Maintaining a Vbe Multiplier's bias value

I have a domain I can use. It is just that it hardly seemed necessary, given the simplicity involved and the fact that this group is quite used to ASCII schematics, given the years I've been watching here. It has advantages in that it is archived for very long times, this way, as well.

However, I will try and take your concerns into account and see about organizing a directory and smoothing through issues of dropping files there which have been massaged and arranged for easier viewing.

Yes, I think that's been well-posted in the thread and I was aware of it, before. It's one of the first things I saw when starting on this trek 2 weeks back. Hard to miss. The issue is more about learning, though. Not picking a specific solution and ignoring the others. I'd like to have some spectrum of options I've looked at well and discarded (as well as retained.)

Jon

:)

Jon

Could you give a quick explanation of how the circuit in the link works? I'm having a bit of trouble following what's going on...

It's at:

It contains just two files, the EXE and a library symbol file. Place both in some directory that is in the path. You need to use DOS, sadly. I didn't set these up for Windows -- wanted to focus on the task, not get mired in Windows sideshows.

If you run the program without a filename, it will say:

There are some options, like the clipboard. But post Win2000, that mechanism was broken and I haven't set up the additional DLL I'd need to remedy it. (Something I may yet take care of.) So under WinXP, for example, I just run it into a file and use notepad to call it up. Something like:

ASC amplify.asc >amplify.txt NOTEPAD amplify.txt

It gets the job done. Under Win98SE, I just use the +c option and then paste the text, as desired, in Windows.

The library is semi-okay. There's some symbols I've probably not yet added to it because I don't use the parts that much. I won't mind extending it (it's not hard to do) if there is anything you use and would like put in. I just use a text editor and hack in the new ASCII and then save it. The program automatically parses it every time it runs.

Jon

You really can't convey much more than a "basics" circuit with ASCII. Post links or use LTspice listings... everyone seems to have that ;-)

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
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| 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.

Start with IB coming out (*) of IN+ and IN- of EACH section of the LM324.

(*) LM324 has as inputs PNP diff pairs.

Then simply compose loop and node equations.

If any of you are struggling with this kind of analysis, I'll mark it up and show how to solve it. ...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.

Examine this mark-up...

...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.

I

ep

you

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.

You can, but it stops being class A for large signal swings or low load impedances.

There's a monolithic structure which bends the output transistors' characteristics so they never cut off. I forget who owns it. I suppose that would count as class A at any signal/load condition.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

If quiescent current exceeds peak load current, it's pretty much Class A. It could be symmetric, like a classic PNP-NPN class AB with tons of bias current, or asymmetric, like an emitter follower with a constant-current sink to ground.

It's silly as much as any class A amp is silly: it wastes a lot of power.

John

The current mirror config, the one that uses the opamp rail currents to drive mirrors, can do that. If the top side mirror is pulling up the load, you can arrange for the bottom side to keep running at its quiescent current, as opposed to turning off and even off-er as the top side drives the load harder. Keeping both side on all the time has some nice dynamics when driving reactive loads.

I don't know if that's still "class A". "A" is just a letter, after all.

John

And then there's SLIDING-class-A, which I've used successfully in hearing aids... better efficiency than class-B at really low voltages. ...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.

Agreed. Good advice, actually. In this case, the schematic _was_ pretty basic. But then the LTspice file would also have been similarly short, too. So it probably just makes sense, period.

...

I had earlier been imagining what would be multiple views offered, so that others can choose their own poison more freely:

(1) ASCII, where not crazy-minded, (2) GIF, JPG, or PNG schematic for browsing via http (3) as (2), but for associated signal/freq plots (4) .ASC file, for LTspice schematic/netlist (5) .NET file, where schematic layout isn't important (6) associated other inputs not in standard LTspice (7) PDF file documenting why and considerations (8) ZIP file boxing all this up in a package (9) .html page with clickable links and PDF info

Looked like _work_ to me, though. ;)

Jon

PNG and PDF are most readable and one or the other is marginally smaller depending on creating software tool.

It always is ;-) ...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.

Have a look -

I noted the author's comment, "If I knew what I was doing, I could probably optimize this circuit to produce even better results with feedback..." The author the also talks about "bias stability", which I might have taken to mean keeping the bias point at the same voltage as Vs1 varies, but could instead mean making it linear, without any "bow" to it. (As I understand it, for simple amplifier stages the collector is usually set somewhere near the midpoint and thus one might actually _want_ the change but want it to be linear in some known fashion and certainly not bending over in a bow as in the second circuit's case!) So the 'goal' isn't clearly stated to me.

By the way, that first circuit _is_ a Vbe multiplier with a resistor limiting current from Vs1. So computing point A on it isn't much different than what I did, earlier, when I first posted.

The first circuit shown there on that web site (and I don't know if you intended this, or not) is basically a Vbe multiplier sourced by a 10k resistor with point A being the Vbe multiplier output. The second circuit shown there is, once again, basically a Vbe multiplier with the collector resistor in place, if you think of point A in that case as being the output, but now with the topside tied directly to a voltage source instead of a resistor or current source -- which obviously isn't the way a Vbe multiplier would operate.

However, the curve he shows for it remains interesting.

Although the author is talking about something else, the importance of NFB instead, the first case he makes actually presents the _problem_ I was talking. In his first circuit case, the variation in the Vbe multiplier's output vs sourcing current through R1 is shown clearly.

I pointed this out in the first post in this thread (with terms now changed to match his first circuit's usage):

V_bias = Vbe*(1+(R2/R3)) + R2*Ic/beta

(Which ignores the tiny kT/q 26mV always present in the emitter.)

He shows a Vbe of 0.633V, R2=98k, R3=5.6k and I think the beta of his Q1 is over 200. Ic is about 770uA from his values. From these, I compute V_bias = 12.0878V. His circuit shows 12.3V there.

Perhaps close enough, but I was interested in seeing what LTspice would show. After duplicating his schematic and running it, I see 12.081V at point A. Much closer to my computed value.

I'd also gone to the trouble, that the author does not, of computing R_ac for the system. In his case, the value works out to around 1200 Ohms. Roughly speaking then, we have a

10k/1.2k divider for small __changes__ in voltage. This suggests about .11V/V while his graph shows something more like .75/5 or .15V/V. However, once again my schematic in LTspice shows instead .61/5 or .122V/V, which is closer to the value I calculated using R_ac as an approximation.

I'd already done some useful analysis for his circuits and I'd not even read his web site, yet.

Another interesting point. In the second circuit's case, although it uses a voltage source at the top -- which is decidedly NOT what I'm considering, obviously -- the _shape_ of his curve is exactly what I _want_ to have.

Obviously driven differently than shown, I would set the collector resistor value to be approximately the R_ac computed without it and that nice curve should appear -- just not the very large magnitude excursions since the drive is different and the collector resistor is smaller in magnitude (it would be set to around 1.1k, not 10k, other things being similar.) In fact, I think I mentioned this either in this thread or the one over in .basics, last week. That curve helps to allow me to tweak for an optimal spot and then minimize voltage output variation over current that is sourcing through it.

So I again modified the schematic to vary a current source instead of a voltage source, from 500uA to 1000uA (roughly centered over the estimated 770uA drive from before), and plotted the voltage curve. Using that 1.1k collector resistor in place, it is a very nice bow centered very sweetly around the target of about 750uA, drooping by only

38mV out at the skirts. Exactly as I predicted the shape should be with that value.

Interesting page, sadly lacking in equation development. What I took away from it may have been different from what the author (or you) perhaps intended. But there it is.

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

[snip]

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

--
| 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.

Like this _very_linear_ differential pair....

:-P ...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.

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