Maintaining a Vbe Multiplier's bias value
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From: miso on 9 Feb 2010 02:47 On Feb 8, 10:06 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: > On Mon, 08 Feb 2010 19:09:28 -0700, Jim Thompson > > <To-Email-Use-The-Envelope-I...(a)My-Web-Site.com> wrote: > >What's a "VAS"? > > Sorry. I read it somewhere regarding audio amplifiers and > the term stuck in my mind, I suppose. It's short-hand for > Voltage Amplifier Stage. It's almost so simple that no one > would bother creating a term for it, except that it seems as > though someone did and folks have used it in places where > I've been reading. > > By the way, if you look at the semi-conceptual schematic at > the top of this page: > > http://en.wikipedia.org/wiki/Electronic_amplifier > > You will see Q3 acting as the VAS. Together with R6 it > converts the beta multiplied current into drive voltage. > > (The Vbe/Ic transfer nasties this up, but I think it may be > survivable. Everything is important, but I'm leaving > worrying about this till later.) > > That schematic isn't entirely realistic, either. R3/R4 are > better replaced with a mirror, regular, Wilson, or otherwise. > R5 is often itself a current source or sink (depending on > which way you flip the schematic polarities) and may be a BJT > and diodes or two BJTs, etc. > > >What exactly are you trying to do? > > If you look again at the schematic mentioned above, note the > function of D1 and D2. They stack to create a bias voltage. > That's used to set the point of operation for the output > stage (two-quadrant emitter follower -- which may be just two > BJTs as in that picture, or more.) Often, this is replaced > with an adjustable BJT configured as a Vbe multiplier. That's > what I'm trying to do. Except that I'd like to have the +V > and -V supply rails (ground is also present in the system) be > unregulated. > > Part of the function of the Vbe multiplier is to also track > the Vbe requirements for the output stage as it heats up and > cools down. The variation of Vbe is quite large, as you > know, where the controlling Eg term in the Is(T) equation > overwhelms the otherwise oppositely-signed dV/dT of the > Shockley equation. Above -2mV/K. And with the exponential > dependance of Ic on Vbe... well, it serves that function as > well. So the Vbe value needs to track temperature in just > such a way that it maintains the design operating point for > the output stage, over temperature, while also ignoring > variations in the current that sources through it. > > I'm trying to keep my options open, regarding the amplifier's > class. If it were operating class-A all the time, my limited > understanding suggests that some variation across the Vbe > multiplier isn't nearly as important as it clearly would be > for, say, class-B operation. I'm not exactly sure where I > want to wind up biasing things. > > So I am slowly learning this stuff and, assuming the Vbe > multiplier has some part within it thermally coupled as > appropriate to some well-chosen part of the output stage, > trying to gather how I'd: (1) stabilize the voltage at some > fixed temperature T against variations in the current flowing > through it, and (2) calibrate it's Vbe multiplication factor > in just the right way so that it tracks well with the > effective Eg found in the Is(T) function of the output stage > needed to hold the operating point steady vs temperature. > > My question here was regarding (1), not (2). I'm not far > enough along on that one to even begin on that one, yet. To > be honest, I just started learning about audio amplifier > design, including terms like VAS, starting around the 26th > last month. So I may be far off the mark in a few places. > > I'm finding it a very interesting education, though, and I'm > glad I started down the road a small bit. But "being exact" > about what I want remains part of the learning process, > itself. So what you see here is as far as I've gotten to. > > >My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe" > >Thompson, because I could pull so much magic with Vbe compensation > >methods ;-) > > Well, I can believe it. And I mean that as a sincere > compliment. If you can suggest something still better than > what I've already posted, I'd like to look at it. > > >(Vbe multipliers generally are used just to create a smaller dead-band > >that is temperature stable. > > In this case, I want it to track the output stage so I'm > going to have to couple it thermally in some useful way. What > I'm considering, right now, is how to make it immune to > unregulated supply variations and VAS output voltage swings. > > >Class AB bias is an art form of which I > >am expert, but cannot divulge publicly at this time :-) > > Well, I want to examine class-AB at some point. It may be > where I want to settle, though class-B would be quite fine > for my needs. > > If you can't help with class-AB, then you can't. I will have > to struggle along. However, anywhere else you can send me a > clue I'd certainly appreciate it. > > There is no interest other than personal. Certainly nothing > commercial in mind. I'm just a hobbyist trying to learn. > > Jon Have you read Randy Slone's power amplifier book? This stuff really isn't rocket science. Nor is AB. ;-) <http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/ dp/0071599258/ref=dp_ob_title_bk> The black art is all in assembly, protection circuitry, and making sure it starts up cleanly. [Most engineers never look at start up, so you get designs that thump when you power them. I have lots of gear with power-on thumps.] I'd pick one of his MOS designs. Bipolar designs often have good intentions, but ring like a bell. MOS is mushy, but predictably mushy.
From: Jon Kirwan on 9 Feb 2010 03:18 On Mon, 8 Feb 2010 23:47:24 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >Have you read Randy Slone's power amplifier book? This stuff really >isn't rocket science. Nor is AB. ;-) I haven't. It's not rocket science. But it is interesting at my level. ><http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/ >dp/0071599258/ref=dp_ob_title_bk> I'll look, but the title appears more on the contruction side. I am using this to educate myself a little better. >The black art is all in assembly, protection circuitry, and making >sure it starts up cleanly. [Most engineers never look at start up, so >you get designs that thump when you power them. I have lots of gear >with power-on thumps.] I recall reading of a recommendation suggesting that all electronic devices use less than 1W when on, but not performing their intended application. This would also seem to require a little added effort to achieve, as well. But I take your point. >I'd pick one of his MOS designs. Bipolar designs often have good >intentions, but ring like a bell. MOS is mushy, but predictably mushy. I'm still learning about BJTs. In fact, that's what this is about for me. MOS later. ICs later. BJTs now. Jon
From: Jon Kirwan on 9 Feb 2010 03:30 On Mon, 08 Feb 2010 23:32:00 -0800, Robert Baer <robertbaer(a)localnet.com> wrote: ><snip> > Have you considered making R2 and/or R3 constant current devices >(depletion FETs are good here)? I've not. Could you be more specific? Jon
From: Jon Kirwan on 9 Feb 2010 03:56 On Tue, 9 Feb 2010 08:36:37 +0100, "Ban" <bansuri(a)web.de> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> schrieb im Newsbeitrag >news:jg91n5d684ru5imsq1cfcjpjd1vddg2b2l(a)4ax.com... >>I think this fits in sci.electronics.design, not .basics. >> >> I'd like to consider the Vbe multiplier often used in audio >> amplifiers to maintain a bias voltage for the output stage. >> The purpose is to better mitigate against ripple in the >> unregulated power supply rails and against the the VAS >> voltage output resulting from amplified signal voltages. >> >No this is not the purpose of this stage. It is used as an adjustable Zener >and is used to create and temperature compensate the bias voltage of the >output stage. A pur DC function. Since the power Transistors draw quite a >bit of quiescent current the ripple of the power supply will be higher then >without. The supply ripple is reduced mainly by negative feedback from >output to the input stage. I think your perspective is more comprehensive than mine, obviously. And I am still building up bits, piecewise. It's how I have to approach this. That said, and I may yet be getting this wrong, but it seems to me that I've seen some serious attention in amplifier schematics; not only looking at global NFB to solve such problems -- though that seems central, of course. But I need to take things one at a time, right now. This is education for me, after all. Not constructing an amplifier to solve some problem I have. I've no problem focusing upon this, a bit, until I subsume it, and then not wind up using all the options I looked at. Learning doesn't only come from taking all the right steps, but also from taking many others that aren't entirely in the right direction. I could only hope to be so perfect as to never choose wrongly. And if so, I probably wouldn't be learning. Anyway, the understanding I wrote can certainly be wrong. However, your later comment seems to say it is wrong because it's "better" to do it using the NFB from output to the diff amp. Yet I still wonder if doing some of this locally is appropriate. In any case, it seems certain that temp comp is one of its functions. Unless I've somehow completely missed things altogether. >snip> >>>: +V >>>: | >>>: resistor or >>>: current source >>>: | >>>: ,---+---, >>>: | | >>>: \ | >>>: / R2 | >>>: \ +-----> upper quadrant >>>: / | ^ >>>: | | | >>>: | |/c Q1 BIAS >>>: +-----| VOLTAGE >>>: | |>e | >>>: \ | | >>>: / R1 | v >>>: \ +-----> lower quadrant >>>: / | >>>: | | >>>: '---+---' >>>: | >>>: VAS ---' >>>: >>>: FIGURE 1 >> >> If I use a resistor as the load for the VAS, it's obvious to >> me that the Vbe multiplier will need to cope with varying >> currents. But even if I use a BJT (or two) to make a current >> source sitting above the Vbe multiplier, it's still not going >> to hold entirely still with +V ripple and with varying VAS >> drive voltages. That variation will ultimately manifest >> itself in a varying Vbe bias voltage. That will change the >> operating point for the output stage. >> >> If it is class-A, I suppose it doesn't matter that much. But >> I don't want to be forced into class-A operation. Nor do I >> want to be forced into regulated rails. So it becomes a >> little more important, I think, to get this nailed down >> better. >> > >In class A you do not use this kind of bias generator If I set it "wide enough" it seems to operate that way. Perhaps I've got that wrong, too? >snip > >I think you should understand before calculating. It is not much of a use to >view this stage in isolation without the output stage and the associated NFB >paths. I think it is useful for me to learn by studying the small building blocks, right now, and considering some thoughts (but not necessarily all the right ones) about larger issues these may need to cope with. I'm in no way ready for the "larger view" you are talking about. Not yet. And only a rare few can start there. I'm not such. For me, it goes from small to large, then back to small, then back out again, and so on until it "gels." I think I will take this structure just a little further in thinking... perhaps a 3rd BJT, I'm thinking. But not more than that. Diminishing returns. I was just wondering if there was more I hadn't come across. Perhaps not. Thanks, Jon
From: Jon Kirwan on 9 Feb 2010 04:19
On Mon, 08 Feb 2010 22:17:31 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 08 Feb 2010 22:11:51 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Mon, 08 Feb 2010 20:49:24 -0800, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan >>><jonk(a)infinitefactors.org> wrote: >>> >>>>On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin >>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>>Hang a big capacitor across it. >>>> >>>>Nice try. >>>> >>>>Jon >>> >>>No, seriously, that solves a bunch of problems. >>> >>>John >> >>Which problems does a slew-dependent, C*dV/dt bypass current >>solve? >> >>Jon > >A big cap across the biasing gadget keeps the voltage drop across it >fairly constant, of course. That nukes some of the problems you >referred to. More peak current is available to the output bases, for >example. What size cap would help with power supply ripple? Seems the dV/dt is so small that a fair sized cap would be required to make any difference. Similarly for low frequency amplified signal out of the VAS. When you say "big," maybe you mean it. Ban is suggesting global NFB from output back to input. You've said as much when you say to apply "lots of NFB." I don't doubt the sincerity of either of you and I'm certain it will do a lot. But right now I'm interested in seeing what can be done right on this local subcircuit and at LF as well as higher frequencies. Unless someone wants to walk me through the thinking towards the larger concepts. I'm good either way, as it's the learning that takes place I'm looking for. But without such guidance, I need to move along at the pace I can handle while guiding myself. Jon |