Maintaining a Vbe Multiplier's bias value
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From: Jon Kirwan on 9 Feb 2010 18:32 On Tue, 09 Feb 2010 16:21:27 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Tue, 09 Feb 2010 15:17:24 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Tue, 09 Feb 2010 16:08:22 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>>On Tue, 09 Feb 2010 14:50:50 -0800, Jon Kirwan >>><jonk(a)infinitefactors.org> wrote: >>> >>>>On Tue, 09 Feb 2010 15:42:48 -0700, Jim Thompson >>>><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >>>> >>>>>What in the world ?:-) >>>> >>>>View in fixed-spaced font. And it's a rendition of the >>>>schematic that Bob Monson had posted, earlier, from EDN. He >>>>wrote, "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." >>>> >>>>I merely re-arranged it in LTspice to be a little more to my >>>>taste and then passed it through a program that generates >>>>ASCII from that. >>>> >>>>Jon >>> >>>Burr-Brown was famous for using bias compensation like that in the >>>front ends of some of their operational amplifiers, but I doubt its >>>efficacy in power output stages. >> >>I am still struggling to understand it. There are very >>obvious parts that I completely understand. For example, the >>divider for a "center" voltage followed by a unity gain >>buffer for drive current compliance. I would guess that the >>gain is determined by the NFB resistor divided by the input >>impedance, which is mostly R4 in this case... so 10. I see >>an opamp looking like an integrator, but I'm frankly >>unfamiliar with the 4-BJT arrangement structure and need to >>think about that one. >> >>>The Burr-Brown scheme is similar to a discussion here a few (seven :-) >>>years ago... >>> >>> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf >> >>I'll download it now and look when I get a moment to engage a >>little thought. >> >>Thanks, >>Jon > >Study up on writing loop and nodal equations and either solving by >simultaneous equations or matrix manipulation. I'm familiar with Norton and Thevenin and the use of three different perspectives, branch-current, mesh, and nodal analyses. I very much prefer to "think" with nodal analysis and have pretty much set aside the other two approaches, now. I'm also familiar with matrix solutions and have developed my own programs for solving them a little easier than my TI calculator allows for and with better accuracy in difficult cases. I can also do Laplace, but frankly I have NOT yet learned the shortcuts often used. So I wind up with pages of partial fractions in the end, converting back to time domain with tables, and seeing how things look there. Being capable at a detailed level does not let me "see" at the top level, just yet. Sometimes, it takes a while. The immediate example making this point is where I didn't _understand_ what the collector resistor _might_ do when I first saw it in a Vbe multiplier. And I initially tried to analyze the circuit with it, included, and I realized then that there was a negative feedback present at the tap-off point. But it was only when I analyzed the simpler circuit, without it, and found the approximate R_ac for it that I then _saw_ that this calculated R_ac closely matched the collector resistor values I saw in the examples. That then immediately told me the _why_!! It's how things sometimes work for me, I guess. Anyway, I am very glad for the suggested examples. Thanks, Jon
From: Jon Kirwan on 9 Feb 2010 18:35 On Tue, 09 Feb 2010 16:24:43 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Tue, 09 Feb 2010 14:30:54 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Tue, 9 Feb 2010 15:48:42 -0600, "Tim Williams" >><tmoranwms(a)charter.net> wrote: >> >>>"Bob Monsen" <rcmonsen(a)gmail.com> wrote in message >>>news:1265746051.657782(a)sj-nntpcache-3.cisco.com... >>>> 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. Here it is: >>> >>>Could you take a screenshot of the schematic? >> >>I'll include an ASCII version here: >> >>>: R2 >>>: +V = 12V ,------/\/\---------------------, >>>: | 1k | >>>: | +V | >>>: | | | >>>: | \ | >>>: | / R1 | >>>: | \ 1k5 | >>>: | / | >>>: | | +V | >>>: C2 | ,----+ | | >>>: || 10uF R4 | | | 2N3904| | >>>: ,------||------/\/\---------+ | | | | >>>: I| || 100 | | Q4 e>| |/c Q3 | >>>: N| | | |-------| | >>>: | | | c/| |>e | >>>: | | C1 --- | | | >>>: --- | 10uF--- |2N3906 | | >>>: - V2 | | | | | >>>: --- SINE(0 .2 1k) | | | | | C3 >>>: - | C5 | | | | || 470uF >>>: | | || 10p| | +----+-||----,O >>>: | +V +---||----+ | | || |U >>>: | | | || | | | |T >>>: gnd | | | | | \ >>>: | | | |2N3904 | / R5 >>>: \ ,-------, | | | | \ 8 >>>: / R3 | | | +V | Q1 c\| |<e Q5 / >>>: \ 1k | +V | | | 2N| |-------| 2N3906 | >>>: / | | | | |\| | e<| |\c | >>>: | | |\| | '-|-\ | | | | >>>: | '--|-\ | | >----+----' | gnd >>>: | | >-+-----|+/ | >>>: +-------|+/ |/| LT6234 | >>>: ,-----+ |/| LT6234 | gnd >>>: | | | gnd >>>: --- C4 \ gnd >>>: --- 1uF/ R9 >>>: | \ 1k >>>: | / >>>: | | >>>: gnd gnd >> >>(This was auto-generated from my LTspice to ASCII program.) >> >>Jon > >Did something get lost in the ASCII? Otherwise collector-to-collector >as in Q1-Q4 is a no-no... one of those devices will saturate. > > ...Jim Thompson I just double-checked. It's just that way in Bob's posted LTspice schematic. And when I simulate the thing, it produces a 2V p-p output into R5 from a .2V p-p input. Takes a few cycles to settle on a DC center level, though. Jon
From: Bob Monsen on 9 Feb 2010 18:44 "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. 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... > > 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. 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: Version 4 SHEET 1 880 680 WIRE 32 -48 -160 -48 WIRE 272 -48 32 -48 WIRE 32 -32 32 -48 WIRE 272 32 272 -48 WIRE 32 80 32 48 WIRE 208 80 32 80 WIRE 96 128 64 128 WIRE 160 128 128 128 WIRE 304 128 272 128 WIRE 400 128 384 128 WIRE 32 144 32 80 WIRE 96 144 96 128 WIRE 160 144 160 128 WIRE -160 176 -160 -48 WIRE 400 192 400 128 WIRE 496 192 400 192 WIRE 496 224 496 192 WIRE 32 240 32 208 WIRE 64 240 64 128 WIRE 64 240 32 240 WIRE 96 240 96 208 WIRE 96 240 80 240 WIRE 128 240 128 128 WIRE 128 240 96 240 WIRE 304 256 272 256 WIRE 400 256 400 192 WIRE 400 256 384 256 WIRE 160 304 160 208 WIRE 208 304 160 304 WIRE -64 320 -112 320 WIRE 160 320 160 304 WIRE 80 352 80 240 WIRE 80 352 32 352 WIRE 96 352 80 352 WIRE -64 368 -64 320 WIRE -32 368 -64 368 WIRE -160 400 -160 256 WIRE -112 400 -160 400 WIRE 160 400 -112 400 WIRE 272 400 272 352 WIRE 272 400 160 400 WIRE 336 400 272 400 WIRE 496 400 496 304 WIRE 496 400 400 400 FLAG 400 192 a FLAG -32 336 a FLAG -160 400 0 SYMBOL npn 208 32 R0 SYMATTR InstName Q1 SYMBOL pnp 208 352 M180 SYMATTR InstName Q2 SYMBOL res 400 112 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R1 SYMATTR Value 1 SYMBOL res 400 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 1 SYMBOL voltage -160 160 R0 SYMATTR InstName V1 SYMATTR Value 20 SYMBOL diode 144 144 R0 SYMATTR InstName D1 SYMBOL diode 80 144 R0 SYMATTR InstName D2 SYMBOL diode 16 144 R0 SYMATTR InstName D3 SYMBOL current 32 -32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 100m SYMBOL Opamps\\opamp 0 288 R0 SYMATTR InstName U1 SYMBOL voltage -112 304 R0 WINDOW 3 24 44 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 10 SYMBOL res 480 208 R0 SYMATTR InstName R3 SYMATTR Value 2 SYMBOL cap 400 384 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 1 SYMBOL current 160 320 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I2 SYMATTR Value 100m TEXT -32 424 Left 0 !.include opamp.sub TEXT -194 444 Left 0 !.dc TEMP 0 150 .01
From: Tim Williams on 9 Feb 2010 18:48 "Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message news:sgr3n5tb6hamiocr5fjb37e5lj5t9asu0l(a)4ax.com... > I'm also familiar with matrix solutions and have developed my > own programs for solving them a little easier than my TI > calculator allows for and with better accuracy in difficult > cases. I can also do Laplace, but frankly I have NOT yet > learned the shortcuts often used. So I wind up with pages of > partial fractions in the end, converting back to time domain > with tables, and seeing how things look there. Find "residues" in your TI, it's exactly what you need. Also, polynomial factorization, if you aren't using it already. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Jim Thompson on 9 Feb 2010 19:01
On Tue, 09 Feb 2010 15:35:14 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Tue, 09 Feb 2010 16:24:43 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Tue, 09 Feb 2010 14:30:54 -0800, Jon Kirwan >><jonk(a)infinitefactors.org> wrote: >> >>>On Tue, 9 Feb 2010 15:48:42 -0600, "Tim Williams" >>><tmoranwms(a)charter.net> wrote: >>> >>>>"Bob Monsen" <rcmonsen(a)gmail.com> wrote in message >>>>news:1265746051.657782(a)sj-nntpcache-3.cisco.com... >>>>> 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. Here it is: >>>> >>>>Could you take a screenshot of the schematic? >>> >>>I'll include an ASCII version here: >>> >>>>: R2 >>>>: +V = 12V ,------/\/\---------------------, >>>>: | 1k | >>>>: | +V | >>>>: | | | >>>>: | \ | >>>>: | / R1 | >>>>: | \ 1k5 | >>>>: | / | >>>>: | | +V | >>>>: C2 | ,----+ | | >>>>: || 10uF R4 | | | 2N3904| | >>>>: ,------||------/\/\---------+ | | | | >>>>: I| || 100 | | Q4 e>| |/c Q3 | >>>>: N| | | |-------| | >>>>: | | | c/| |>e | >>>>: | | C1 --- | | | >>>>: --- | 10uF--- |2N3906 | | >>>>: - V2 | | | | | >>>>: --- SINE(0 .2 1k) | | | | | C3 >>>>: - | C5 | | | | || 470uF >>>>: | | || 10p| | +----+-||----,O >>>>: | +V +---||----+ | | || |U >>>>: | | | || | | | |T >>>>: gnd | | | | | \ >>>>: | | | |2N3904 | / R5 >>>>: \ ,-------, | | | | \ 8 >>>>: / R3 | | | +V | Q1 c\| |<e Q5 / >>>>: \ 1k | +V | | | 2N| |-------| 2N3906 | >>>>: / | | | | |\| | e<| |\c | >>>>: | | |\| | '-|-\ | | | | >>>>: | '--|-\ | | >----+----' | gnd >>>>: | | >-+-----|+/ | >>>>: +-------|+/ |/| LT6234 | >>>>: ,-----+ |/| LT6234 | gnd >>>>: | | | gnd >>>>: --- C4 \ gnd >>>>: --- 1uF/ R9 >>>>: | \ 1k >>>>: | / >>>>: | | >>>>: gnd gnd >>> >>>(This was auto-generated from my LTspice to ASCII program.) >>> >>>Jon >> >>Did something get lost in the ASCII? Otherwise collector-to-collector >>as in Q1-Q4 is a no-no... one of those devices will saturate. >> >> ...Jim Thompson > >I just double-checked. It's just that way in Bob's posted >LTspice schematic. And when I simulate the thing, it >produces a 2V p-p output into R5 from a .2V p-p input. Takes >a few cycles to settle on a DC center level, though. > >Jon A demonstration of why not to trust simulators. What does LTspice show for the voltages at Q1:c and Q1:e? Q4:c and Q4:e? ...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. |