Maintaining a Vbe Multiplier's bias value
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From: Jon Kirwan on 9 Feb 2010 15:59 On Tue, 09 Feb 2010 08:33:45 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: ><snip> >Useless nonsense.... Are you talking about my comments, those of others here, or the link you posted below? >http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf I'll take a look, today. Thanks, Jon
From: bg on 9 Feb 2010 16:05 Jon Kirwan wrote in message ... >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. > >(The only active device under consideration is a BJT, though. >No JFETs or MOSFETS or opamps or other ICs.) > >The basic starting form for a Vbe multiplier is shown in Fig. >1 and the bias voltage output is indicated there. Assume Q1 >is thermally coupled in some magic way, for now, in just the >right way so that if the current through the Vbe multiplier >were perfectly stable, that the bias voltage would track just >as needed (The 'Eg' of Q1 is exactly what's needed for the >output stage's temperature tracking in some nice way and the >values of R1 and R2 are set correctly and the thermal >coupling and location is somehow where it needs to be.) The >focus is on the Vbe multiplier's variation of bias in the >face of changes in sourcing current at the top of Fig. 1. > >>: +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. > >There's the problem, anyway. > >To quantify how bad all this really is, I tried my hand at >figuring out the small signal analysis of the Vbe multiplier. >If I got a first approximation about right, it is based >squarely upon the small-re of the BJT. The very familiar >value for (kT/q)/Ic. > >There is also the value of R2 shown in Fig. 1, but since its >effect is only affected by the change in base current, I >believe it's contribution is divided by Q1's beta. So the >actual equation is something like: > > R_ac = (1/Ic)*(kT/q)*(1+R2/R1) + R2/beta > >For a 2X multiplier where R2 is about R1, this is: > > R_ac = (2/Ic)*(kT/q) + R2/beta > >The Vbe multipler value is: > > V_bias = Vbe*(1+(R2/R1)) + R2*Ic/beta > >(The latter term being a correction for base current.) > >Ignoring base current for now and assuming I had Ic set >around 5mA and placed R1=R2=1k for the 2X factor, this R_ac >value works out to about 15.4 ohms. > >A variation of half an mA in Ic yields about 7.7mV change in >the bias point. > >I decided to see if the Early effect made much of a >difference. The adjustment appears to be something like >this: > > R_early = dV/dI = -Ic/VA*R^2 > >If I'm interpreting it right, it really does show as negative >resistance added to R_ac. The fuller equation, then, >including the Early effect, would be: > > R_ac = (2/Ic)*(kT/q) + R2/beta - Ic/VA*R^2 > >(Which requires a quadratic solution to solve for R.) > >If R_ac is 15.4 ohms and Ic is around 5mA, a VA of 100V would >suggest about R_early=-10mOhms. Which is roughly a factor of >1500 less than 15.4 Ohms. Since it now appears to be on the >order of 0.1% or so for typical Ic, VA, and, R_ac values, I >think I can ignore it for these considerations. > >So drop it, I will. > >I had scouted around a few weeks back (not for this reason) >and found what is shown in Fig. 2. I remembered it, but >didn't understand it then. > >>: +V >>: | >>: resistor or >>: current source >>: | >>: ,---+---, <-- node A >>: | | >>: | \ >>: | / R3 >>: \ \ >>: / R2 / >>: \ | >>: / +-----> upper quadrant >>: | | ^ >>: | |/c Q1 | >>: +-----| BIAS >>: | |>e VOLTAGE >>: \ | | >>: / R1 | v >>: \ +-----> lower quadrant >>: / | >>: | | >>: '---+---' >>: | >>: VAS ---' >>: >>: FIGURE 2 > >I think I now understand why R3 was there. Changes in Ic >create changes in Q1's collector voltage, per Ic*R3. The >result is that dV=dI*R3. If R3 is on the order of the above >computed R_ac, then variations at node A caused by changing >currents through the Vbe multipler (most of which are seen as >Ic changes) will be neatly compensated for the change in the >voltage drop caused by R3. > >However, that can only be set for some assumed Ic. Nearby >changes will work pretty well. But further deviations will >start to show problems again. Also, the Fig. 2 version will >use a slightly higher multiplier value to get node A up high >enough for the R3 drop to hit the right place required to >bias the output stage. That higher multiplier means that >while, let's say, the two (or four, if that's it) output >BJT's Vbe values vary over temp and the thermally coupled Q1 >above also varies it's own Vbe value, the multiplier other >than 2 (or 4) will mean the variation of the bias will match >at only one place -- if it ever did more than one spot. How >important that is, I've not considered yet. > >I'm wondering about additional topology changes to improve >the performance still more. Obviously, if they are crazy and >wild, I'm probably going to live with the above and be done >with it. But I think there's got to be something still >better. Another BJT as a bypass route across Q1 and R3? > >Getting this nailed down should help mitigate against both >unreg supply ripple (on one side, anyway) putting hum into >the output and also against large scale changes in the VAS >amplified signal voltage (which means distortion.) > >Jon Your circuit is an example of collector feedback. Collector feedback does not work well with large signal swings and it lowers the input impedance. A lower input impedance means that the drivers will also need a lower output impedance. The bottom line is that you will not see this bias method used in a power output stages. Another option is to use emitter feedback but for the emitter resistors to be effective, they will drop alot of signal output and waste power, which explains why those resistors are usually very low values, They have very little effect unless the emitter current is large. At the DC bias current level, they don't do a thing. Local feedback (emitter feedback or collector feedback) both create more problems then they solve for stabilizing the power output stage bias point. To keep the output stage bias point stabilized, the use of overall feedback is the standard practise. A simple typical amplifier might have a differential input stage, followerd by a voltage amplifier, followed by the power output stage. The output from the power stage is fed back to the differential input stage. High open loop gain with large feedback is the key to better stabilization of the operating points. Fix it with feedback is a term to remember. The typical bias chain using diodes can be made with resistors as well, but diodes have the advantage of dropping the bias voltage while having a lower impedance to the signal. Sometimes you will see those bypassed with a large cap if the impedance causes to much signal loss. Diodes can also offer temperature compensation. In any case, an output stage will have way more current flowing in that bias chain than is actually needed as base bias current. The voltage drops developed in the bias chain will not be greatly affected by changes in the base emitter junction because the base bias current is small compared to the current in the bias chain. And remember, that " Fix it with feedback " applies here too. So variations in the power supply have a very reduced effect on the bias point. The feedback signal is a voltage, and enough feedback will compensate to keep the output voltage offset at zero. It will not compensate for for excessve collector currents or power dissaption if the offset voltage remains low. That is why temperature compensation is used too. In the early years of transistors, it was common to see transistor stages using many of the techniques used with vacuum tubes. Dc coupled amplifiers were rare, because any bias shift was amplified in further stages. Feedback was applied locally, and overall feedback had no effect on the DC operating points. The trend now is to stabilize everything with feedback. It works, and it works well. Unless you are a purist and have some religious reason to avoid this technique, there is no sense in reinventing the wheel.
From: Jon Kirwan on 9 Feb 2010 16:05 On Tue, 9 Feb 2010 11:35:36 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >On Feb 9, 5:39�am, Jon Kirwan <j...(a)infinitefactors.org> wrote: >> On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold >> >> <ggher...(a)gmail.com> wrote: >> >><snip> >> >"I'm wondering about additional topology changes to improve >> >the performance still more." >> >> >Hi Jon, �I've been 'sorta' following your thread on s.e.basics. �I >> >wonder if you abandoned class �A operation too early? �Why not keep >> >things linear evreywhere and avoid the �dead band�? �So what if you >> >need a bigger heat sink. �It�s certainly a lot simpler. >> >> >George H. >> >> Well, George... No, I've not abandoned it. �Actually, it's my >> hope to wind up building the amplifier and then operating it >> (by hopefully choosing a design where that is possible) in >> different modes for the learning experience of it. �I hope >> that is in the cards. �I really do. >> >> But to make a sharp point on it, although it's probably just >> an extreme case, I remember reading about a 10W amplifier, >> single channel, dissipating 120W! �Creeps me out. �So I >> definitely _want_ to consider other classes of operation. And >> cripes, I want to learn, anyway. �So why not keep my options >> open? >> >> Jon > >" I remember reading about a 10W amplifier, >> single channel, dissipating 120W! " > >It might have been here, >http://www.passdiy.com/default.html >I got to reading about amplifiers on the above site... Do in part to >your interest. > >George H. Egads. Loads of PDF files. Now I have to create a directory, download them one by one, and then call them up with my slow machine to look. Any particular page or file where you saw it? (No, that isn't where I saw the comment.) But thanks for the link. I'll add it to those I read, also. Jon
From: Jim Thompson on 9 Feb 2010 16:07 On Tue, 09 Feb 2010 12:59:01 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 09 Feb 2010 08:33:45 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >><snip> >>Useless nonsense.... > >Are you talking about my comments, those of others here, or >the link you posted below? > >>http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf > >I'll take a look, today. > >Thanks, >Jon The link. Do the math, it's a hoax, good only at one current and temperature pair... besides being Beta sensitive. ...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: Jon Kirwan on 9 Feb 2010 16:18
On Tue, 9 Feb 2010 11:40:45 -0800, "Bob Monsen" <rcmonsen(a)gmail.com> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:jg91n5d684ru5imsq1cfcjpjd1vddg2b2l(a)4ax.com... >> I think this fits in sci.electronics.design, not .basics. >> >> >> Jon > >Sorry, I didn't read the entire message... > >However, if you want a stiff multiplier, use a TLV431 instead of a BJT. >Somewhat more expensive, but it'll be VERY stiff. I'm still in "discrete" mode. For example, I am _less_ interested in opamp topologies and design techniques than I am in _how_ to design opamps. There is nothing like knowing the details about how they are designed inside to understand the gotchas that aren't readily accessible to someone using them. A comparison here might be like "using a handgun" vs "understanding how handguns are designed and built." A gunsmith requires a very detailed knowledge and while this level of detailed knowledge may not make them a better shooter, that knowledge still informs them about the handgun in ways that most shooters have little idea about. And I think it prepares them for certain unusual circumstances a little better. I'm at the gunsmith level, right now. I am NOT wanting to go shooting, just yet. >However, you don't really want to hold that value constant. You want the >voltage to compensate for the temperature of the output transistors. Yes. >You might be able to use a diode to track the temperature change, and then use >that in the feedback loop to compensate the TLV431. No ICs. I might like to thoroughly _understand_ the internal design of the TLV431, first. Then I'm willing to use it. >A honking big capacitor, one that has very low impedance at your frequencies >of interest, is probably the best idea I've seen on the thread. Well, I'm interested in focusing on the crafted design of Vbe multipliers, right now. I can _always_ slap a cap on whatever that winds up being, later on. So set that aside. What also bugs me is how that darned thing is going to interact with the larger system, eventually. I don't like ignorantly littering a schematic with poles and zeros and phase delays where right now I have very little idea right what then happens when I close the outer NFB loop. I'm still "in the trenches" and trying to understand each piece in detail and think at that level. The capacitor is at the next level above and is outside my "view." Besides, it doesn't do much for LF. The Z is too high and in parallel, ignorable. >On a related note, there was an article in a recent EDN about a self biasing >preamp which was kinda cool. Instead of trying to track the difference using >diodes or a multiplier, it used a couple of transistors and an opamp to set >the correct values at the bases of the pass transistors. It was so novel (at >least to me) that I typed it into LTSpice. ><snip> Okay. I'm going to save it, too. I'm not ready to assimilate it, of course. But I definitely want it around when I _am_ ready for it. Thanks, Jon |