Maintaining a Vbe Multiplier's bias value

[Jon Kirwan]

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

 

[Jon Kirwan]

Have you read Randy Slone's power amplifier book? This stuff really
isn't rocket science. Nor is AB. ;-)

I haven't.  It's not rocket science.  But it is interesting
at my level.

<http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/
dp/0071599258/ref=dp_ob_title_bk>

I'll look, but the title appears more on the contruction
side.  I am using this to educate myself a little better.

The black art is all in assembly, protection circuitry, and making
sure it starts up cleanly. [Most engineers never look at start up, so
you get designs that thump when you power them. I have lots of gear
with power-on thumps.]

I recall reading of a recommendation suggesting that all
electronic devices use less than 1W when on, but not
performing their intended application.  This would also seem
to require a little added effort to achieve, as well.  But I
take your point.

I'd pick one of his MOS designs. Bipolar designs often have good
intentions, but ring like a bell. MOS is mushy, but predictably mushy.

I'm still learning about BJTs.  In fact, that's what this is
about for me.  MOS later.  ICs later.  BJTs now.

Jon

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.

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

 

[Tim Williams]

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

They don't make diodes with pots varying Eg. ;-)

Tim

 

[Jon Kirwan]

<snip>
Less words and real schematics would get you more readers. [The only
thing worse than ascii equations are ascii schematics.]

ASCII is what I'll post.  It's the only way to get them
archived or properly posted to a text newsgroup.  I no longer
have access to the binary for schematics, sadly.  If I lose
some people because they cannot manage fixed-spaced fonts, I
guess I lose them.  I could place links up on my domain, I
suppose.  But in this case, the schematics are really very
basic and not overly burdensome in ASCII.  Besides, Win Hill
posted some really nice examples here, before.  Folks seemed
to live with that.  Not sure why you are picking on me, here.

In any event, just google improved vbe multiplier. I've seen all sorts
of circuits published to get lower impedance at the nodes.

Okay.  I'll do that if folks here aren't interested at all in
talking about it.

Jon

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.

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.

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

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 Kirwan]

They don't make diodes with pots varying Eg. ;-)

Tim

Jon

 

[Jon Kirwan]

No hurry. Thanks, Bob Monsen

It's at:

http://www.infinitefactors.org/misc/asc.zip

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:

: asc version 1.2.1, (library C:\TOOLS\BIN\ASC.SYM found)
:
: This program converts LTSpice schematics into ASCII schematic output (or
: files.) If you specify no files at all, it accepts the LTSpice schematic
: from its standard input device. If you specify exactly one file, it dis-
: plays the ASCII schematic output to the standard output device. If you
: specify more than one file, it then generates .TXT files otherwise having
: the same name as the specified schematics.
:
: These options are supported:
: +h requests this help message, -h disables it.
: +r enables rectangle drawing, -r disables it (default is -r)
: +c enables clipboard copying, -c disables it (default is -c)
: +c<char> enables clipboard copying and prepends <char> to each line
:
: Usage: asc <options> <filename> [<filename>]...

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

 

[Tim Williams]

Say can you make a push-pull stage run class A? (Or is that just a
silly idea?)

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

 

[Jon Kirwan]

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

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

 

[Jon Kirwan]

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

 

[Jon Kirwan]

On Wed, 10 Feb 2010 15:37:00 -0800, Jon Kirwan

[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

 

[Phil Allison]

"Tim Williams"

"George Herold"

** Push-pull class A is the MOST COMMON method use in tube and transistor
audio power amplifiers.

With transistor amps, it is only necessary to set to standing bias current
to a high value like an amp or two.

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

** No need for either thing to happen.

One picks the load to suit the amplifier and the peak ( class A) current is
double the bias setting current.

Eg:

With a bias of 2 amps, peak load current is 4amps which allows +/-32 volts
into 8 ohms.

Class A power is then 64 watts rms.

The DC rails need to be about +/- 35 volts and the supply current a steady 2
amps.


...... Phil

 

[Jon Kirwan]

Jon Kirwan wrote in message ...

On Wed, 10 Feb 2010 15:37:00 -0800, Jon Kirwan

[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

Jim hit the nail right on the head, make it linear as possible with local
feedback first. Add global later if it applies -

Thanks. Now why do I instead find myself sometimes having to
actually _argue_ about understanding building blocks well?
Why is the answer so often, "throw gobs of global NFB at it?"

I wonder if the availability of all-too-perfect A_ol = 1E15
opamps (not really, but what's the difference?) is part of
the problem, here. Maybe it's making things too easy.

I put that page together

Ah. That was you? Thanks for the effort, then!!

as a quick reponse so that you might see for
yourself what I mean by feedback being used to stabilize things. It is far
from an in depth analysis and there very well could be something desperately
wrong with the circuit, but it does reduce the drift to 1/4 of the circuit
without feedback and that is the point of my post.

I think I already understood the majestic power of global
NFB. It's so important, I am sure, that if NFB didn't _also_
wrap the output stage itself, the results would be indeed
very lousy no matter how good the earlier part of it turned
out to be. So it is not an option.

I did learne the basic gain/feedback equation years ago:
Vout/Vin=A/(1+A*B), with B being the feedback and A the open
loop gain. With gobs of A available in these all-too-perfect
opamps these days, the whole thing drops back to 1/B 'real
fast.' Which is nice because then just set B and get handed
a fixed gain on the so-called silver platter.

I have to still believe, as broadly ignorant as I indeed am
about these things, that crafted design with localized NFB
remains useful even in the case of audio amplifiers. That
doesn't mean the power of global NFB isn't of the overarching
importance that it is. There is no option there. But there
remains more to life than merely that, too. Local NFB seems
to remain important to me. And it was nice that Jim took a
moment to confirm that impression, when so few had done so
beforehand.

.... You also noticed that I took the web page in a totally
different direction?

Speaking of which, what spice model did you use for that
2N3904? Can you post it? I'd like to stick it into LTspice
and see why I got different results. Since my calculations
didn't depend too highly on wrong estimates of beta and since
kT/q doesn't care about the BJT, I'm curious about exploring
it a little more.

Again , try to find the book, it is the best advice I can give.

Thanks,
Jon

 

[Jon Kirwan]

Jon Kirwan wrote in message ...
@My-Web-Site.com> wrote:

On Wed, 10 Feb 2010 15:37:00 -0800, Jon Kirwan

[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
Jim hit the nail right on the head, make it linear as possible with local
feedback first. Add global later if it applies -

Thanks.  Now why do I instead find myself sometimes having to
actually _argue_ about understanding building blocks well?
Why is the answer so often, "throw gobs of global NFB at it?"

I wonder if the availability of all-too-perfect A_ol = 1E15
opamps (not really, but what's the difference?) is part of
the problem, here.  Maybe it's making things too easy.

I put that page together

Ah.  That was you?  Thanks for the effort, then!!

as a quick reponse so that you might see for
yourself what I mean by feedback being used to stabilize things. It is far
from an in depth analysis and there very well could be something desperately
wrong with the circuit, but it does reduce the drift to 1/4 of the circuit
without feedback and that is the point of my post.

I think I already understood the majestic power of global
NFB.  It's so important, I am sure, that if NFB didn't _also_
wrap the output stage itself, the results would be indeed
very lousy no matter how good the earlier part of it turned
out to be.  So it is not an option.

I did learne the basic gain/feedback equation years ago:
Vout/Vin=A/(1+A*B), with B being the feedback and A the open
loop gain.  With gobs of A available in these all-too-perfect
opamps these days, the whole thing drops back to 1/B 'real
fast.'  Which is nice because then just set B and get handed
a fixed gain on the so-called silver platter.

I have to still believe, as broadly ignorant as I indeed am
about these things, that crafted design with localized NFB
remains useful even in the case of audio amplifiers.  That
doesn't mean the power of global NFB isn't of the overarching
importance that it is.  There is no option there.  But there
remains more to life than merely that, too.  Local NFB seems
to remain important to me.  And it was nice that Jim took a
moment to confirm that impression, when so few had done so
beforehand.

... You also noticed that I took the web page in a totally
different direction?  

Speaking of which, what spice model did you use for that
2N3904?  Can you post it?  I'd like to stick it into LTspice
and see why I got different results.  Since my calculations
didn't depend too highly on wrong estimates of beta and since
kT/q doesn't care about the BJT, I'm curious about exploring
it a little more.

Again , try to find the book, it is the best advice I can give.

Thanks,
Jon- Hide quoted text -

- Show quoted text -

Hi Jon, I'm loving your threads...

Thanks.

(did any one ever tell you your kinda long winded?)

Not nearly as kindly.

I wanted to take exception to your opamp statement,

"I wonder if the availability of all-too-perfect A_ol = 1E15
opamps (not really, but what's the difference?) is part of
the problem, here. Maybe it's making things too easy. "

As the frequency of interest approachs the GBP the gain goes to 1 and
things get intresting...

George H.

Well, I was thinking exactly this when I was earlier
writing... imagining instead of writing this way, "... of all
too perfect A_ol*BW = 1E15" rather than as I did write. Just
to get that in. But it wasn't important to the point at hand
and I edited it back out as I figured "it goes without
saying."

But yes.

Jon

 

[Phil Allison]

"George Herold"
"Phil Allison"

** Push-pull class A is the MOST COMMON method use in tube and transistor
audio power amplifiers.

With transistor amps, it is only necessary to set to standing bias current
to a high value like an amp or two.


** No need for either thing to happen.

One picks the load to suit the amplifier and the peak ( class A) current
is
double the bias setting current.

Eg:

With a bias of 2 amps, peak load current is 4amps which allows +/-32 volts
into 8 ohms.

Class A power is then 64 watts rms.

The DC rails need to be about +/- 35 volts and the supply current a steady
2
amps.

Do you just use a current source as bias as in the Amps from Pass labs?

** No - doing that is utterly STUPID.

The only difference between a class A and class B push-pull amp IS the
bias setting.

One just turns the bias trim pot to get the desired standing current or with
tubes adjust the grid bias to get the same result.

Tubes can't do push-pull because "it's hard to get positrons from the
filament" to parapharse what I read.

** How pathetic.



..... Phil

 

[Jon Kirwan]

I'm perhaps more of a novice than you...

Somehow, I doubt that. I barely rate "hobbyist."

but I find opamp circuits complicated enough....

Don't sweat it. While some opamps leave _some_ issues nearly
ignorable, there is always some tough problem at that scale
that makes it non-trivial and interesting to work on, I
imagine. Each macroscale view has it's own complexity.
Telescoping levels, where the complexity at one stage doesn't
take away from interesting complexity at another level.

(I would say more but I'm keeping in mind your warning about
long-windedness and will now muzzle myself.)

And tend to stick transistors
only on the edges of things. (Mostly on the output side... on the
input you have to 'know more' than the guys who designed the opamp..
hard to do for a novice.)

I do the same things except that I enjoy math and BJTs give
me an excuse, perhaps. Maybe that's the only difference.

I guess if I was designing an audio amp I'd figure on an opamp driving
some sort of FET output stage. The question of how to bias the output
stage is interesting. And also of how all the NFB works.

Might as well just get a power opamp like the OPA502 and be
done with it. Give it two rails, feed the input, and just
drive the hell out of a speaker. Or get two of them and do a
bridge amplifier. But where is the enjoyment in that? Or
the learning? Someone else already did most of the fun stuff
and there's nothing really left to do except some hook up and
heat sinking. It's not at all satisfying to me, anyway.

An audio amplifier is basically a power opamp. Using an
opamp to make one feels to me like building a car by first
buying a car without the tires, selecting and installing some
tires, and then saying you designed and built yourself a car.

Jon

 

[Jon Kirwan]

This is the model that came with circuitmaker - -
*2N3904
*Si 310mW 40V 200mA 300MHz pkg:TO-92B 1,2,3
.MODEL 2N3904 NPN(IS=1.4E-14 BF=300 VAF=100 IKF=0.025 ISE=3E-13
+ BR=7.5 RC=2.4 CJE=4.5E-12 TF=4E-10 CJC=3.5E-12 TR=2.1E-8 XTB=1.5
KF=9E-16 )

Thanks!! LTspice's model is:

..model 2N3904 NPN(IS=1E-14 VAF=100 Bf=300 IKF=0.4 XTB=1.5
+ BR=4 CJC=4E-12 CJE=8E-12 RB=20 RC=0.1 RE=0.1 TR=250E-9
+ TF=350E-12 ITF=1 VTF=2 XTF=3 Vceo=40 Icrating=200m
+ mfg=Philips)

Note that it does NOT include a figure for ISE, which your
model does. LTspice is setting it to zero, instead. Your
model may very well be right. If so, it affects the beta
value (degrades it due to some recombination cause) and that
affects my calculations. In fact, that value of ISE cuts it
down to around 100, or so. Which about doubles the
contribution to the final term in the equation I gave where a
term is divided by beta, so that adds back a few tenths of a
volt to the prediction! Great!

Now, with your model, LTspice shows the same results you got.

I don't use LT so I'm not sure if the commented (*) text needs to be
removed.

No, that is pretty much the universal spice comment
character. It's fine.

These spice model files might be of interest but possibly obsolete - google
for them
OnSemiconductorAllModels.zip
OnSemiconductorDiscreteModels.zip
OnSemiconductorIntegratedFunctionsModels.zip

I already had the first one laying about. I downloaded the
second one just to have it and extracted 2N3904.LIB from it,
using that model, as well. It works much more like the
LTspice one and perhaps even more different than the one you
were using. However, it does produce about the same _beta_
as your model does.

Very interesting. I need to consider the reasons carefully.

Thanks,
Jon

P.S. Just for completeness, the OnSemi model is this:

..MODEL 2N3904 NPN(
+IS=1.26532e-10 BF=206.302 NF=1.5 VAF=1000
+IKF=0.0272221 ISE=2.30771e-09 NE=3.31052 BR=20.6302
+NR=2.89609 VAR=9.39809 IKR=0.272221 ISC=2.30771e-09
+NC=1.9876 RB=5.8376 IRB=50.3624 RBM=0.634251
+RE=0.0001 RC=2.65711 XTB=0.1 XTI=1
+EG=1.05 CJE=4.64214e-12 VJE=0.4 MJE=0.256227
+TF=4.19578e-10 XTF=0.906167 VTF=8.75418 ITF=0.0105823
+CJC=3.76961e-12 VJC=0.4 MJC=0.238109 XCJC=0.8
+FC=0.512134 CJS=0 VJS=0.75 MJS=0.5
+TR=6.82023e-08 PTF=0 KF=0 AF=1)

 

[Spehro Pefhany]

"Tim Williams"

** Push-pull class A is the MOST COMMON method use in tube and transistor
audio power amplifiers.

With transistor amps, it is only necessary to set to standing bias current
to a high value like an amp or two.



** No need for either thing to happen.

One picks the load to suit the amplifier and the peak ( class A) current is
double the bias setting current.

Eg:

With a bias of 2 amps, peak load current is 4amps which allows +/-32 volts
into 8 ohms.

Class A power is then 64 watts rms.

"watts rms"?

The DC rails need to be about +/- 35 volts and the supply current a steady 2
amps.


..... Phil

Best regards,
Spehro Pefhany

 

[Tim Williams]

And a 2.5V "dead-band", but it _is_ precisely known, and temperature
stable. Interesting thought if you have high enough power supplies.

Bonus: the dead band allows you to use that TL431 "Vbe" mentinoed earlier.

Too bad they're so slow (hardly capable for audio). Does anyone make "fast"
regulators (without being stupid LDOs)?

Tim

 

[Jon Kirwan]

Go with the OnSemi model.

...Jim Thompson

Instead of a blanket Is and BF, or a VA set at a default 100,
and Ise seems more realistically close to Is... Well, it
appears that it was actually a developed model done by
measuring (and with Is, likely extrapolated from
measurements) and deals with the high level injection knee.
It looks solid and actually uses something other than the
default value for Eg, which is taken as 1.11 in LTspice. I
cannot recall ever getting a beta near 300 with one, either.

What I completely lack is an understanding of the processes
used to make these and how those processes vary between
alternate manufacturers and which parameters are likely to be
very similar between them and which may be quite different
for the same part designation.

In other words, although OnSemi has a realistic model for
their own parts, which is fine for simulating their parts
more accurately -- is there a reason to shop around and
actually _select_ someone else's parts for some application
reason. And in what cases would you not bother wasting time
shopping around and for what other cases would you decide to
spend the time, because you know enough about how they are
made and what differences that can make to be worth that
effort to test and verify when making a selection?

Jon

 

[Jon Kirwan]

<snip>
In other words, although OnSemi has a realistic model for
their own parts, which is fine for simulating their parts
more accurately -- is there a reason to shop around and
actually _select_ someone else's parts for some application
reason. And in what cases would you not bother wasting time
shopping around and for what other cases would you decide to
spend the time, because you know enough about how they are
made and what differences that can make to be worth that
effort to test and verify when making a selection?

To further clarify this question, I already know that some
manufacturers provide 2N2222A's with 40V and with 60V max Vce
specifications. I assume this is a function of differences
in the FAB processes they choose to apply in making their
parts. That's what I'm talking about... not only for
differences in model parameters but _useful_ differences,
too.

And how do I learn the salient details of various FAB
processes?

Thanks,
Jon