Maintaining a Vbe Multiplier's bias value
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From: Jim Thompson on 9 Feb 2010 19:46 On Tue, 09 Feb 2010 16:39:07 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 09 Feb 2010 16:36:40 -0800, I wrote: > >>I know your own preference to include diagrams, rather than >>just words. So here is a link: >> >>>: http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1 e+c.gif > >Sorry. I missed a %20 in there, so it may or may not work in >your browser. Try: > >http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1%20e+c.gif > >Jon The original worked, pasted rather than directly clicked. Something smelly there. I'll try it tomorrow... the Moo Goo Gai Pan is just about ready to eat... plus I've already had some Dry Sack ;-) ...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 20:13 On Tue, 9 Feb 2010 15:44:30 -0800, "Bob Monsen" <rcmonsen(a)gmail.com> wrote: >"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message >news:hhj3n5lpqucim8nmik9eijrc2h9kmo6enn(a)4ax.com... >> 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. > >TLV431s are very simple. They are a bandgap that sucks current until the >'ref' input voltage matches the bandgap output. I've modeled the TLV431 >using the datasheet, and it is a fun exercise. I'll take a crack at it. AofE also talks about bandgaps and I think I understand them, given the nice discussion there. >BTW, do you have a link to that cool LTSpice -> ASCII program? I'd forgotten >that you wrote it. I've been laboring over a hot 'andy's ascii' program for >schematics that I already have in LTSpice... Hehe. Sure. >> 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 > >Well, the thing about these horrible power output stages is that they can >get dicey if they get too hot. When they heat up, the Vbe goes down for >both, which tends to pass more 'shoot through' current, which heats them >more... You get this. Yes, I _get_ it. I earlier ignored temperature when thinking about BJT analysis, as a rule. Too much to include all at once, I suppose. But now I keep it in some part of my mind. >So, using three diodes may actually be better than a single transistor, >assuming that they are thermally coupled with the output devices. Then, the >diodes will have a higher TC than the devices (since there are three rather >than two). So, the output current goes down when it gets hot. > >Here is a simulation that shows it. Look at the shoot-through current as the >temperature goes from 0 to 150C: ><snip> I love the fact that you are shooting these schematics to me. But I'm not sure what "shoot-through" current to look at. What I _do_ see is your point about the output current going downwards as temperature increases, supporting what you suggest about stacking the diodes 3-deep. And I like the general lesson in the schematic, too, allowing you to focus on the output stage. The opamp basically represents the input stage up through, but not including, the final stage. I need to do that for other sections as I proceed around understanding each detail individually and then together as a whole. Kind of a behavioral thing for everything but the section under the microscope. I need to take that approach more often than I do. Jon
From: Jon Kirwan on 9 Feb 2010 20:38 On Tue, 09 Feb 2010 17:13:50 -0800, I wrote: >>BTW, do you have a link to that cool LTSpice -> ASCII program? I'd forgotten >>that you wrote it. I've been laboring over a hot 'andy's ascii' program for >>schematics that I already have in LTSpice... > >Hehe. Sure. Sorry... I didn't stick in the link. Because it isn't set up well, right now. Let me fix that and post it up and then I will add the link. Jon
From: Bob Monsen on 9 Feb 2010 20:58 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:1g34n513mhonq98prjqeggc6h15mtplfu2(a)4ax.com... > On Tue, 09 Feb 2010 17:13:50 -0800, I wrote: > >>>BTW, do you have a link to that cool LTSpice -> ASCII program? I'd >>>forgotten >>>that you wrote it. I've been laboring over a hot 'andy's ascii' program >>>for >>>schematics that I already have in LTSpice... >> >>Hehe. Sure. > > Sorry... I didn't stick in the link. Because it isn't set > up well, right now. Let me fix that and post it up and then > I will add the link. > > Jon No hurry. Thanks, Bob Monsen
From: Bob Monsen on 9 Feb 2010 21:58
"Jim Thompson" <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote in message news:4604n5lppq5jocclt3si1gdjveeu345den(a)4ax.com... > > Something smelly there. I'll try it tomorrow... the Moo Goo Gai Pan > I love to cook with wine. Sometimes I even put it in the food. The original is from EDN, Oct 22, 2009, page 45. "Class B amplifier has automatic bias". Regards, Bob Monsen |