Maintaining a Vbe Multiplier's bias value
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From: miso on 10 Feb 2010 23:49 On Feb 10, 7:34 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "George Herold" > "Phil Allison" > > > > > > > ** Push-pull class A is the MOST COMMON method use in tube and transistor > > audio power amplifiers. > > > With transistor amps, it is only necessary to set to standing bias current > > to a high value like an amp or two. > > > > You can, but it stops being class A for large signal swings or low load > > > impedances. > > > ** No need for either thing to happen. > > > One picks the load to suit the amplifier and the peak ( class A) current > > is > > double the bias setting current. > > > Eg: > > > With a bias of 2 amps, peak load current is 4amps which allows +/-32 volts > > into 8 ohms. > > > Class A power is then 64 watts rms. > > > The DC rails need to be about +/- 35 volts and the supply current a steady > > 2 > > amps. > > Do you just use a current source as bias as in the Amps from Pass labs? > > ** No - doing that is utterly STUPID. > > The only difference between a class A and class B push-pull amp IS the > bias setting. > > One just turns the bias trim pot to get the desired standing current or with > tubes adjust the grid bias to get the same result. > > > Tubes can't do push-pull because "it's hard to get positrons from the > > filament" to parapharse what I read. > > ** How pathetic. > > .... Phil Did you mean AB versus B? And can you answer without cursing?
From: John Larkin on 10 Feb 2010 23:55 On Wed, 10 Feb 2010 20:47:33 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Feb 10, 8:35�am, George Herold <ggher...(a)gmail.com> wrote: >> On Feb 9, 4:05�pm, Jon Kirwan <j...(a)infinitefactors.org> wrote: >> >> >> >> > On Tue, 9 Feb 2010 11:35:36 -0800 (PST), George Herold >> >> > <ggher...(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- Hide quoted text - >> >> > - Show quoted text - >> >> Ohh sorry about that... Don't bother reading them... He (Nelson Pass) >> has designs for Class A amps using a FET. �(named Zen) �One of his >> variations (son of Zen?) was made with no NFB and I think wasted your >> stated 120 Watts of power for 10W into the speaker. �But this seemed >> pretty pointless to me... some audio guys wanted an amp with out >> NFB.... Don't aks me why! >> >> Say can you make a push-pull stage run class A? �(Or is that just a >> silly idea?) >> >> George H. > >Pass designs are interesting reading. Nobody says you have to build >them. ;-) Nelson is the opposite of Randy Slone, who is Mr. Practical. >Doug Self is somewhere in the middle. How about an amp using complementary voltage regulators as the output stage, like LM317 and LM337? Nice current and thermal protections. John
From: bg on 11 Feb 2010 01:24 Jon Kirwan wrote in message ... >On Wed, 10 Feb 2010 18:18:21 -0700, "bg" <bg(a)nospam.com> >wrote: > >>Jon Kirwan wrote in message ... >>>On Wed, 10 Feb 2010 16:42:47 -0700, Jim Thompson >>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>> >>>>On Wed, 10 Feb 2010 15:37:00 -0800, Jon Kirwan >>>><jonk(a)infinitefactors.org> wrote: >>>> >>>>[snip] >>>>> >>>>>I still _get_ the idea of NFB!! So I don't mean to argue >>>>>against that! I just went somewhere else with that page. >>>>> >>>>>Jon >>>> >>>>First rule of "NFB": Make it as good as you possibly can without NFB, >>>>_then_ apply NFB ;-) >>>> >>>>But it's sort of a trick and a lie... you use _local_ feedback to make >>>>the individual pieces as linear as you can, then add overall _global_ >>>>feedback. >>>> >>>> ...Jim Thompson >>> >>>Now _this_ is what I wanted to hear. >>> >>>Many seem to just tell me "use global NFB to fix things" >>>almost, it seems, to simply stop me from bothering to >>>struggle at all or even care about understanding things. >>> >>>Maybe it is just because it _takes work_ to actually engage a >>>quantitative discussion and the lazy way out is to just hand >>>wave and tell me to "move on by." >>> >>>But it was my sense at the outset, and it is my motivation >>>for starting this thread as well, to do exactly what you are >>>talking about here. I'm so glad to see it said. "Make it as >>>good as you can without NFB, then apply NFB." Yes! >>> >>>For example, the Sziklai pair is really a BJT wrapped with a >>>local NFB using the other BJT for that purpose. Nice. >>> >>>I couldn't state it this clearly because I'm just learning >>>things. But what you said is what my instincts tell me, >>>despite attempts to say "move on, there's nothing to see >>>here." >>> >>>Jon > >>Jim hit the nail right on the head, make it linear as possible with local >>feedback first. Add global later if it applies - > >Thanks. Now why do I instead find myself sometimes having to >actually _argue_ about understanding building blocks well? >Why is the answer so often, "throw gobs of global NFB at it?" > >I wonder if the availability of all-too-perfect A_ol = 1E15 >opamps (not really, but what's the difference?) is part of >the problem, here. Maybe it's making things too easy. > >>I put that page together > >Ah. That was you? Thanks for the effort, then!! > >>as a quick reponse so that you might see for >>yourself what I mean by feedback being used to stabilize things. It is far >>from an in depth analysis and there very well could be something desperately >>wrong with the circuit, but it does reduce the drift to 1/4 of the circuit >>without feedback and that is the point of my post. > >I think I already understood the majestic power of global >NFB. It's so important, I am sure, that if NFB didn't _also_ >wrap the output stage itself, the results would be indeed >very lousy no matter how good the earlier part of it turned >out to be. So it is not an option. > >I did learne the basic gain/feedback equation years ago: >Vout/Vin=A/(1+A*B), with B being the feedback and A the open >loop gain. With gobs of A available in these all-too-perfect >opamps these days, the whole thing drops back to 1/B 'real >fast.' Which is nice because then just set B and get handed >a fixed gain on the so-called silver platter. > >I have to still believe, as broadly ignorant as I indeed am >about these things, that crafted design with localized NFB >remains useful even in the case of audio amplifiers. That >doesn't mean the power of global NFB isn't of the overarching >importance that it is. There is no option there. But there >remains more to life than merely that, too. Local NFB seems >to remain important to me. And it was nice that Jim took a >moment to confirm that impression, when so few had done so >beforehand. > >... You also noticed that I took the web page in a totally >different direction? ;) > >Speaking of which, what spice model did you use for that >2N3904? Can you post it? I'd like to stick it into LTspice >and see why I got different results. Since my calculations >didn't depend too highly on wrong estimates of beta and since >kT/q doesn't care about the BJT, I'm curious about exploring >it a little more. > >> Again , try to find the book, it is the best advice I can give. > >Thanks, >Jon This is the model that came with circuitmaker - - *2N3904 *Si 310mW 40V 200mA 300MHz pkg:TO-92B 1,2,3 ..MODEL 2N3904 NPN(IS=1.4E-14 BF=300 VAF=100 IKF=0.025 ISE=3E-13 + BR=7.5 RC=2.4 CJE=4.5E-12 TF=4E-10 CJC=3.5E-12 TR=2.1E-8 XTB=1.5 KF=9E-16 ) I don't use LT so I'm not sure if the commented (*) text needs to be removed. These spice model files might be of interest but possibly obsolete - google for them OnSemiconductorAllModels.zip OnSemiconductorDiscreteModels.zip OnSemiconductorIntegratedFunctionsModels.zip
From: Jon Kirwan on 11 Feb 2010 03:24 On Wed, 10 Feb 2010 19:55:44 -0800 (PST), George Herold <ggherold(a)gmail.com> wrote: >I'm perhaps more of a novice than you... Somehow, I doubt that. I barely rate "hobbyist." >but I find opamp circuits complicated enough.... Don't sweat it. While some opamps leave _some_ issues nearly ignorable, there is always some tough problem at that scale that makes it non-trivial and interesting to work on, I imagine. Each macroscale view has it's own complexity. Telescoping levels, where the complexity at one stage doesn't take away from interesting complexity at another level. (I would say more but I'm keeping in mind your warning about long-windedness and will now muzzle myself.) >And tend to stick transistors >only on the edges of things. (Mostly on the output side... on the >input you have to 'know more' than the guys who designed the opamp.. >hard to do for a novice.) I do the same things except that I enjoy math and BJTs give me an excuse, perhaps. Maybe that's the only difference. >I guess if I was designing an audio amp I'd figure on an opamp driving >some sort of FET output stage. The question of how to bias the output >stage is interesting. And also of how all the NFB works. Might as well just get a power opamp like the OPA502 and be done with it. Give it two rails, feed the input, and just drive the hell out of a speaker. Or get two of them and do a bridge amplifier. But where is the enjoyment in that? Or the learning? Someone else already did most of the fun stuff and there's nothing really left to do except some hook up and heat sinking. It's not at all satisfying to me, anyway. An audio amplifier is basically a power opamp. Using an opamp to make one feels to me like building a car by first buying a car without the tires, selecting and installing some tires, and then saying you designed and built yourself a car. Jon
From: Jon Kirwan on 11 Feb 2010 04:07
On Wed, 10 Feb 2010 23:24:04 -0700, "bg" <bg(a)nospam.com> wrote: >This is the model that came with circuitmaker - - >*2N3904 >*Si 310mW 40V 200mA 300MHz pkg:TO-92B 1,2,3 >.MODEL 2N3904 NPN(IS=1.4E-14 BF=300 VAF=100 IKF=0.025 ISE=3E-13 >+ BR=7.5 RC=2.4 CJE=4.5E-12 TF=4E-10 CJC=3.5E-12 TR=2.1E-8 XTB=1.5 >KF=9E-16 ) Thanks!! LTspice's model is: ..model 2N3904 NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5 + BR=4 CJC=4E-12 CJE=8E-12 RB=20 RC=0.1 RE=0.1 TR=250E-9 + TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40 Icrating=200m + mfg=Philips) Note that it does NOT include a figure for ISE, which your model does. LTspice is setting it to zero, instead. Your model may very well be right. If so, it affects the beta value (degrades it due to some recombination cause) and that affects my calculations. In fact, that value of ISE cuts it down to around 100, or so. Which about doubles the contribution to the final term in the equation I gave where a term is divided by beta, so that adds back a few tenths of a volt to the prediction! Great! Now, with your model, LTspice shows the same results you got. >I don't use LT so I'm not sure if the commented (*) text needs to be >removed. No, that is pretty much the universal spice comment character. It's fine. >These spice model files might be of interest but possibly obsolete - google >for them >OnSemiconductorAllModels.zip >OnSemiconductorDiscreteModels.zip >OnSemiconductorIntegratedFunctionsModels.zip I already had the first one laying about. I downloaded the second one just to have it and extracted 2N3904.LIB from it, using that model, as well. It works much more like the LTspice one and perhaps even more different than the one you were using. However, it does produce about the same _beta_ as your model does. Very interesting. I need to consider the reasons carefully. Thanks, Jon P.S. Just for completeness, the OnSemi model is this: ..MODEL 2N3904 NPN( +IS=1.26532e-10 BF=206.302 NF=1.5 VAF=1000 +IKF=0.0272221 ISE=2.30771e-09 NE=3.31052 BR=20.6302 +NR=2.89609 VAR=9.39809 IKR=0.272221 ISC=2.30771e-09 +NC=1.9876 RB=5.8376 IRB=50.3624 RBM=0.634251 +RE=0.0001 RC=2.65711 XTB=0.1 XTI=1 +EG=1.05 CJE=4.64214e-12 VJE=0.4 MJE=0.256227 +TF=4.19578e-10 XTF=0.906167 VTF=8.75418 ITF=0.0105823 +CJC=3.76961e-12 VJC=0.4 MJC=0.238109 XCJC=0.8 +FC=0.512134 CJS=0 VJS=0.75 MJS=0.5 +TR=6.82023e-08 PTF=0 KF=0 AF=1) |