From: bg on

Jon Kirwan wrote in message <9kk3n5tjcntp1b1itrlbldjgta7st0oai8(a)4ax.com>...
>On Tue, 9 Feb 2010 14:05:32 -0700, "bg" <bg(a)nospam.com>
>wrote:
>
>><snip>
Have a look -
http://www.xmission.com/~bmge/Feedback.htm


From: Jon Kirwan on
On Wed, 10 Feb 2010 14:20:43 -0700, "bg" <bg(a)nospam.com>
wrote:

>
>Jon Kirwan wrote in message <9kk3n5tjcntp1b1itrlbldjgta7st0oai8(a)4ax.com>...
>>On Tue, 9 Feb 2010 14:05:32 -0700, "bg" <bg(a)nospam.com>
>>wrote:
>>
>>><snip>
>Have a look -
>http://www.xmission.com/~bmge/Feedback.htm

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
From: Jim Thompson on
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
--
| 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: Jim Thompson on
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.
>

Like this _very_linear_ differential pair....

http://analog-innovations.com/SED/TL431DiffPair.pdf

:-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.
From: Jon Kirwan on
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