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Jim Thompson
Guest
Guest
Tue Feb 09, 2010 4:03 pm
On Mon, 08 Feb 2010 22:06:54 -0800, Jon Kirwan
<jonk_at_infinitefactors.org> wrote:
Quote:
On Mon, 08 Feb 2010 19:09:28 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:
What's a "VAS"?
Sorry. I read it somewhere regarding audio amplifiers and
the term stuck in my mind, I suppose. It's short-hand for
Voltage Amplifier Stage. It's almost so simple that no one
would bother creating a term for it, except that it seems as
though someone did and folks have used it in places where
I've been reading.
By the way, if you look at the semi-conceptual schematic at
the top of this page:
http://en.wikipedia.org/wiki/Electronic_amplifier
You will see Q3 acting as the VAS. Together with R6 it
converts the beta multiplied current into drive voltage.
(The Vbe/Ic transfer nasties this up, but I think it may be
survivable. Everything is important, but I'm leaving
worrying about this till later.)
That schematic isn't entirely realistic, either. R3/R4 are
better replaced with a mirror, regular, Wilson, or otherwise.
R5 is often itself a current source or sink (depending on
which way you flip the schematic polarities) and may be a BJT
and diodes or two BJTs, etc.
What exactly are you trying to do?
If you look again at the schematic mentioned above, note the
function of D1 and D2. They stack to create a bias voltage.
That's used to set the point of operation for the output
stage (two-quadrant emitter follower -- which may be just two
BJTs as in that picture, or more.) Often, this is replaced
with an adjustable BJT configured as a Vbe multiplier. That's
what I'm trying to do. Except that I'd like to have the +V
and -V supply rails (ground is also present in the system) be
unregulated.
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down. The variation of Vbe is quite large, as you
know, where the controlling Eg term in the Is(T) equation
overwhelms the otherwise oppositely-signed dV/dT of the
Shockley equation. Above -2mV/K. And with the exponential
dependance of Ic on Vbe... well, it serves that function as
well. So the Vbe value needs to track temperature in just
such a way that it maintains the design operating point for
the output stage, over temperature, while also ignoring
variations in the current that sources through it.
I'm trying to keep my options open, regarding the amplifier's
class. If it were operating class-A all the time, my limited
understanding suggests that some variation across the Vbe
multiplier isn't nearly as important as it clearly would be
for, say, class-B operation. I'm not exactly sure where I
want to wind up biasing things.
So I am slowly learning this stuff and, assuming the Vbe
multiplier has some part within it thermally coupled as
appropriate to some well-chosen part of the output stage,
trying to gather how I'd: (1) stabilize the voltage at some
fixed temperature T against variations in the current flowing
through it, and (2) calibrate it's Vbe multiplication factor
in just the right way so that it tracks well with the
effective Eg found in the Is(T) function of the output stage
needed to hold the operating point steady vs temperature.
My question here was regarding (1), not (2). I'm not far
enough along on that one to even begin on that one, yet. To
be honest, I just started learning about audio amplifier
design, including terms like VAS, starting around the 26th
last month. So I may be far off the mark in a few places.
I'm finding it a very interesting education, though, and I'm
glad I started down the road a small bit. But "being exact"
about what I want remains part of the learning process,
itself. So what you see here is as far as I've gotten to.
My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe"
Thompson, because I could pull so much magic with Vbe compensation
methods ;-)
Well, I can believe it. And I mean that as a sincere
compliment. If you can suggest something still better than
what I've already posted, I'd like to look at it.
(Vbe multipliers generally are used just to create a smaller dead-band
that is temperature stable.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Class AB bias is an art form of which I
am expert, but cannot divulge publicly at this time :-)
Well, I want to examine class-AB at some point. It may be
where I want to settle, though class-B would be quite fine
for my needs.
If you can't help with class-AB, then you can't. I will have
to struggle along. However, anywhere else you can send me a
clue I'd certainly appreciate it.
There is no interest other than personal. Certainly nothing
commercial in mind. I'm just a hobbyist trying to learn.
Jon
To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:
What's a "VAS"?
Sorry. I read it somewhere regarding audio amplifiers and
the term stuck in my mind, I suppose. It's short-hand for
Voltage Amplifier Stage. It's almost so simple that no one
would bother creating a term for it, except that it seems as
though someone did and folks have used it in places where
I've been reading.
By the way, if you look at the semi-conceptual schematic at
the top of this page:
http://en.wikipedia.org/wiki/Electronic_amplifier
You will see Q3 acting as the VAS. Together with R6 it
converts the beta multiplied current into drive voltage.
(The Vbe/Ic transfer nasties this up, but I think it may be
survivable. Everything is important, but I'm leaving
worrying about this till later.)
That schematic isn't entirely realistic, either. R3/R4 are
better replaced with a mirror, regular, Wilson, or otherwise.
R5 is often itself a current source or sink (depending on
which way you flip the schematic polarities) and may be a BJT
and diodes or two BJTs, etc.
What exactly are you trying to do?
If you look again at the schematic mentioned above, note the
function of D1 and D2. They stack to create a bias voltage.
That's used to set the point of operation for the output
stage (two-quadrant emitter follower -- which may be just two
BJTs as in that picture, or more.) Often, this is replaced
with an adjustable BJT configured as a Vbe multiplier. That's
what I'm trying to do. Except that I'd like to have the +V
and -V supply rails (ground is also present in the system) be
unregulated.
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down. The variation of Vbe is quite large, as you
know, where the controlling Eg term in the Is(T) equation
overwhelms the otherwise oppositely-signed dV/dT of the
Shockley equation. Above -2mV/K. And with the exponential
dependance of Ic on Vbe... well, it serves that function as
well. So the Vbe value needs to track temperature in just
such a way that it maintains the design operating point for
the output stage, over temperature, while also ignoring
variations in the current that sources through it.
I'm trying to keep my options open, regarding the amplifier's
class. If it were operating class-A all the time, my limited
understanding suggests that some variation across the Vbe
multiplier isn't nearly as important as it clearly would be
for, say, class-B operation. I'm not exactly sure where I
want to wind up biasing things.
So I am slowly learning this stuff and, assuming the Vbe
multiplier has some part within it thermally coupled as
appropriate to some well-chosen part of the output stage,
trying to gather how I'd: (1) stabilize the voltage at some
fixed temperature T against variations in the current flowing
through it, and (2) calibrate it's Vbe multiplication factor
in just the right way so that it tracks well with the
effective Eg found in the Is(T) function of the output stage
needed to hold the operating point steady vs temperature.
My question here was regarding (1), not (2). I'm not far
enough along on that one to even begin on that one, yet. To
be honest, I just started learning about audio amplifier
design, including terms like VAS, starting around the 26th
last month. So I may be far off the mark in a few places.
I'm finding it a very interesting education, though, and I'm
glad I started down the road a small bit. But "being exact"
about what I want remains part of the learning process,
itself. So what you see here is as far as I've gotten to.
My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe"
Thompson, because I could pull so much magic with Vbe compensation
methods ;-)
Well, I can believe it. And I mean that as a sincere
compliment. If you can suggest something still better than
what I've already posted, I'd like to look at it.
(Vbe multipliers generally are used just to create a smaller dead-band
that is temperature stable.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Class AB bias is an art form of which I
am expert, but cannot divulge publicly at this time :-)
Well, I want to examine class-AB at some point. It may be
where I want to settle, though class-B would be quite fine
for my needs.
If you can't help with class-AB, then you can't. I will have
to struggle along. However, anywhere else you can send me a
clue I'd certainly appreciate it.
There is no interest other than personal. Certainly nothing
commercial in mind. I'm just a hobbyist trying to learn.
Jon
(1) Split R1, bypass that junction to ground
(2) Make R5 and R6 into mirrors, resistor feed from VDD, but split and
bypassed.
(3) As you said, replace R4:R3 with a mirror, I don't think a compound
device mirror, such as Wilson, is necessary. Study this if you want
more info....
http://analog-innovations.com/SED/EnhancedCurrentMirrors.pdf
(4) Since you're on a learning curve, just replace D1/D2 with 1.5*Vbe,
losing about 1/5 of the Q3 quiescent current in the resistors.
Bypassing base-to-base (of Q4-Q5) will help at all but very low
frequencies.
(5) Long haul as you "oomph" the power:
Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more
complicated (Darlington extension of Vbe multiplier).
Start simple, then grow it, that way you learn before you flame 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.
John Larkin
Guest
Guest
Tue Feb 09, 2010 4:03 pm
On Mon, 08 Feb 2010 22:43:19 -0800, Jon Kirwan
<jonk_at_infinitefactors.org> wrote:
Quote:
On Mon, 08 Feb 2010 22:35:05 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 9 Feb 2010 00:28:27 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:
"Jon Kirwan" <jonk_at_infinitefactors.org> wrote in message
news:okr1n55h5dvjjklg760dllkqq50v7s38ib_at_4ax.com...
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down.
The general idea is to put the Vbe transistor on the same heatsink as the
outputs, if not glued to a transistor directly.
Unfortunately, for widely mismatched current densities, this doesn't work.
http://webpages.charter.net/dawill/Images/Ampere.gif
In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
tempco to compensate the far beefier (= lower current density??) output
darlingtons.
I was thinking of adding another CCS so a constant voltage drop appears on
the Vbe's base divider resistor. Algebraically subtracting a fairly stable
voltage results in the effective tempco (percentwise) increasing. The base
divider ratio has to be changed to compensate.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Don't worry about stability -- as John said, bypass and forget about it.
Most of the dynamic VAS/CCS current flows into the output stage, since
that's what it's there for anyway. The capacitor helps turn on the N side /
turn off the P side for rising edges and vice versa.
As for PSRR, the CCS's and gobs of feedback keep that in check. Of course,
in principle you need something to start the CCS's. ICs do this with a JFET
(i.e. current regulating diode) or bandgap reference (e.g., TL431), or
sometimes both, to set a master current, from which everything else is
mirrored. Most discrete circuits just use a resistor, which is "0%" PSRR,
but it's not all that bad because the currents are balanced (*on average*,
which means you'll see IMD products when it's moving).
Tim
This topology, thermally coupled Vbe multiplier, was mediocre 50 years
ago. And still is.
A trek of a thousand miles starts with but the first step.
Jon
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 9 Feb 2010 00:28:27 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:
"Jon Kirwan" <jonk_at_infinitefactors.org> wrote in message
news:okr1n55h5dvjjklg760dllkqq50v7s38ib_at_4ax.com...
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down.
The general idea is to put the Vbe transistor on the same heatsink as the
outputs, if not glued to a transistor directly.
Unfortunately, for widely mismatched current densities, this doesn't work.
http://webpages.charter.net/dawill/Images/Ampere.gif
In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
tempco to compensate the far beefier (= lower current density??) output
darlingtons.
I was thinking of adding another CCS so a constant voltage drop appears on
the Vbe's base divider resistor. Algebraically subtracting a fairly stable
voltage results in the effective tempco (percentwise) increasing. The base
divider ratio has to be changed to compensate.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Don't worry about stability -- as John said, bypass and forget about it.
Most of the dynamic VAS/CCS current flows into the output stage, since
that's what it's there for anyway. The capacitor helps turn on the N side /
turn off the P side for rising edges and vice versa.
As for PSRR, the CCS's and gobs of feedback keep that in check. Of course,
in principle you need something to start the CCS's. ICs do this with a JFET
(i.e. current regulating diode) or bandgap reference (e.g., TL431), or
sometimes both, to set a master current, from which everything else is
mirrored. Most discrete circuits just use a resistor, which is "0%" PSRR,
but it's not all that bad because the currents are balanced (*on average*,
which means you'll see IMD products when it's moving).
Tim
This topology, thermally coupled Vbe multiplier, was mediocre 50 years
ago. And still is.
A trek of a thousand miles starts with but the first step.
Jon
A trek of 23,000 miles starts with but the first step in the wrong
direction.
John
John Larkin
Guest
Guest
Tue Feb 09, 2010 4:05 pm
On Mon, 8 Feb 2010 23:47:24 -0800 (PST), "miso_at_sushi.com"
<miso_at_sushi.com> wrote:
Quote:
On Feb 8, 10:06 pm, Jon Kirwan <j...@infinitefactors.org> wrote:
On Mon, 08 Feb 2010 19:09:28 -0700, Jim Thompson
To-Email-Use-The-Envelope-I...@My-Web-Site.com> wrote:
What's a "VAS"?
Sorry. I read it somewhere regarding audio amplifiers and
the term stuck in my mind, I suppose. It's short-hand for
Voltage Amplifier Stage. It's almost so simple that no one
would bother creating a term for it, except that it seems as
though someone did and folks have used it in places where
I've been reading.
By the way, if you look at the semi-conceptual schematic at
the top of this page:
http://en.wikipedia.org/wiki/Electronic_amplifier
You will see Q3 acting as the VAS. Together with R6 it
converts the beta multiplied current into drive voltage.
(The Vbe/Ic transfer nasties this up, but I think it may be
survivable. Everything is important, but I'm leaving
worrying about this till later.)
That schematic isn't entirely realistic, either. R3/R4 are
better replaced with a mirror, regular, Wilson, or otherwise.
R5 is often itself a current source or sink (depending on
which way you flip the schematic polarities) and may be a BJT
and diodes or two BJTs, etc.
What exactly are you trying to do?
If you look again at the schematic mentioned above, note the
function of D1 and D2. They stack to create a bias voltage.
That's used to set the point of operation for the output
stage (two-quadrant emitter follower -- which may be just two
BJTs as in that picture, or more.) Often, this is replaced
with an adjustable BJT configured as a Vbe multiplier. That's
what I'm trying to do. Except that I'd like to have the +V
and -V supply rails (ground is also present in the system) be
unregulated.
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down. The variation of Vbe is quite large, as you
know, where the controlling Eg term in the Is(T) equation
overwhelms the otherwise oppositely-signed dV/dT of the
Shockley equation. Above -2mV/K. And with the exponential
dependance of Ic on Vbe... well, it serves that function as
well. So the Vbe value needs to track temperature in just
such a way that it maintains the design operating point for
the output stage, over temperature, while also ignoring
variations in the current that sources through it.
I'm trying to keep my options open, regarding the amplifier's
class. If it were operating class-A all the time, my limited
understanding suggests that some variation across the Vbe
multiplier isn't nearly as important as it clearly would be
for, say, class-B operation. I'm not exactly sure where I
want to wind up biasing things.
So I am slowly learning this stuff and, assuming the Vbe
multiplier has some part within it thermally coupled as
appropriate to some well-chosen part of the output stage,
trying to gather how I'd: (1) stabilize the voltage at some
fixed temperature T against variations in the current flowing
through it, and (2) calibrate it's Vbe multiplication factor
in just the right way so that it tracks well with the
effective Eg found in the Is(T) function of the output stage
needed to hold the operating point steady vs temperature.
My question here was regarding (1), not (2). I'm not far
enough along on that one to even begin on that one, yet. To
be honest, I just started learning about audio amplifier
design, including terms like VAS, starting around the 26th
last month. So I may be far off the mark in a few places.
I'm finding it a very interesting education, though, and I'm
glad I started down the road a small bit. But "being exact"
about what I want remains part of the learning process,
itself. So what you see here is as far as I've gotten to.
My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe"
Thompson, because I could pull so much magic with Vbe compensation
methods ;-)
Well, I can believe it. And I mean that as a sincere
compliment. If you can suggest something still better than
what I've already posted, I'd like to look at it.
(Vbe multipliers generally are used just to create a smaller dead-band
that is temperature stable.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Class AB bias is an art form of which I
am expert, but cannot divulge publicly at this time :-)
Well, I want to examine class-AB at some point. It may be
where I want to settle, though class-B would be quite fine
for my needs.
If you can't help with class-AB, then you can't. I will have
to struggle along. However, anywhere else you can send me a
clue I'd certainly appreciate it.
There is no interest other than personal. Certainly nothing
commercial in mind. I'm just a hobbyist trying to learn.
Jon
Have you read Randy Slone's power amplifier book? This stuff really
isn't rocket science. Nor is AB.
http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/
dp/0071599258/ref=dp_ob_title_bk
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'd pick one of his MOS designs. Bipolar designs often have good
intentions, but ring like a bell. MOS is mushy, but predictably mushy.
On Mon, 08 Feb 2010 19:09:28 -0700, Jim Thompson
To-Email-Use-The-Envelope-I...@My-Web-Site.com> wrote:
What's a "VAS"?
Sorry. I read it somewhere regarding audio amplifiers and
the term stuck in my mind, I suppose. It's short-hand for
Voltage Amplifier Stage. It's almost so simple that no one
would bother creating a term for it, except that it seems as
though someone did and folks have used it in places where
I've been reading.
By the way, if you look at the semi-conceptual schematic at
the top of this page:
http://en.wikipedia.org/wiki/Electronic_amplifier
You will see Q3 acting as the VAS. Together with R6 it
converts the beta multiplied current into drive voltage.
(The Vbe/Ic transfer nasties this up, but I think it may be
survivable. Everything is important, but I'm leaving
worrying about this till later.)
That schematic isn't entirely realistic, either. R3/R4 are
better replaced with a mirror, regular, Wilson, or otherwise.
R5 is often itself a current source or sink (depending on
which way you flip the schematic polarities) and may be a BJT
and diodes or two BJTs, etc.
What exactly are you trying to do?
If you look again at the schematic mentioned above, note the
function of D1 and D2. They stack to create a bias voltage.
That's used to set the point of operation for the output
stage (two-quadrant emitter follower -- which may be just two
BJTs as in that picture, or more.) Often, this is replaced
with an adjustable BJT configured as a Vbe multiplier. That's
what I'm trying to do. Except that I'd like to have the +V
and -V supply rails (ground is also present in the system) be
unregulated.
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down. The variation of Vbe is quite large, as you
know, where the controlling Eg term in the Is(T) equation
overwhelms the otherwise oppositely-signed dV/dT of the
Shockley equation. Above -2mV/K. And with the exponential
dependance of Ic on Vbe... well, it serves that function as
well. So the Vbe value needs to track temperature in just
such a way that it maintains the design operating point for
the output stage, over temperature, while also ignoring
variations in the current that sources through it.
I'm trying to keep my options open, regarding the amplifier's
class. If it were operating class-A all the time, my limited
understanding suggests that some variation across the Vbe
multiplier isn't nearly as important as it clearly would be
for, say, class-B operation. I'm not exactly sure where I
want to wind up biasing things.
So I am slowly learning this stuff and, assuming the Vbe
multiplier has some part within it thermally coupled as
appropriate to some well-chosen part of the output stage,
trying to gather how I'd: (1) stabilize the voltage at some
fixed temperature T against variations in the current flowing
through it, and (2) calibrate it's Vbe multiplication factor
in just the right way so that it tracks well with the
effective Eg found in the Is(T) function of the output stage
needed to hold the operating point steady vs temperature.
My question here was regarding (1), not (2). I'm not far
enough along on that one to even begin on that one, yet. To
be honest, I just started learning about audio amplifier
design, including terms like VAS, starting around the 26th
last month. So I may be far off the mark in a few places.
I'm finding it a very interesting education, though, and I'm
glad I started down the road a small bit. But "being exact"
about what I want remains part of the learning process,
itself. So what you see here is as far as I've gotten to.
My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe"
Thompson, because I could pull so much magic with Vbe compensation
methods ;-)
Well, I can believe it. And I mean that as a sincere
compliment. If you can suggest something still better than
what I've already posted, I'd like to look at it.
(Vbe multipliers generally are used just to create a smaller dead-band
that is temperature stable.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Class AB bias is an art form of which I
am expert, but cannot divulge publicly at this time :-)
Well, I want to examine class-AB at some point. It may be
where I want to settle, though class-B would be quite fine
for my needs.
If you can't help with class-AB, then you can't. I will have
to struggle along. However, anywhere else you can send me a
clue I'd certainly appreciate it.
There is no interest other than personal. Certainly nothing
commercial in mind. I'm just a hobbyist trying to learn.
Jon
Have you read Randy Slone's power amplifier book? This stuff really
isn't rocket science. Nor is AB.
http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/
dp/0071599258/ref=dp_ob_title_bk
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'd pick one of his MOS designs. Bipolar designs often have good
intentions, but ring like a bell. MOS is mushy, but predictably mushy.
Can you explain "mushy" in any more technical terms?
John
Jim Thompson
Guest
Guest
Tue Feb 09, 2010 4:33 pm
On Tue, 09 Feb 2010 02:44:31 -0800, Jon Kirwan
<jonk_at_infinitefactors.org> wrote:
Quote:
On Mon, 8 Feb 2010 18:23:16 -0800 (PST), "miso_at_sushi.com"
miso_at_sushi.com> wrote:
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
miso_at_sushi.com> wrote:
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
Useless nonsense....
http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf
...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.
Bob Monsen
Guest
Guest
Tue Feb 09, 2010 8:20 pm
"John Larkin" <jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote in message
news:c7u2n5ldssoc0drl22u1rrif02t7jmkc0s_at_4ax.com...
Quote:
On Mon, 08 Feb 2010 22:43:19 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:
A trek of a thousand miles starts with but the first step.
Jon
A trek of 23,000 miles starts with but the first step in the wrong
direction.
jonk_at_infinitefactors.org> wrote:
A trek of a thousand miles starts with but the first step.
Jon
A trek of 23,000 miles starts with but the first step in the wrong
direction.
Not while I'm drinking my coffee, please... :)
Regards,
Bob Monsen
Bob Monsen
Guest
Guest
Tue Feb 09, 2010 8:40 pm
"Jon Kirwan" <jonk_at_infinitefactors.org> wrote in message
news:jg91n5d684ru5imsq1cfcjpjd1vddg2b2l_at_4ax.com...
Quote:
I think this fits in sci.electronics.design, not .basics.
Jon
Jon
Sorry, I didn't read the entire message...
However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
Somewhat more expensive, but it'll be VERY stiff.
However, you don't really want to hold that value constant. You want the
voltage to compensate for the temperature of the output transistors. You
might be able to use a diode to track the temperature change, and then use
that in the feedback loop to compensate the TLV431.
A honking big capacitor, one that has very low impedance at your frequencies
of interest, is probably the best idea I've seen on the thread.
On a related note, there was an article in a recent EDN about a self biasing
preamp which was kinda cool. Instead of trying to track the difference using
diodes or a multiplier, it used a couple of transistors and an opamp to set
the correct values at the bases of the pass transistors. It was so novel (at
least to me) that I typed it into LTSpice. Here it is:
Version 4
SHEET 1 948 680
WIRE -288 -304 -608 -304
WIRE -16 -304 -288 -304
WIRE 144 -304 -16 -304
WIRE 320 -304 144 -304
WIRE 512 -304 320 -304
WIRE -288 -272 -288 -304
WIRE -608 -240 -608 -304
WIRE 320 -240 320 -304
WIRE 320 -128 320 -160
WIRE 320 -128 240 -128
WIRE 512 -128 512 -304
WIRE 448 -80 384 -80
WIRE 240 -64 240 -128
WIRE -608 -16 -608 -160
WIRE -160 -16 -608 -16
WIRE -128 32 -464 32
WIRE 48 32 -48 32
WIRE 112 32 48 32
WIRE 512 32 512 -32
WIRE 512 32 192 32
WIRE 560 32 512 32
WIRE 656 32 624 32
WIRE 672 32 656 32
WIRE -464 96 -464 32
WIRE 48 96 48 32
WIRE 128 96 48 96
WIRE 240 96 240 0
WIRE 240 96 192 96
WIRE 320 96 320 -32
WIRE 512 96 512 32
WIRE 672 128 672 32
WIRE -608 144 -608 -16
WIRE -608 144 -704 144
WIRE 32 144 -96 144
WIRE 448 144 384 144
WIRE 144 160 144 -304
WIRE -464 176 -464 160
WIRE -16 176 -16 -304
WIRE 48 176 48 96
WIRE 112 176 48 176
WIRE -96 192 -96 144
WIRE -48 192 -96 192
WIRE 240 192 240 96
WIRE 240 192 176 192
WIRE 320 192 240 192
WIRE -464 208 -464 176
WIRE 32 208 32 144
WIRE 32 208 16 208
WIRE 112 208 32 208
WIRE -160 224 -160 -16
WIRE -48 224 -160 224
WIRE -608 240 -608 144
WIRE -704 256 -704 144
WIRE -704 352 -704 320
WIRE -608 352 -608 320
WIRE -608 352 -704 352
WIRE -464 352 -464 288
WIRE -464 352 -608 352
WIRE -288 352 -288 -192
WIRE -288 352 -464 352
WIRE -272 352 -288 352
WIRE -16 352 -16 240
WIRE -16 352 -272 352
WIRE 144 352 144 224
WIRE 144 352 -16 352
WIRE 512 352 512 192
WIRE 512 352 144 352
WIRE 672 352 672 208
WIRE 672 352 512 352
FLAG -272 352 0
FLAG 656 32 out
FLAG -464 176 in
SYMBOL npn 384 96 M0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn 448 -128 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL pnp 384 -32 R180
SYMATTR InstName Q4
SYMATTR Value 2N3906
SYMBOL pnp 448 192 M180
SYMATTR InstName Q5
SYMATTR Value 2N3906
SYMBOL voltage -288 -288 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL cap 224 -64 R0
SYMATTR InstName C1
SYMATTR Value 10µF
SYMBOL res 208 16 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res -32 16 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 100
SYMBOL voltage -464 192 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(0 .2 1k)
SYMBOL cap -480 96 R0
SYMATTR InstName C2
SYMATTR Value 10µF
SYMBOL cap 624 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value 470µ
SYMBOL res 656 112 R0
SYMATTR InstName R5
SYMATTR Value 8
SYMBOL res 304 -256 R0
SYMATTR InstName R1
SYMATTR Value 1k5
SYMBOL Opamps\\LT6234 144 192 R0
SYMATTR InstName U1
SYMBOL res -624 224 R0
SYMATTR InstName R9
SYMATTR Value 1k
SYMBOL cap 192 80 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C5
SYMATTR Value 10pF
SYMBOL res -624 -256 R0
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL Opamps\\LT6234 -16 208 R0
SYMATTR InstName U2
SYMBOL cap -720 256 R0
SYMATTR InstName C4
SYMATTR Value 1µF
TEXT 552 -296 Left 0 !.tran 0 .1 0 1u
TEXT 552 -256 Left 0 !.four 1k 10 v(out)
TEXT 552 -216 Left 0 ;.noise V(out) V2 oct 1001 1 100k
Regards,
Bob Monsen
Jon Kirwan
Guest
Guest
Tue Feb 09, 2010 9:25 pm
On Tue, 09 Feb 2010 06:49:11 -0500, Bitrex
<bitrex_at_de.lete.earthlink.net> wrote:
Quote:
Jon Kirwan wrote:
On Mon, 08 Feb 2010 22:17:31 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 08 Feb 2010 22:11:51 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:
On Mon, 08 Feb 2010 20:49:24 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:
On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
Hang a big capacitor across it.
Nice try.
Jon
No, seriously, that solves a bunch of problems.
John
Which problems does a slew-dependent, C*dV/dt bypass current
solve?
Jon
A big cap across the biasing gadget keeps the voltage drop across it
fairly constant, of course. That nukes some of the problems you
referred to. More peak current is available to the output bases, for
example.
What size cap would help with power supply ripple? Seems the
dV/dt is so small that a fair sized cap would be required to
make any difference. Similarly for low frequency amplified
signal out of the VAS. When you say "big," maybe you mean
it.
Ban is suggesting global NFB from output back to input.
You've said as much when you say to apply "lots of NFB." I
don't doubt the sincerity of either of you and I'm certain it
will do a lot. But right now I'm interested in seeing what
can be done right on this local subcircuit and at LF as well
as higher frequencies. Unless someone wants to walk me
through the thinking towards the larger concepts. I'm good
either way, as it's the learning that takes place I'm looking
for. But without such guidance, I need to move along at the
pace I can handle while guiding myself.
Jon
Hey Jon, I found a derivation of the input impedance of the two-resistor
/transistor Vbe multiplier you might be interested in looking at:
http://paginas.fe.up.pt/~fff/eBook/MDA/Mult_Vbe.html
On Mon, 08 Feb 2010 22:17:31 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 08 Feb 2010 22:11:51 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:
On Mon, 08 Feb 2010 20:49:24 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:
On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
Hang a big capacitor across it.
Nice try.
Jon
No, seriously, that solves a bunch of problems.
John
Which problems does a slew-dependent, C*dV/dt bypass current
solve?
Jon
A big cap across the biasing gadget keeps the voltage drop across it
fairly constant, of course. That nukes some of the problems you
referred to. More peak current is available to the output bases, for
example.
What size cap would help with power supply ripple? Seems the
dV/dt is so small that a fair sized cap would be required to
make any difference. Similarly for low frequency amplified
signal out of the VAS. When you say "big," maybe you mean
it.
Ban is suggesting global NFB from output back to input.
You've said as much when you say to apply "lots of NFB." I
don't doubt the sincerity of either of you and I'm certain it
will do a lot. But right now I'm interested in seeing what
can be done right on this local subcircuit and at LF as well
as higher frequencies. Unless someone wants to walk me
through the thinking towards the larger concepts. I'm good
either way, as it's the learning that takes place I'm looking
for. But without such guidance, I need to move along at the
pace I can handle while guiding myself.
Jon
Hey Jon, I found a derivation of the input impedance of the two-resistor
/transistor Vbe multiplier you might be interested in looking at:
http://paginas.fe.up.pt/~fff/eBook/MDA/Mult_Vbe.html
That one takes an approach that I'm not familiar with and
didn't take. I'll have to consider the approach more.
However, I did take a look at the end of it. It says:
R = (R1+(R2||re)) / (1+(1/R1+gm)*(R2||re))
If I understand the value gm, and I may not, it's just 1/re
or else re=1/gm. Basically, just the (kT/q)/Ic I'd mentioned
when I wrote. If that is the case, I used these to see how
that page predicts:
ic=.005
vt=k*300/q
gm=ic/vt
re=1/gm
r1=1000
r2=1000
r2p=r2*re/(r2+re)
and then computed:
(r1+r2p)/(1+(1/r1+gm)*r2p)
and got:
502.5719049 Ohms.
This is so far from my own calculations of about 15.4 Ohms
that I just _had_ to put this into LTspice and test it. To
do that, I simply set up the basic circuit with the two
resistors and BJT and then hooked up a variable current
source to the topside. I set it up as an AC source of 5mA
with peaks of 500uA, and then ran a .TRAN on it and plotted
the upper rail of the structure's voltage. I used a 2N2222
BJT, as well. Convenient, and I have them laying about.
Anyway, so I ran the sims and got 17.44mV, peak to peak.
Divided by the peak to peak current variation of 1mA gives an
apparent R of 17.44 Ohms. My calculations arrived at 15.4
Ohms, or so.
All this could be operator error. I may be operating the web
page you suggested incorrectly, so that the 503 Ohms I get is
because I didn't know what I was plugging in and where. I
may be operating LTspice incorrectly, so that it's results
aren't usable and it's just luck that the numbers worked out
in my favor.
But there it is.
Here is the LTspice file:
Version 4
SHEET 1 880 680
WIRE 128 0 16 0
WIRE 224 0 128 0
WIRE 288 0 224 0
WIRE 128 32 128 0
WIRE 16 112 16 0
WIRE 224 112 224 0
WIRE 128 160 128 112
WIRE 160 160 128 160
WIRE 128 208 128 160
WIRE 16 224 16 192
WIRE 128 320 128 288
WIRE 224 320 224 208
WIRE 224 320 128 320
WIRE 128 336 128 320
FLAG 128 336 0
FLAG 288 0 V_rail
FLAG 16 224 0
SYMBOL npn2 160 112 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL res 112 192 R0
SYMATTR InstName R1
SYMATTR Value 1k
SYMBOL res 112 16 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL current 16 192 R180
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value SINE(5m 500u 50)
TEXT -76 296 Left 0 !.tran 1
Quote:
For bypassing purposes the rule of thumb I've always heard is to make
the impedance of the capacitor 1/10th the value of the impedance looking
in to the circuit at the lowest audio frequency.
the impedance of the capacitor 1/10th the value of the impedance looking
in to the circuit at the lowest audio frequency.
Well, let's assume that I got lucky and LTspice and I agree
on the figure of about 16 Ohms. With a signal at 20Hz, we
are talking:
C = 1/(2 PI f (R_ac/10)) = 5000uF
Yikes. John L. wasn't kidding when he wrote "big." Luckily,
in steady state it could be a low voltage cap!
Jon
Jon Kirwan
Guest
Guest
Tue Feb 09, 2010 9:26 pm
On Tue, 09 Feb 2010 07:03:18 -0800, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
Quote:
On Mon, 08 Feb 2010 22:43:19 -0800, Jon Kirwan
jonk_at_infinitefactors.org> wrote:
On Mon, 08 Feb 2010 22:35:05 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 9 Feb 2010 00:28:27 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:
"Jon Kirwan" <jonk_at_infinitefactors.org> wrote in message
news:okr1n55h5dvjjklg760dllkqq50v7s38ib_at_4ax.com...
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down.
The general idea is to put the Vbe transistor on the same heatsink as the
outputs, if not glued to a transistor directly.
Unfortunately, for widely mismatched current densities, this doesn't work.
http://webpages.charter.net/dawill/Images/Ampere.gif
In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
tempco to compensate the far beefier (= lower current density??) output
darlingtons.
I was thinking of adding another CCS so a constant voltage drop appears on
the Vbe's base divider resistor. Algebraically subtracting a fairly stable
voltage results in the effective tempco (percentwise) increasing. The base
divider ratio has to be changed to compensate.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Don't worry about stability -- as John said, bypass and forget about it.
Most of the dynamic VAS/CCS current flows into the output stage, since
that's what it's there for anyway. The capacitor helps turn on the N side /
turn off the P side for rising edges and vice versa.
As for PSRR, the CCS's and gobs of feedback keep that in check. Of course,
in principle you need something to start the CCS's. ICs do this with a JFET
(i.e. current regulating diode) or bandgap reference (e.g., TL431), or
sometimes both, to set a master current, from which everything else is
mirrored. Most discrete circuits just use a resistor, which is "0%" PSRR,
but it's not all that bad because the currents are balanced (*on average*,
which means you'll see IMD products when it's moving).
Tim
This topology, thermally coupled Vbe multiplier, was mediocre 50 years
ago. And still is.
A trek of a thousand miles starts with but the first step.
Jon
A trek of 23,000 miles starts with but the first step in the wrong
direction.
jonk_at_infinitefactors.org> wrote:
On Mon, 08 Feb 2010 22:35:05 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 9 Feb 2010 00:28:27 -0600, "Tim Williams"
tmoranwms_at_charter.net> wrote:
"Jon Kirwan" <jonk_at_infinitefactors.org> wrote in message
news:okr1n55h5dvjjklg760dllkqq50v7s38ib_at_4ax.com...
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down.
The general idea is to put the Vbe transistor on the same heatsink as the
outputs, if not glued to a transistor directly.
Unfortunately, for widely mismatched current densities, this doesn't work.
http://webpages.charter.net/dawill/Images/Ampere.gif
In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
tempco to compensate the far beefier (= lower current density??) output
darlingtons.
I was thinking of adding another CCS so a constant voltage drop appears on
the Vbe's base divider resistor. Algebraically subtracting a fairly stable
voltage results in the effective tempco (percentwise) increasing. The base
divider ratio has to be changed to compensate.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
Don't worry about stability -- as John said, bypass and forget about it.
Most of the dynamic VAS/CCS current flows into the output stage, since
that's what it's there for anyway. The capacitor helps turn on the N side /
turn off the P side for rising edges and vice versa.
As for PSRR, the CCS's and gobs of feedback keep that in check. Of course,
in principle you need something to start the CCS's. ICs do this with a JFET
(i.e. current regulating diode) or bandgap reference (e.g., TL431), or
sometimes both, to set a master current, from which everything else is
mirrored. Most discrete circuits just use a resistor, which is "0%" PSRR,
but it's not all that bad because the currents are balanced (*on average*,
which means you'll see IMD products when it's moving).
Tim
This topology, thermally coupled Vbe multiplier, was mediocre 50 years
ago. And still is.
A trek of a thousand miles starts with but the first step.
Jon
A trek of 23,000 miles starts with but the first step in the wrong
direction.
There is no wrong direction at the start. It's all good.
Jon
George Herold
Guest
Guest
Tue Feb 09, 2010 9:35 pm
On Feb 9, 5:39 am, Jon Kirwan <j...@infinitefactors.org> wrote:
Quote:
On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold
ggher...@gmail.com> wrote:
snip
"I'm wondering about additional topology changes to improve
the performance still more."
Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
wonder if you abandoned class A operation too early? Why not keep
things linear evreywhere and avoid the ‘dead band’? So what if you
need a bigger heat sink. It’s certainly a lot simpler.
George H.
Well, George... No, I've not abandoned it. Actually, it's my
hope to wind up building the amplifier and then operating it
(by hopefully choosing a design where that is possible) in
different modes for the learning experience of it. I hope
that is in the cards. I really do.
But to make a sharp point on it, although it's probably just
an extreme case, I remember reading about a 10W amplifier,
single channel, dissipating 120W! Creeps me out. So I
definitely _want_ to consider other classes of operation. And
cripes, I want to learn, anyway. So why not keep my options
open?
Jon
ggher...@gmail.com> wrote:
snip
"I'm wondering about additional topology changes to improve
the performance still more."
Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
wonder if you abandoned class A operation too early? Why not keep
things linear evreywhere and avoid the ‘dead band’? So what if you
need a bigger heat sink. It’s certainly a lot simpler.
George H.
Well, George... No, I've not abandoned it. Actually, it's my
hope to wind up building the amplifier and then operating it
(by hopefully choosing a design where that is possible) in
different modes for the learning experience of it. I hope
that is in the cards. I really do.
But to make a sharp point on it, although it's probably just
an extreme case, I remember reading about a 10W amplifier,
single channel, dissipating 120W! Creeps me out. So I
definitely _want_ to consider other classes of operation. And
cripes, I want to learn, anyway. So why not keep my options
open?
Jon
" I remember reading about a 10W amplifier,
Quote:
single channel, dissipating 120W! "
It might have been here,
http://www.passdiy.com/default.html
I got to reading about amplifiers on the above site... Do in part to
your interest.
George H.
Jon Kirwan
Guest
Guest
Tue Feb 09, 2010 9:56 pm
On Tue, 09 Feb 2010 08:03:14 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:
Quote:
(1) Split R1, bypass that junction to ground
Understood.
Quote:
(2) Make R5 and R6 into mirrors, resistor feed from VDD, but split and
bypassed.
bypassed.
I had been thinking more like this structure:
Quote:
: to input to voltage
: stage mirror amp stage
: | ,---, |
: | | | |
: | | | |
: | gnd | |
: | \ |
: | / R4 |
: | \ |
: | / |
: | | |
: | ,------+ |
: | | | |
: | --- C1 | |
: | --- | |
: | | \ |
: | | / R3 |
: | | \ |
: | | / |
: | Vdd | |
: Q2 c\| | R5 |/c Q3
: |-----------+----/\/\-----|
: e<| | |>e
: | | |
: | R1 |/c Q1 |
: +---/\/\----| |
: | |>e \
: | | / R6
: | | \
: \ | /
: / R2 | |
: \ | |
: / | |
: | | |
: Vdd Vdd Vdd
: stage mirror amp stage
: | ,---, |
: | | | |
: | | | |
: | gnd | |
: | \ |
: | / R4 |
: | \ |
: | / |
: | | |
: | ,------+ |
: | | | |
: | --- C1 | |
: | --- | |
: | | \ |
: | | / R3 |
: | | \ |
: | | / |
: | Vdd | |
: Q2 c\| | R5 |/c Q3
: |-----------+----/\/\-----|
: e<| | |>e
: | | |
: | R1 |/c Q1 |
: +---/\/\----| |
: | |>e \
: | | / R6
: | | \
: \ | /
: / R2 | |
: \ | |
: / | |
: | | |
: Vdd Vdd Vdd
However, I take your point.
Quote:
(3) As you said, replace R4:R3 with a mirror, I don't think a compound
device mirror, such as Wilson, is necessary.
device mirror, such as Wilson, is necessary.
Understood. Although I'm not able to make my own decisions
on this, yet, I've read repeatedly that the distortions to
deal with are not at the input stage. The input stage can be
made better, the improvements are small in comparison to what
remains in the rest of a well-designed system. Point taken.
Quote:
Thanks, I will!
Quote:
(4) Since you're on a learning curve, just replace D1/D2 with 1.5*Vbe,
losing about 1/5 of the Q3 quiescent current in the resistors.
losing about 1/5 of the Q3 quiescent current in the resistors.
Thanks for taking a moment to confirm the "1/5th" division.
I'd already figured that was commonly done and had some ideas
of my own about why that makes sense. (I could talk about
that, but I'm sure you already know and I think I know, too.)
Quote:
Bypassing base-to-base (of Q4-Q5) will help at all but very low
frequencies.
frequencies.
Okay. That's how I see it, too.
Quote:
(5) Long haul as you "oomph" the power:
Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more
complicated (Darlington extension of Vbe multiplier).
Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more
complicated (Darlington extension of Vbe multiplier).
This is what I'd like to explore, right now. Extensions.
It's because it is where my mind is at, right now. And I
want to explore this more fully before walking away from it
and moving on.
Quote:
Start simple, then grow it, that way you learn before you flame it
hehe. Good advice, of course. As I'm still struggling to
make sure I understand each piece, right now, I'm just not
yet ready to put it all together -- not even in a low power
system. I might be able to vaguely grasp what I would be
doing, but I prefer taking each part and thoroughly looking
at its function before moving on. Then, when I look once
again at the whole, I can better "read" what I see and that
helps a lot in terms of gaining a global view. I'm still in
the trenches, right now, and not allowing myself to raise my
head much above that until I get some of the details nailed
down.
Speaking of that, can you confirm (or correct) the equation I
developed for the simple Vbe multiplier's small signal R? Or
the relative scale and _sign_ of the Early effect correction
to it, which I peg near -1 part per thousand in the case I
cited?
All this is good for me to go through.
Thanks,
Jon
Jon Kirwan
Guest
Guest
Tue Feb 09, 2010 9:59 pm
On Tue, 09 Feb 2010 08:33:45 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:
Quote:
snip
Useless nonsense....
Useless nonsense....
Are you talking about my comments, those of others here, or
the link you posted below?
Quote:
http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf
I'll take a look, today.
Thanks,
Jon
Jon Kirwan
Guest
Guest
Tue Feb 09, 2010 10:05 pm
On Tue, 9 Feb 2010 11:35:36 -0800 (PST), George Herold
<ggherold_at_gmail.com> wrote:
Quote:
On Feb 9, 5:39 am, Jon Kirwan <j...@infinitefactors.org> wrote:
On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold
ggher...@gmail.com> wrote:
snip
"I'm wondering about additional topology changes to improve
the performance still more."
Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
wonder if you abandoned class A operation too early? Why not keep
things linear evreywhere and avoid the ‘dead band’? So what if you
need a bigger heat sink. It’s certainly a lot simpler.
George H.
Well, George... No, I've not abandoned it. Actually, it's my
hope to wind up building the amplifier and then operating it
(by hopefully choosing a design where that is possible) in
different modes for the learning experience of it. I hope
that is in the cards. I really do.
But to make a sharp point on it, although it's probably just
an extreme case, I remember reading about a 10W amplifier,
single channel, dissipating 120W! Creeps me out. So I
definitely _want_ to consider other classes of operation. And
cripes, I want to learn, anyway. So why not keep my options
open?
Jon
" I remember reading about a 10W amplifier,
single channel, dissipating 120W! "
It might have been here,
http://www.passdiy.com/default.html
I got to reading about amplifiers on the above site... Do in part to
your interest.
George H.
On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold
ggher...@gmail.com> wrote:
snip
"I'm wondering about additional topology changes to improve
the performance still more."
Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
wonder if you abandoned class A operation too early? Why not keep
things linear evreywhere and avoid the ‘dead band’? So what if you
need a bigger heat sink. It’s certainly a lot simpler.
George H.
Well, George... No, I've not abandoned it. Actually, it's my
hope to wind up building the amplifier and then operating it
(by hopefully choosing a design where that is possible) in
different modes for the learning experience of it. I hope
that is in the cards. I really do.
But to make a sharp point on it, although it's probably just
an extreme case, I remember reading about a 10W amplifier,
single channel, dissipating 120W! Creeps me out. So I
definitely _want_ to consider other classes of operation. And
cripes, I want to learn, anyway. So why not keep my options
open?
Jon
" I remember reading about a 10W amplifier,
single channel, dissipating 120W! "
It might have been here,
http://www.passdiy.com/default.html
I got to reading about amplifiers on the above site... Do in part to
your interest.
George H.
Egads. Loads of PDF files. Now I have to create a
directory, download them one by one, and then call them up
with my slow machine to look. Any particular page or file
where you saw it? (No, that isn't where I saw the comment.)
But thanks for the link. I'll add it to those I read, also.
Jon
bg
Guest
Guest
Tue Feb 09, 2010 10:05 pm
Jon Kirwan wrote in message ...
Quote:
I think this fits in sci.electronics.design, not .basics.
I'd like to consider the Vbe multiplier often used in audio
amplifiers to maintain a bias voltage for the output stage.
The purpose is to better mitigate against ripple in the
unregulated power supply rails and against the the VAS
voltage output resulting from amplified signal voltages.
(The only active device under consideration is a BJT, though.
No JFETs or MOSFETS or opamps or other ICs.)
The basic starting form for a Vbe multiplier is shown in Fig.
1 and the bias voltage output is indicated there. Assume Q1
is thermally coupled in some magic way, for now, in just the
right way so that if the current through the Vbe multiplier
were perfectly stable, that the bias voltage would track just
as needed (The 'Eg' of Q1 is exactly what's needed for the
output stage's temperature tracking in some nice way and the
values of R1 and R2 are set correctly and the thermal
coupling and location is somehow where it needs to be.) The
focus is on the Vbe multiplier's variation of bias in the
face of changes in sourcing current at the top of Fig. 1.
: +V
: |
: resistor or
: current source
: |
: ,---+---,
: | |
: \ |
: / R2 |
: \ +-----> upper quadrant
: / | ^
: | | |
: | |/c Q1 BIAS
: +-----| VOLTAGE
: | |>e |
: \ | |
: / R1 | v
: \ +-----> lower quadrant
: / |
: | |
: '---+---'
: |
: VAS ---'
:
: FIGURE 1
If I use a resistor as the load for the VAS, it's obvious to
me that the Vbe multiplier will need to cope with varying
currents. But even if I use a BJT (or two) to make a current
source sitting above the Vbe multiplier, it's still not going
to hold entirely still with +V ripple and with varying VAS
drive voltages. That variation will ultimately manifest
itself in a varying Vbe bias voltage. That will change the
operating point for the output stage.
If it is class-A, I suppose it doesn't matter that much. But
I don't want to be forced into class-A operation. Nor do I
want to be forced into regulated rails. So it becomes a
little more important, I think, to get this nailed down
better.
There's the problem, anyway.
To quantify how bad all this really is, I tried my hand at
figuring out the small signal analysis of the Vbe multiplier.
If I got a first approximation about right, it is based
squarely upon the small-re of the BJT. The very familiar
value for (kT/q)/Ic.
There is also the value of R2 shown in Fig. 1, but since its
effect is only affected by the change in base current, I
believe it's contribution is divided by Q1's beta. So the
actual equation is something like:
R_ac = (1/Ic)*(kT/q)*(1+R2/R1) + R2/beta
For a 2X multiplier where R2 is about R1, this is:
R_ac = (2/Ic)*(kT/q) + R2/beta
The Vbe multipler value is:
V_bias = Vbe*(1+(R2/R1)) + R2*Ic/beta
(The latter term being a correction for base current.)
Ignoring base current for now and assuming I had Ic set
around 5mA and placed R1=R2=1k for the 2X factor, this R_ac
value works out to about 15.4 ohms.
A variation of half an mA in Ic yields about 7.7mV change in
the bias point.
I decided to see if the Early effect made much of a
difference. The adjustment appears to be something like
this:
R_early = dV/dI = -Ic/VA*R^2
If I'm interpreting it right, it really does show as negative
resistance added to R_ac. The fuller equation, then,
including the Early effect, would be:
R_ac = (2/Ic)*(kT/q) + R2/beta - Ic/VA*R^2
(Which requires a quadratic solution to solve for R.)
If R_ac is 15.4 ohms and Ic is around 5mA, a VA of 100V would
suggest about R_early=-10mOhms. Which is roughly a factor of
1500 less than 15.4 Ohms. Since it now appears to be on the
order of 0.1% or so for typical Ic, VA, and, R_ac values, I
think I can ignore it for these considerations.
So drop it, I will.
I had scouted around a few weeks back (not for this reason)
and found what is shown in Fig. 2. I remembered it, but
didn't understand it then.
: +V
: |
: resistor or
: current source
: |
: ,---+---, <-- node A
: | |
: | \
: | / R3
: \ \
: / R2 /
: \ |
: / +-----> upper quadrant
: | | ^
: | |/c Q1 |
: +-----| BIAS
: | |>e VOLTAGE
: \ | |
: / R1 | v
: \ +-----> lower quadrant
: / |
: | |
: '---+---'
: |
: VAS ---'
:
: FIGURE 2
I think I now understand why R3 was there. Changes in Ic
create changes in Q1's collector voltage, per Ic*R3. The
result is that dV=dI*R3. If R3 is on the order of the above
computed R_ac, then variations at node A caused by changing
currents through the Vbe multipler (most of which are seen as
Ic changes) will be neatly compensated for the change in the
voltage drop caused by R3.
However, that can only be set for some assumed Ic. Nearby
changes will work pretty well. But further deviations will
start to show problems again. Also, the Fig. 2 version will
use a slightly higher multiplier value to get node A up high
enough for the R3 drop to hit the right place required to
bias the output stage. That higher multiplier means that
while, let's say, the two (or four, if that's it) output
BJT's Vbe values vary over temp and the thermally coupled Q1
above also varies it's own Vbe value, the multiplier other
than 2 (or 4) will mean the variation of the bias will match
at only one place -- if it ever did more than one spot. How
important that is, I've not considered yet.
I'm wondering about additional topology changes to improve
the performance still more. Obviously, if they are crazy and
wild, I'm probably going to live with the above and be done
with it. But I think there's got to be something still
better. Another BJT as a bypass route across Q1 and R3?
Getting this nailed down should help mitigate against both
unreg supply ripple (on one side, anyway) putting hum into
the output and also against large scale changes in the VAS
amplified signal voltage (which means distortion.)
Jon
Your circuit is an example of collector feedback. Collector feedback does
I'd like to consider the Vbe multiplier often used in audio
amplifiers to maintain a bias voltage for the output stage.
The purpose is to better mitigate against ripple in the
unregulated power supply rails and against the the VAS
voltage output resulting from amplified signal voltages.
(The only active device under consideration is a BJT, though.
No JFETs or MOSFETS or opamps or other ICs.)
The basic starting form for a Vbe multiplier is shown in Fig.
1 and the bias voltage output is indicated there. Assume Q1
is thermally coupled in some magic way, for now, in just the
right way so that if the current through the Vbe multiplier
were perfectly stable, that the bias voltage would track just
as needed (The 'Eg' of Q1 is exactly what's needed for the
output stage's temperature tracking in some nice way and the
values of R1 and R2 are set correctly and the thermal
coupling and location is somehow where it needs to be.) The
focus is on the Vbe multiplier's variation of bias in the
face of changes in sourcing current at the top of Fig. 1.
: +V
: |
: resistor or
: current source
: |
: ,---+---,
: | |
: \ |
: / R2 |
: \ +-----> upper quadrant
: / | ^
: | | |
: | |/c Q1 BIAS
: +-----| VOLTAGE
: | |>e |
: \ | |
: / R1 | v
: \ +-----> lower quadrant
: / |
: | |
: '---+---'
: |
: VAS ---'
:
: FIGURE 1
If I use a resistor as the load for the VAS, it's obvious to
me that the Vbe multiplier will need to cope with varying
currents. But even if I use a BJT (or two) to make a current
source sitting above the Vbe multiplier, it's still not going
to hold entirely still with +V ripple and with varying VAS
drive voltages. That variation will ultimately manifest
itself in a varying Vbe bias voltage. That will change the
operating point for the output stage.
If it is class-A, I suppose it doesn't matter that much. But
I don't want to be forced into class-A operation. Nor do I
want to be forced into regulated rails. So it becomes a
little more important, I think, to get this nailed down
better.
There's the problem, anyway.
To quantify how bad all this really is, I tried my hand at
figuring out the small signal analysis of the Vbe multiplier.
If I got a first approximation about right, it is based
squarely upon the small-re of the BJT. The very familiar
value for (kT/q)/Ic.
There is also the value of R2 shown in Fig. 1, but since its
effect is only affected by the change in base current, I
believe it's contribution is divided by Q1's beta. So the
actual equation is something like:
R_ac = (1/Ic)*(kT/q)*(1+R2/R1) + R2/beta
For a 2X multiplier where R2 is about R1, this is:
R_ac = (2/Ic)*(kT/q) + R2/beta
The Vbe multipler value is:
V_bias = Vbe*(1+(R2/R1)) + R2*Ic/beta
(The latter term being a correction for base current.)
Ignoring base current for now and assuming I had Ic set
around 5mA and placed R1=R2=1k for the 2X factor, this R_ac
value works out to about 15.4 ohms.
A variation of half an mA in Ic yields about 7.7mV change in
the bias point.
I decided to see if the Early effect made much of a
difference. The adjustment appears to be something like
this:
R_early = dV/dI = -Ic/VA*R^2
If I'm interpreting it right, it really does show as negative
resistance added to R_ac. The fuller equation, then,
including the Early effect, would be:
R_ac = (2/Ic)*(kT/q) + R2/beta - Ic/VA*R^2
(Which requires a quadratic solution to solve for R.)
If R_ac is 15.4 ohms and Ic is around 5mA, a VA of 100V would
suggest about R_early=-10mOhms. Which is roughly a factor of
1500 less than 15.4 Ohms. Since it now appears to be on the
order of 0.1% or so for typical Ic, VA, and, R_ac values, I
think I can ignore it for these considerations.
So drop it, I will.
I had scouted around a few weeks back (not for this reason)
and found what is shown in Fig. 2. I remembered it, but
didn't understand it then.
: +V
: |
: resistor or
: current source
: |
: ,---+---, <-- node A
: | |
: | \
: | / R3
: \ \
: / R2 /
: \ |
: / +-----> upper quadrant
: | | ^
: | |/c Q1 |
: +-----| BIAS
: | |>e VOLTAGE
: \ | |
: / R1 | v
: \ +-----> lower quadrant
: / |
: | |
: '---+---'
: |
: VAS ---'
:
: FIGURE 2
I think I now understand why R3 was there. Changes in Ic
create changes in Q1's collector voltage, per Ic*R3. The
result is that dV=dI*R3. If R3 is on the order of the above
computed R_ac, then variations at node A caused by changing
currents through the Vbe multipler (most of which are seen as
Ic changes) will be neatly compensated for the change in the
voltage drop caused by R3.
However, that can only be set for some assumed Ic. Nearby
changes will work pretty well. But further deviations will
start to show problems again. Also, the Fig. 2 version will
use a slightly higher multiplier value to get node A up high
enough for the R3 drop to hit the right place required to
bias the output stage. That higher multiplier means that
while, let's say, the two (or four, if that's it) output
BJT's Vbe values vary over temp and the thermally coupled Q1
above also varies it's own Vbe value, the multiplier other
than 2 (or 4) will mean the variation of the bias will match
at only one place -- if it ever did more than one spot. How
important that is, I've not considered yet.
I'm wondering about additional topology changes to improve
the performance still more. Obviously, if they are crazy and
wild, I'm probably going to live with the above and be done
with it. But I think there's got to be something still
better. Another BJT as a bypass route across Q1 and R3?
Getting this nailed down should help mitigate against both
unreg supply ripple (on one side, anyway) putting hum into
the output and also against large scale changes in the VAS
amplified signal voltage (which means distortion.)
Jon
Your circuit is an example of collector feedback. Collector feedback does
not work well with large signal swings and it lowers the input impedance. A
lower input impedance means that the drivers will also need a lower output
impedance. The bottom line is that you will not see this bias method used in
a power output stages. Another option is to use emitter feedback but for the
emitter resistors to be effective, they will drop alot of signal output and
waste power, which explains why those resistors are usually very low values,
They have very little effect unless the emitter current is large. At the DC
bias current level, they don't do a thing.
Local feedback (emitter feedback or collector feedback) both create more
problems then they solve for stabilizing the power output stage bias point.
To keep the output stage bias point stabilized, the use of overall feedback
is the standard practise. A simple typical amplifier might have a
differential input stage, followerd by a voltage amplifier, followed by the
power output stage. The output from the power stage is fed back to the
differential input stage. High open loop gain with large feedback is the key
to better stabilization of the operating points. Fix it with feedback is a
term to remember.
The typical bias chain using diodes can be made with resistors as well, but
diodes have the advantage of dropping the bias voltage while having a lower
impedance to the signal. Sometimes you will see those bypassed with a large
cap if the impedance causes to much signal loss. Diodes can also offer
temperature compensation. In any case, an output stage will have way more
current flowing in that bias chain than is actually needed as base bias
current. The voltage drops developed in the bias chain will not be greatly
affected by changes in the base emitter junction because the base bias
current is small compared to the current in the bias chain. And remember,
that " Fix it with feedback " applies here too. So variations in the power
supply have a very reduced effect on the bias point. The feedback signal is
a voltage, and enough feedback will compensate to keep the output voltage
offset at zero. It will not compensate for for excessve collector currents
or power dissaption if the offset voltage remains low. That is why
temperature compensation is used too.
In the early years of transistors, it was common to see transistor stages
using many of the techniques used with vacuum tubes. Dc coupled amplifiers
were rare, because any bias shift was amplified in further stages. Feedback
was applied locally, and overall feedback had no effect on the DC operating
points. The trend now is to stabilize everything with feedback. It works,
and it works well. Unless you are a purist and have some religious reason to
avoid this technique, there is no sense in reinventing the wheel.
Jim Thompson
Guest
Guest
Tue Feb 09, 2010 10:07 pm
On Tue, 09 Feb 2010 12:59:01 -0800, Jon Kirwan
<jonk_at_infinitefactors.org> wrote:
Quote:
On Tue, 09 Feb 2010 08:33:45 -0700, Jim Thompson
To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:
snip
Useless nonsense....
Are you talking about my comments, those of others here, or
the link you posted below?
http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf
I'll take a look, today.
Thanks,
Jon
To-Email-Use-The-Envelope-Icon_at_My-Web-Site.com> wrote:
snip
Useless nonsense....
Are you talking about my comments, those of others here, or
the link you posted below?
http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf
I'll take a look, today.
Thanks,
Jon
The link. Do the math, it's a hoax, good only at one current and
temperature pair... besides being Beta sensitive.
...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.
Jon Kirwan
Guest
Guest
Tue Feb 09, 2010 10:18 pm
On Tue, 9 Feb 2010 11:40:45 -0800, "Bob Monsen"
<rcmonsen_at_gmail.com> wrote:
Quote:
"Jon Kirwan" <jonk_at_infinitefactors.org> wrote in message
news:jg91n5d684ru5imsq1cfcjpjd1vddg2b2l_at_4ax.com...
I think this fits in sci.electronics.design, not .basics.
Jon
Sorry, I didn't read the entire message...
However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
Somewhat more expensive, but it'll be VERY stiff.
news:jg91n5d684ru5imsq1cfcjpjd1vddg2b2l_at_4ax.com...
I think this fits in sci.electronics.design, not .basics.
Jon
Sorry, I didn't read the entire message...
However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
Somewhat more expensive, but it'll be VERY stiff.
I'm still in "discrete" mode. For example, I am _less_
interested in opamp topologies and design techniques than I
am in _how_ to design opamps. There is nothing like knowing
the details about how they are designed inside to understand
the gotchas that aren't readily accessible to someone using
them.
A comparison here might be like "using a handgun" vs
"understanding how handguns are designed and built." A
gunsmith requires a very detailed knowledge and while this
level of detailed knowledge may not make them a better
shooter, that knowledge still informs them about the handgun
in ways that most shooters have little idea about. And I
think it prepares them for certain unusual circumstances a
little better.
I'm at the gunsmith level, right now. I am NOT wanting to go
shooting, just yet.
Quote:
However, you don't really want to hold that value constant. You want the
voltage to compensate for the temperature of the output transistors.
voltage to compensate for the temperature of the output transistors.
Yes.
Quote:
You might be able to use a diode to track the temperature change, and then use
that in the feedback loop to compensate the TLV431.
that in the feedback loop to compensate the TLV431.
No ICs. I might like to thoroughly _understand_ the internal
design of the TLV431, first. Then I'm willing to use it.
Quote:
A honking big capacitor, one that has very low impedance at your frequencies
of interest, is probably the best idea I've seen on the thread.
of interest, is probably the best idea I've seen on the thread.
Well, I'm interested in focusing on the crafted design of Vbe
multipliers, right now. I can _always_ slap a cap on
whatever that winds up being, later on. So set that aside.
What also bugs me is how that darned thing is going to
interact with the larger system, eventually. I don't like
ignorantly littering a schematic with poles and zeros and
phase delays where right now I have very little idea right
what then happens when I close the outer NFB loop. I'm still
"in the trenches" and trying to understand each piece in
detail and think at that level. The capacitor is at the next
level above and is outside my "view."
Besides, it doesn't do much for LF. The Z is too high and in
parallel, ignorable.
Quote:
On a related note, there was an article in a recent EDN about a self biasing
preamp which was kinda cool. Instead of trying to track the difference using
diodes or a multiplier, it used a couple of transistors and an opamp to set
the correct values at the bases of the pass transistors. It was so novel (at
least to me) that I typed it into LTSpice.
snip
preamp which was kinda cool. Instead of trying to track the difference using
diodes or a multiplier, it used a couple of transistors and an opamp to set
the correct values at the bases of the pass transistors. It was so novel (at
least to me) that I typed it into LTSpice.
snip
Okay. I'm going to save it, too. I'm not ready to
assimilate it, of course. But I definitely want it around
when I _am_ ready for it.
Thanks,
Jon
elektroda.net NewsGroups Forum Index - Electronics Design - Maintaining a Vbe Multiplier's bias value
