggest) will help some. George H.
From: pimpom on 29 Jan 2010 14:41 George Herold wrote: > On Jan 27, 9:51 pm, Jon Kirwan <j...(a)infinitefactors.org> > 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.)" > > First Jon I know less about amplifier design than you do... > That said, > I would be careful about replacing the diodes in the push-pull > stage. > Way back in college I had a Sony stero amp that I had to fix. > It came > with a nice circuit diagram. I seem to recall that the bias > diodes > in the push pull stage were thermally attached to the same heat > sink > that held the output transistors. As the output transistors > warm up > their Vbe drop decreases. You want the bias diodes to track > this > change. Or else the whole thing could 'run-away' on you. ... > degenerative emmiter resistors (as you suggest) will help some. > I like the biasing scheme mentioned by Jon and use it for all my designs except the early ones using germanium transistors, though I don't know the name either. The biasing transistor can be mounted on the output transistors' heatsink for temperature tracking. I like it because it's versatile and a single transistor can be used to bias several transistors with their b-e junctions in series as long as they are mounted on a common heatsink. http://img691.imageshack.us/img691/2075/bias.png My personal preference is to place the bias adjustment pot R3 in this position rather than with R1. It ensures that any accidental loss of contact by the pot's wiper arm will reduce the total bias whereas placing it with R1 will have the opposite effect and could cause excessive quiescent current in the output transistors, possibly getting them to overheat.
From: Jon Kirwan on 29 Jan 2010 15:11
On Fri, 29 Jan 2010 09:19:31 -0800 (PST), George Herold <ggherold(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 |