Maker Pro
Maker Pro

Discussing audio amplifier design -- BJT, discrete

J

Jon Kirwan

Jan 1, 1970
0
I'm leaving all the sniping to you because your the one who knows what
you want to investigate further.

"Sniping" in the US has a negative connotation, which I'm not
sure you intended. I hope I'm not coming across in some
negative way. If so, I do apologize and will try for better.
I really do appreciate the time you've offered me.
Also, for today, I am using mostly using "perfect components" and
theoretical efficiencies etc to make my life easier - they will show the
point and I am talking about an amp spec'd for consumer audio.
Okay.


No, that's the power of a 12.7 volt (peak) sine wave into an 8 ohm load.
An instantaneous peak power figure would be (Vmax**2)/R or 20watts.

Clearly understood. I just wasn't thinking well at the
moment. I'm exactly with you on this.
Think of it this way:

You build an amplifier that puts out a 10 watt sine wave into 8 ohms
100% of the time. For a power transformer you will need something like a
30 volt CT rated at 40VA for this design (this one is a realistic not
theoretical estimate).

Use this amp in a consumer environment and the customer is happy about
everything except the cost.

Can you lower the cost without damaging the output quality?

Yes. As mentioned earlier the long term average output power of the
amplifier will be about 2 watts and any transformer will have a very
long thermal time constant compared to any other component in the
amplifier, so there is no danger of overheating during a peak in the
music output. 40VA x 0.2 = 8VA. An 8VA transformer is big enough (in
real life you would use a little bigger because of the increased I**2 x
R losses 10VA would probably be a good choice, (if you had more
information you could make a better choice but the result would be very
close to 10VA)

A 10VA transformer costs a whole lot less than 40VA and all you have
done is removed an unnecessary over-specification of a component and
that will have zero effect to the consumer.

You can do the same thing with the heatsink, but it is not so dramatic a
change, and it needs more care. On a small amp like this where the cost
of a heatsink is low I wouldn't bother - except as an exercise or if you
were making hundreds of them. You won't be able to proceed here until
you have a more finalized design.

I think I follow all of this. I guess my earlier writing was
about my own realizations and nothing else. I wrote more
strongly then because I'm just "seeing" a little better, is
all.
recheck everyone's figures

Tentatively, I'm lumping his tabled results into effects I'm
less aware of, for now. Context will become clearer, later.
20 Watts - you were right. My figure wrong.
Thanks.


You seem to be doing OK. Maybe we should brush up and compare notes on
the meanings of average and RMS but that's about it I think.

A class A amp say at 10 watts into 8 ohms will have an output stage with
a constant current sink (or source) set at 1.59 amps.

I know there are a number of structures, but I like to think
in terms of two BJTs, one NPN and one PNP, in a push-pull
arrangement to the rails -- for class-A -- with a Vbe
multiplier set to cause both BJTs to have at least some
emitter current at all times.

I know that there is also a single-ended arrangement. But I
never give that one more than a very slight nod. It's way,
way too inefficient to care about. I'm wondering if that is
what you are talking about here.

If so, then you'd indeed set Iq to be 1.59 amps, either to
the (+) or (-) rails, as I read you saying here. Because
then the single-ended BJT can either source/sink nothing
causing the -1.59 (or +1.59) amps to flow from speaker to
rail or the single-ended BJT can source/sink 3.18 amps, only
1.59 of which gets wasted via Iq and the rest going to the
speaker.

However, I don't think much about that arrangement and I'm
sure that Self wasn't talking about an amplifier designed
that way. I'm pretty sure he was discussing a push-pull
class-A amplifier.
If the speaker
load changes to 4 ohms the maximum current into and out of the speaker
is still 1.59 amps.

In the single-ended case, _very_ generally, yes. But let me
walk you through my single-ended thoughts. (I hate single
ended designs, so I hope I don't have to think about them
again!)

Let's assume we have (+), (-), and ground rails. Let's
arrange it so that the Iq current is a sink, as you stated.
So it goes from (-) to the speaker pin. There is a BJT that
goes from the speaker pin and up to (+). It's base is driven
by the VAS and the emitter simply follows that. The speaker
load goes to ground. That's what I'm imagining you are
talking about.

At 8 ohms and Iq = -1.59A, lets say that the emitter can rise
up to the point where Iload = +1.59A. (So about +12.7V at
the emitter, as discussed regularly.) This means the BJT's
emitter must be sourcing 3.18 amps, enough to supply both the
speaker and Iq. If the BJT effectively turns off and its
emitter current goes to about 0, then Iq causes Iload =
-1.59A.

A 4 ohm load would still "see" no lower than -1.59A, since
that is all that Iq can do. However, when the emitter rises
again to it's +12.7V (driven by the same exact signal at its
base, by assumption), then we will have 12.7V across 4 ohms
and +3.18A into the speaker. The BJT will not only have to
supply that, but also the 1.59A required by Iq. That's 4.77A
total. So it will operate in class-AB, now.

So I'm pretty sure I'm _not_ following you when you say the
output current is the same. Which I take to mean there is
something very wrong with the way I'm seeing this.
How's the power now?

Best to wait for your knock on my head about this. I would
have first preferred to talk about a push-pull class-A case,
which seems fundamentally different. But I'm still learning
and have to assume I am getting all of this wrong.
Self is talking about his practical results and if you dig around you
will see/find he believes in over-biasing the output stage current
source by 50% - 100% hence the apparent anomaly.
<snip>

I think he was NOT talking about single-ended class-A
designs, but instead push-pull class-A. I get your point, I
think, about over-biasing -- it is something I already feel
I'd want to do, too... though I'm not sure 50% is right and
even then I don't think he spent much of any time at all
talking about single-ended designs (for reasons I think I
agree with.)

I feel like I probably failed to get some point I should have
and so I'll stop here and wait.

Very much appreciated,
Jon
 
J

Jon Kirwan

Jan 1, 1970
0
I'm not sure that is right, pure Class A doesn't degrade

In my mind it can. But I only think in terms of class-A push
pull class-A, not single-ended. Maybe I should be more
detailed when I write?
- does it have a Vbe multiplier?

It can. It doesn't have to have one, though. I was just
thinking about the case where there is one -- a push-pull
that looks a lot like a class-B stage but with higher Vbias.
Class AB can degrade into pure class B if the Vbe
multiplier is not set properly.

Right.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
I am/was thinking of a single supply class A amp. It has a great big
output capacitor.

Then I think class-AB remains the mode of operation when the
8 ohm is replaced with the 4 ohm output, per your question to
me about that. A single-sided class-A with a 1.59A sink and
an appropriately sized rails would barely work class-A with 8
ohms. And would move into AB, driving 4. I think.

I talked a bit about the topology I was considering, earlier,
so hopefully I didn't get that part wrong even if I did fail
to add the output cap to the description. Just to be clear,
here is what I'm imagining right now:
: V+
: |
: |
: |/c Q1
: VAS ----| TIP3055
: |>e
: | C1
: | || BIG
: +----||----,
: | || |
: | \
: | / R1
: / \ \ 8 or 4
: | I1 /
: v 1.59A |
: \ / |
: | gnd
: |
: gnd

Just an emitter follower feeding a sink and the speaker via a
cap. Maybe I'm getting that wrong, though.

Jon
 
P

Paul E. Schoen

Jan 1, 1970
0
Jon Kirwan said:
Then I think class-AB remains the mode of operation when the
8 ohm is replaced with the 4 ohm output, per your question to
me about that. A single-sided class-A with a 1.59A sink and
an appropriately sized rails would barely work class-A with 8
ohms. And would move into AB, driving 4. I think.

I talked a bit about the topology I was considering, earlier,
so hopefully I didn't get that part wrong even if I did fail
to add the output cap to the description. Just to be clear,
here is what I'm imagining right now:


Just an emitter follower feeding a sink and the speaker via a
cap. Maybe I'm getting that wrong, though.

My conception of a class A amplifier is one where instead of an active
current sink, there is just a resistor. It may be in the form of an emitter
follower as in this case with a unity gain, or the resistor may be in the
collector to obtain a voltage gain greater than one. But in these cases,
large signal linearity is not realized. So such a configuration is used for
very small signals that are at least an order of magnitude smaller than the
supply rails, and power levels in the milliwatt range. With a resistor
load, the maximum power output is where the output impedance equals the
resistor, and the maximum voltage that can be achieved is about half the
supply rails.

When one adds an active current source or sink, it involves another
transistor and the circuit becomes essentially a half-bridge. For the
circuit shown above, with I1 = 0.75V and V+ = 12 VDC, a sine wave of 6V
amplitude will be reproduced across R1 = 8 ohms. With R1 = 4 ohms, the
current source must be set to 1.5A. The efficiency under these conditions
is 25%, and 4.5 W output. But this assumes that the current source can pull
the output below ground, which is not possible with any practical
component. And I did not factor in the power provided by the current
source, so actual efficiency will be lower.

If I use a 4 ohm resistor as the emitter load, and bias Q1 so that there is
equal clipping at the output, I can get an output of about 7.6 volts P-P,
or 1.8 W. The efficiency is about 8%. If I bias the resistor for 1/2 the
supply rail (6 V), I can get at most about 2.2 VRMS into 4 ohms, or 1.2 W.
Efficiency is 6.6%.

With a realizable current source made from a 2N3055 and a 0.2 ohm emitter
resistor, set at 1.55A, I can get about 3.9 watts into 4 ohms, and an
efficiency of 20%.

Now, I decided to see if I could get better efficiency by adding a variable
current sink. Essentially I am now making a push-pull circuit where the
lower half is not pulling so much when the upper half is pushing, and then
it pulls harder when it needs to do so during the negative excursions of
the signal. It simply required two additional components. My LTSpice
simulation shows an output of about 3.9 W into 4 ohms, with an efficiency
of about 27%.

This design is similar to class A in that it burns up about 15 watts with
low level signals. As such, maybe it is not so much and amplifier as an
"Apple-fryer" :)

And under those conditions the output stage is running 1.2A. But that is
better than running 1.5 A as was the case with the original design. There
does not appear to be any crossover distortion, and at high signal levels
you just get clipping, and that occurs within 1 volt of the supply rails.
As Scotty might say, "Cap'n, she just caint give ye no more!"

OK, I've played around enough. I've attached the ASC file if anyone wants
to play with it or criticize it. I just used a "shotgun" approach with
simplicity in mind. Maybe it's worth building, but I'm happy enough with
the usual Class B or AB amplifiers that don't function as space heaters
when they're just sitting there. And a class A power amplifier will never
win an Energy Star! Go green! Use PWM!

Paul

============================================================

Version 4
SHEET 1 880 680
WIRE 16 -80 -160 -80
WIRE 240 -80 16 -80
WIRE 240 -64 240 -80
WIRE 160 -16 -80 -16
WIRE 176 -16 160 -16
WIRE 16 0 16 -80
WIRE 16 96 16 80
WIRE -160 112 -160 -80
WIRE 240 128 240 32
WIRE 288 128 240 128
WIRE 416 128 352 128
WIRE 160 144 160 -16
WIRE 240 160 240 128
WIRE -80 208 -80 -16
WIRE 176 208 112 208
WIRE 416 208 416 128
WIRE 16 240 16 192
WIRE 112 240 112 208
WIRE 112 240 16 240
WIRE 112 256 112 240
WIRE 240 272 240 256
WIRE -160 352 -160 192
WIRE -80 352 -80 288
WIRE -80 352 -160 352
WIRE 112 352 112 336
WIRE 112 352 -80 352
WIRE 240 352 112 352
WIRE 416 352 416 288
WIRE 416 352 240 352
WIRE 416 368 416 352
FLAG 416 368 0
SYMBOL npn 176 -64 R0
SYMATTR InstName Q1
SYMATTR Value 2N3055
SYMBOL cap 352 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 5000µ
SYMBOL res 400 192 R0
SYMATTR InstName R1
SYMATTR Value 4
SYMBOL voltage -80 192 R0
WINDOW 3 -71 180 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(6.75 5.6 400 0 0 0 1000)
SYMBOL voltage -160 96 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 12
SYMBOL res 224 256 R0
SYMATTR InstName R2
SYMATTR Value .2
SYMBOL npn 176 160 R0
SYMATTR InstName Q2
SYMATTR Value 2N3055
SYMBOL res 96 240 R0
SYMATTR InstName R3
SYMATTR Value 15
SYMBOL pnp 80 192 R180
WINDOW 0 52 29 Left 0
WINDOW 3 60 68 Left 0
SYMATTR InstName Q3
SYMATTR Value 2N3906
SYMBOL res 0 -16 R0
SYMATTR InstName R4
SYMATTR Value 50
SYMBOL res 176 128 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 100
TEXT -66 392 Left 0 !.tran .1 startup
 
J

Jon Kirwan

Jan 1, 1970
0
My conception of a class A amplifier is one where instead of an active
current sink, there is just a resistor.

I first thought of that, as well. But for the purposes at
hand, it seemed a lot easier to plop a current sink in there.

David had written this to me:

"A class A amp say at 10 watts into 8 ohms will
have an output stage with a constant current
sink (or source) set at 1.59 amps. If the speaker
load changes to 4 ohms the maximum current into
and out of the speaker is still 1.59 amps. How's
the power now?"

I didn't want to wind up "getting corrected" for failing to
read well what he wrote, introducing some pre-conception of
mine.

That's why you see my schematic which _uses_ a current sink.
I'm trying to read David as accurately as I can and construct
from his words what I think he may be talking about. To do
otherwise would be to _change_ the subject on him and talk at
cross-purposes.
It may be in the form of an emitter
follower as in this case with a unity gain,

Yes, that's clear -- now that we are quickly moving to change
the subject. :)
or the resistor may be in the
collector to obtain a voltage gain greater than one.

Um. After moving the speaker/cap connection up there, too?
Right?
But in these cases,
large signal linearity is not realized.

You mean in the case where a resistor is used in the emitter
and where a collector resistor may (or may not) be used.
Right? In the case of the current sink I attempted, when
trying to follow David, it seems 'large signal linear' -ish
to me. (Speaking loosely. Except for Vbe variations on Ic
and maybe also the Early effect, anyway.)
So such a configuration is used for
very small signals that are at least an order of magnitude smaller than the
supply rails, and power levels in the milliwatt range. With a resistor
load, the maximum power output is where the output impedance equals the
resistor, and the maximum voltage that can be achieved is about half the
supply rails.

The configurations you now brought up? Or the one that I was
talking about, earlier, when trying to deal with David's
question to me?
When one adds an active current source or sink, it involves another
transistor

Yes, that's a given of sorts. And that is why I'd almost
certainly prefer to go with a push-pull style class-A of some
kind. It seems crazy to go single-ended under the
circumstances.
and the circuit becomes essentially a half-bridge.

A term I need to follow a little better, I suppose. I would
use it in the case of two diodes instead of four in a
full-wave, center-tapped PS with CT to ground and only one
other rail. You are using it differently than that, here.
Which makes me feel behind the terms-curve, still.
For the
circuit shown above, with I1 = 0.75V and V+ = 12 VDC,

Um... I1=.75A? Not 'V', right? (I assume we are getting
back to my ASCII schematic, now.)
a sine wave of 6V
amplitude will be reproduced across R1 = 8 ohms.

Yes, assuming as I know you must be that the drive is nicely
centered on +6V so that it goes from 0V to +12V -- which is
what I take you to mean here.

Actually, maybe 5.2V or so would be better, so that the
emitter can follow up and down well.
With R1 = 4 ohms, the
current source must be set to 1.5A.

Yes, this much I understand.

The question that David was asking me, if I understood him
accurately, didn't permit me to arbitrarily change the
current sink value. As such, the example case you are
bringing up would be a more accurate analogy to his question
if you kept the current source at .75A and changed R1 to 4
ohms. My reply, at least, was made on that basis.
The efficiency under these conditions
is 25%, and 4.5 W output. But this assumes that the current source can pull
the output below ground,

Two issues here. One with and one without the output cap
that David wisely mentioned in his response to me.

In the case without the output cap, the current sink needs
access to a rail _below_ that used by the speaker load's
other end. Otherwise, if they are common to each other, then
there is a DC bias current flowing through the speaker and
that's not really a good thing.

In the case with it, the cap provides the necessary 'most-
negative' side for the speaker and allows, after a few cycles
to pump up an equilibrium voltage on it, a DC center of 0A
for the speaker.
which is not possible with any practical
component.

Without the cap. With the cap, you are still right in that a
0V on the base of Q1 does not mean that the emitter can sink
to -0.8V or whatever, since there is no rail there for it.
(Unless some extra windings are added to the transformer to
get it, of course.)
And I did not factor in the power provided by the current
source, so actual efficiency will be lower.

Yup. Understood. I think Self says 12.5% is the best to be
hoped. I've not done my own double-check. But with your
estimate and adding in an equal amount for the sink, that
seems to get to about there.
If I use a 4 ohm resistor as the emitter load, and bias Q1 so that there is
equal clipping at the output, I can get an output of about 7.6 volts P-P,
or 1.8 W. The efficiency is about 8%. If I bias the resistor for 1/2 the
supply rail (6 V), I can get at most about 2.2 VRMS into 4 ohms, or 1.2 W.
Efficiency is 6.6%.

With a realizable current source made from a 2N3055 and a 0.2 ohm emitter
resistor, set at 1.55A, I can get about 3.9 watts into 4 ohms, and an
efficiency of 20%.

Now, I decided to see if I could get better efficiency by adding a variable
current sink. Essentially I am now making a push-pull circuit where the
lower half is not pulling so much when the upper half is pushing, and then
it pulls harder when it needs to do so during the negative excursions of
the signal. It simply required two additional components. My LTSpice
simulation shows an output of about 3.9 W into 4 ohms, with an efficiency
of about 27%.

Thanks for the circuit. I also ran it under LTspice.
Selecting from 100ms to 500ms as a range by which things have
settled out well, the resistor shows about 3.9 watts and 14.1
watts from the rail supply. Which gets to your number. As
you hoped, most of the 14 watts is in Q1, at about 6W. Q2
shows about 2.9W.
This design is similar to class A in that it burns up about 15 watts with
low level signals. As such, maybe it is not so much and amplifier as an
"Apple-fryer" :)
:)

And under those conditions the output stage is running 1.2A. But that is
better than running 1.5 A as was the case with the original design. There
does not appear to be any crossover distortion, and at high signal levels
you just get clipping, and that occurs within 1 volt of the supply rails.
As Scotty might say, "Cap'n, she just caint give ye no more!"

OK, I've played around enough. I've attached the ASC file if anyone wants
to play with it or criticize it. I just used a "shotgun" approach with
simplicity in mind. Maybe it's worth building, but I'm happy enough with
the usual Class B or AB amplifiers that don't function as space heaters
when they're just sitting there. And a class A power amplifier will never
win an Energy Star! Go green! Use PWM!

Thanks, Paul. All discussion is most welcome to me. I
appreciate it very much.

Jon
 
P

Paul E. Schoen

Jan 1, 1970
0
Paul E. Schoen said:
When one adds an active current source or sink, it involves another
transistor and the circuit becomes essentially a half-bridge. For the
circuit shown above, with I1 = 0.75V and V+ = 12 VDC, a sine wave of 6V
amplitude will be reproduced across R1 = 8 ohms. With R1 = 4 ohms, the
current source must be set to 1.5A. The efficiency under these conditions
is 25%, and 4.5 W output. But this assumes that the current source can
pull the output below ground, which is not possible with any practical
component. And I did not factor in the power provided by the current
source, so actual efficiency will be lower.

Like 13%. The magic active current source was contributing about half the
power.

Paul
 
J

Jon Kirwan

Jan 1, 1970
0
Your picture is what I was thinking about as a class A amp - a single
supply class A amp. I have checked through my notes and I have
absolutely *no* experience with push-pull class A.

Okay. At first, I wasn't understanding what you meant well.
I guess this says we are on the same page, now.

The push-pull version I was thinking about at first looks
exactly like (and is, if the bias is set that way) what I've
see as a class-B push-pull stage. The only difference is
that the Vbias value is set "high" enough to cause both BJTs
to always conduct.

This is what I'm thinking about, regarding class-A push-pull:
: V+
: |
: |
: |/c Q1
: ,-----| TIP3055
: | |>e
: + | |
: --- |
: - Vbias +---- to speaker
: --- |
: - |
: | |<e Q2
: +-----| TIP2955
: | |\c
: | |
: | |
: VAS V-

In the above case, Vbias can be set about right for near
class-B operation (is anything truly exactly class-B?) from
each BJT, with some cross-over distortion. If you increase
Vbias above that, you start getting class-AB operation from
each BJT and eventually with high enough Vbias both BJTs
always have some non-zero collector currents and are each
operating class-A.

Given my vague undertanding of things, anyway.

There are a few other push-pull class-A output stage
topologies, too. For example:
: V+
: |
: |
: / \
: | I1
: v V+
: \ / |
: | |
: | |/c Q1
: +-----|
: | |>e
: | |
: |/c Q3 |
: VAS ----| +----- to speaker
: |>e |
: | |
: | |/c Q2
: +-----|
: | |>e
: | |
: \ |
: / R1 V-
: \
: /
: |
: |
: V-

And at least one other one, as well.

Thanks,
Jon
 
J

Jon Kirwan

Jan 1, 1970
0
Like 13%. The magic active current source was contributing about half the
power.

Self says 12.5% is the best to hope for.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
I don't get wound up very easily by this sort of thing - I know you
aren't trolling. I do hop I do not sound too harsh when I disagree - it
is not my intention to bully. When I see the other person is sincere I
try to put my cynical factor on hold.
<snip>

Sometimes, the fastest way from A to B is through some
'oscillation' in discussion. I think of this as critically
damped in the best of cases and maybe a little underdamped
more than would be liked, most of the time. I don't mind
that if you don't. It would be a slower discussion if we
spoke way overdamped and never oscillated.

Yes, I am seriously studying and seriously trying to engage
myself to every sentence I'm gifted with by you or Paul or
pimpom (and others, too.) I'm pouring through Self's latest
book every night, working some equations on my own. So this
is a serious attempt on my part. Whether or not it is
serious to anyone else, of course, is a different matter. I'm
pretty hard to teach, at times, and won't pretend otherwise.

I do appreciate everything you add. I just hope that my
responses aren't off-putting.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
No. Just keep going

One thing that's become pretty obvious is that audio
amplifiers are roughly the same thing as big, monster opamps.
The main difference is that they hard-wire the power supply
design instead of offering connections, tend to require a
ground reference (though that isn't necessary, I'm seeing
that it can help a little bit), limit one to single-ended
drive and hard-wire their feedback.

Some of the thoughts about how to design in better CMRR in an
audio amplifier may very well be almost the same thoughts
used with designing some opamps, for example. So in ways
what I'm trying to learn is also applicable to opamps in near
equal measure.

Does anyone build and sell a giant 500W opamp? ;)

This is actually very useful for me and not the least bit of
waste, I think. Glad to have started about a month back.
Glad for occasional company, too.

Jon
 
P

Paul E. Schoen

Jan 1, 1970
0
Jon Kirwan said:
One thing that's become pretty obvious is that audio
amplifiers are roughly the same thing as big, monster opamps.
The main difference is that they hard-wire the power supply
design instead of offering connections, tend to require a
ground reference (though that isn't necessary, I'm seeing
that it can help a little bit), limit one to single-ended
drive and hard-wire their feedback.

Some of the thoughts about how to design in better CMRR in an
audio amplifier may very well be almost the same thoughts
used with designing some opamps, for example. So in ways
what I'm trying to learn is also applicable to opamps in near
equal measure.

Does anyone build and sell a giant 500W opamp? ;)

This is actually very useful for me and not the least bit of
waste, I think. Glad to have started about a month back.
Glad for occasional company, too.

This company sells op amps to 450W:
http://www.powerampdesign.net/downloadfullcatalog.html

Apex has been around a long time. Here is a 400W amp (40A, 200V):
http://www.cirrus.com/en/products/pro/detail/P1150.html

A list of all their amps, including a 250W PWM device:
http://www.cirrus.com/en/products/pro/areas/PA139.html#PA142_open

National has had the LM12 for quite a while. It'll do 90W in a TO-3
package:
http://cache.national.com/an/AN/AN-446B.pdf

Around 1982 I used an RCA monolithic 100W power Op-Amp for the amplifier in
a frequency test set that had to produce 0-140 VAC at 45-450 Hz. It was in
a large square package with flying leads of perhaps #16 AWG. I still have
one that I cut open after it fried.

Paul
 
J

Jon Kirwan

Jan 1, 1970
0
This company sells op amps to 450W:
http://www.powerampdesign.net/downloadfullcatalog.html

Apex has been around a long time. Here is a 400W amp (40A, 200V):
http://www.cirrus.com/en/products/pro/detail/P1150.html

A list of all their amps, including a 250W PWM device:
http://www.cirrus.com/en/products/pro/areas/PA139.html#PA142_open

National has had the LM12 for quite a while. It'll do 90W in a TO-3
package:
http://cache.national.com/an/AN/AN-446B.pdf

Around 1982 I used an RCA monolithic 100W power Op-Amp for the amplifier in
a frequency test set that had to produce 0-140 VAC at 45-450 Hz. It was in
a large square package with flying leads of perhaps #16 AWG. I still have
one that I cut open after it fried.

Wow. Thanks, Paul. It's really fun going through this.

I'm feeling just a little bit like I'm gradually gaining a
better instinct for opamps -- not just the idealized
examination -- but a realistic feel for practical design
issues and tradeoffs. In the end, that will hold me in even
better stead for _choosing_ an opamp for an application, I
think.

I'm kind of tempted to leave the global NFB as an external
thing I can easily modify and use a completely separate power
supply for the rails and ground wired to the amplifier, but
replaceable with something else. So long as I clearly
understand the voltages the amplifier can stand off and don't
exceed those, I should be able to play a little. Expose the
two inputs just like an opamp and, of course, use a diff type
structure for the input (I'm tempted, because it isn't
expensive, just time consuming) to use a dual cascode
arrangement for the diff-pair, too, so that I can drive
differentially into the VAS, as well.) The unit could be
kind of modularized, really, allowing me to experiment in
sections.

Crap, I'm beginning to see this as carefully crafted
sections, now. Maybe that's good.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
For anyone interested

I have placed some interesting app notes here

http://www.filedropper.com/amps
Thanks.

It is a free file host, so the files won't be around that long. I am
sorry the total package comes to just shy of 18 MBytes.

I got it in time, I guess.
One is an Hitachi Mosfet app note that pretty much redefined most amp's
topology.

Thanks. I scanned over it and will read it in more detail,
later on. Just by way of demonstrating that I've at least
seen it, I immediately saw Figure 3-5 (b) as typical for
push-pull class-B output stages.
The other is an RCA app note, sorry each page is a separate file, which
has a practical approach to the output stages and other useful.

I've converted it to a single, 16-page PDF file. If you
want, I can post it up on the web and make it available at my
site.
The last "thing" is a JPG of an amp design posted on the internet.
According to the blogger, it has been extensively simulated - how many
mistakes can you find?

Thanks for that. I'll look, a little later on. That will be
good for me to do. It uses "current mirror" structures as a
method where constant current is desired and with varying Vee
these will _not_ be constant current, at all, as layed out
there. R4 and R9 will experience varying voltages and since
they set the currents for the mirror, that might be a problem
for the circuit unless a well-regulated power rail is applied
and is maintained at the exact value those mirrors were
designed for. So fixed, regulated rails. Which then also
suggests that its PSRR, at least for the negative rail, will
be a mess -- even low impedance at the negative rail will
still allow output signal (which hauls large currents around)
to feed back into the current sinks and that will feed back
into the amplifier, I think.

But that's two minute's thought. I need to go look more
seriously.

Jon
 
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