Maintaining a Vbe Multiplier's bias value
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From: John Larkin on 9 Feb 2010 10:03 On Mon, 08 Feb 2010 22:43:19 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Mon, 08 Feb 2010 22:35:05 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Tue, 9 Feb 2010 00:28:27 -0600, "Tim Williams" >><tmoranwms(a)charter.net> wrote: >> >>>"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >>>news:okr1n55h5dvjjklg760dllkqq50v7s38ib(a)4ax.com... >>>> 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 general idea is to put the Vbe transistor on the same heatsink as the >>>outputs, if not glued to a transistor directly. >>> >>>Unfortunately, for widely mismatched current densities, this doesn't work. >>>http://webpages.charter.net/dawill/Images/Ampere.gif >>>In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough >>>tempco to compensate the far beefier (= lower current density??) output >>>darlingtons. >>> >>>I was thinking of adding another CCS so a constant voltage drop appears on >>>the Vbe's base divider resistor. Algebraically subtracting a fairly stable >>>voltage results in the effective tempco (percentwise) increasing. The base >>>divider ratio has to be changed to compensate. >>> >>>> 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. >>> >>>Don't worry about stability -- as John said, bypass and forget about it. >>>Most of the dynamic VAS/CCS current flows into the output stage, since >>>that's what it's there for anyway. The capacitor helps turn on the N side / >>>turn off the P side for rising edges and vice versa. >>> >>>As for PSRR, the CCS's and gobs of feedback keep that in check. Of course, >>>in principle you need something to start the CCS's. ICs do this with a JFET >>>(i.e. current regulating diode) or bandgap reference (e.g., TL431), or >>>sometimes both, to set a master current, from which everything else is >>>mirrored. Most discrete circuits just use a resistor, which is "0%" PSRR, >>>but it's not all that bad because the currents are balanced (*on average*, >>>which means you'll see IMD products when it's moving). >>> >>>Tim >> >>This topology, thermally coupled Vbe multiplier, was mediocre 50 years >>ago. And still is. > >A trek of a thousand miles starts with but the first step. > >Jon A trek of 23,000 miles starts with but the first step in the wrong direction. John
From: John Larkin on 9 Feb 2010 10:05 On Mon, 8 Feb 2010 23:47:24 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >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. Can you explain "mushy" in any more technical terms? John
From: Jim Thompson on 9 Feb 2010 10:03 On Mon, 08 Feb 2010 22:06:54 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Mon, 08 Feb 2010 19:09:28 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(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 (1) Split R1, bypass that junction to ground (2) Make R5 and R6 into mirrors, resistor feed from VDD, but split and bypassed. (3) As you said, replace R4:R3 with a mirror, I don't think a compound device mirror, such as Wilson, is necessary. Study this if you want more info.... http://analog-innovations.com/SED/EnhancedCurrentMirrors.pdf (4) Since you're on a learning curve, just replace D1/D2 with 1.5*Vbe, losing about 1/5 of the Q3 quiescent current in the resistors. Bypassing base-to-base (of Q4-Q5) will help at all but very low frequencies. (5) Long haul as you "oomph" the power: Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more complicated (Darlington extension of Vbe multiplier). Start simple, then grow it, that way you learn before you flame it ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jim Thompson on 9 Feb 2010 10:33 On Tue, 09 Feb 2010 02:44:31 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Mon, 8 Feb 2010 18:23:16 -0800 (PST), "miso(a)sushi.com" ><miso(a)sushi.com> wrote: > >><snip> >>Less words and real schematics would get you more readers. [The only >>thing worse than ascii equations are ascii schematics.] > >ASCII is what I'll post. It's the only way to get them >archived or properly posted to a text newsgroup. I no longer >have access to the binary for schematics, sadly. If I lose >some people because they cannot manage fixed-spaced fonts, I >guess I lose them. I could place links up on my domain, I >suppose. But in this case, the schematics are really very >basic and not overly burdensome in ASCII. Besides, Win Hill >posted some really nice examples here, before. Folks seemed >to live with that. Not sure why you are picking on me, here. > >>In any event, just google improved vbe multiplier. I've seen all sorts >>of circuits published to get lower impedance at the nodes. > >Okay. I'll do that if folks here aren't interested at all in >talking about it. > >Jon Useless nonsense.... http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Bob Monsen on 9 Feb 2010 14:20
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:c7u2n5ldssoc0drl22u1rrif02t7jmkc0s(a)4ax.com... > On Mon, 08 Feb 2010 22:43:19 -0800, Jon Kirwan > <jonk(a)infinitefactors.org> wrote: > >>A trek of a thousand miles starts with but the first step. >> >>Jon > > A trek of 23,000 miles starts with but the first step in the wrong > direction. > Not while I'm drinking my coffee, please... :) Regards, Bob Monsen |