Maintaining a Vbe Multiplier's bias value
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From: Phil Allison on 10 Feb 2010 20:07 "Tim Williams" > "George Herold" >> Say can you make a push-pull stage run class A? (Or is that just a >> silly idea?) ** 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. ...... Phil
From: bg on 10 Feb 2010 20:18 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 - I put that page together 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. Again , try to find the book, it is the best advice I can give.
From: Jim Thompson on 10 Feb 2010 20:23 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 - >I put that page together 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. > Again , try to find the book, it is the best advice I can give. > Good application of local feedback has other advantages... it generally makes it easier to apply global feedback without getting into stability and ringing nightmares. ...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 10 Feb 2010 21:18 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
From: George Herold on 10 Feb 2010 21:48
On Feb 10, 8:07 pm, "Phil Allison" <phi...(a)tpg.com.au> wrote: > "Tim Williams" > > > "George Herold" > >> Say can you make a push-pull stage run class A? (Or is that just a > >> silly idea?) > > ** 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. > > ..... Phil Excellent! Thanks Phil. Do you just use a current source as bias as in the Amps from Pass labs? (Do I need to repost the previous link?) I don't quite see how this works for push-pull. Do you have any schematics? (Hmm thinking that a few resistors should take care of things...) Say I thought I read that tubes can't do push-pull because "it's hard to get positrons from the filament" to parapharse what I read. George H. |