Maintaining a Vbe Multiplier's bias value

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

"Jon Kirwan" wrote in message news: snipped-for-privacy@4ax.com...

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...

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

Find "residues" in your TI, it's exactly what you need. Also, polynomial factorization, if you aren't using it already.

Tim

R5

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

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

It's a current mirror, based on hFE instead of Vbe (yuck!). When Q1 or Q4 saturates, bias current (or op-amp current) is diverted to the output transistors, driving the load.

Tim

There are valid reasons for doing so. High bandwidth is one: when you need high GBW, you don't have time to wait for the signal to propagate through the 5 or 10 or 100 transistors you have[1]. Those kinds of systems are usually built with a lot of low-gain, high bandwidth stages -- the feedback is local, so the phase shift per stage is small, meanwhile the overall phase shift (from input to output) can be arbitrarily large. The disadvantage is there's nothing global to account for distortion or DC offset. However, you can add low frequency servos to stabilize it, getting the best of both worlds.

Tim

That's something I know I have yet to study and so I have NOT looked them up in the TI manual. I'll need to study and understand them much better than now, before trying that.

I use it sometimes when I have the TI nearby. I don't carry it, everywhere, though. And I do think about problems lots more places than where I have a TI nearby.

Besides, it's nice to have it in my head and to keep on working on that part with practice. I remember some of the conversion table and can actually _do_ some derivations the hard way when I am lacking both tables _and_ a calculator. I do like keeping up a personal skill, so that I'm not overly frustrated when the mood strikes and the rare tools are not handy at the moment.

I always have a finger, my brain, and some dirt no matter where I am, unless I wind up chained to some prison wall. Then I'll have to work on my memory, too. ;)

With your recommendation, it's now off to find a good book on complex analysis.

Jon

R5

Among many other such demonstrations -- and a good part about why _I_ need to understand these things, for myself.

I know your own preference to include diagrams, rather than just words. So here is a link:

Forgive the coloring. I didn't mess with it to make it prettier.

Jon

Sorry. I missed a %20 in there, so it may or may not work in your browser. Try:

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

I'll take a crack at it. AofE also talks about bandgaps and I think I understand them, given the nice discussion there.

Hehe. Sure.

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.

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

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

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

The original is from EDN, Oct 22, 2009, page 45. "Class B amplifier has automatic bias".

Regards, Bob Monsen

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

Right. Maybe I'll get another job of yours to fix.

There are all sorts of free places to post images. Imageshack comes to mind. I just lose interest if I have to look at ascii circuits.

You realize you can just string diodes. Nobody says you have to VBE multiply. It's just one of many biasing techinques.

There is plenty of circuit design in the book. It is the assembly stuff (ground loops, thermal tracking, etc.) that people screw up. Anyway, I'd wait for the new edition.

ons,

Self has published many papers too if you have access to an engineering library. I think his stuff is worth reading. Nelson Pass runs a DIY forum

Okay, will do. Turns out the 5th edition of Self's book on amplifiers contains many typos. Lots of references to parts that aren't in the schematic or clearly are not the one under discussion. Same with some of the terms in equations. I'm correcting as I go.

Jon