Class AB common source mosfet power amplifier

[D from BC]

Hi,

OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------

Any ideas?

This is not for audio, so don't need super low distortion - but I
don't want any cross-over distortion to be *too* obvious either.

Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

Thanks,

I found an old schematic I created in 1997.
I quickly drew it in Ltspice..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg
351Kbyte

Don't ask about the output network.. I've completely forgotten why I
did that and it's looks weird to me now..


D from BC

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

 

[D from BC]

Hi,

OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------

Any ideas?

This is not for audio, so don't need super low distortion - but I
don't want any cross-over distortion to be *too* obvious either.

Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

Thanks,

I found an old schematic I created in 1997.
I quickly drew it in Ltspice..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg
351Kbyte

Don't ask about the output network.. I've completely forgotten why I
did that and it's looks weird to me now..


D from BC

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

 

[Winfield]

Hi,

OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------

Any ideas?

This is not for audio, so don't need super low distortion - but I
don't want any cross-over distortion to be *too* obvious either.

Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

Thanks,

I found an old schematic I created in 1997.
I quickly drew it in Ltspice..http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg
351Kbyte

Don't ask about the output network.. I've completely forgotten why I
did that and it's looks weird to me now..

D from BC

Class B

 

[John Devereux]

LOL. So, I'm reading this thread, and the guy is asking if he can solve
this problem where the transistors all have to be mounted on the same
heatsink (or two). And then another guy chimes in with a solution that
completely removes the problem of heat, and the guy shoots it down...

;-)

C'mon...switching ain't all that hard...hell, I did it with tubes!
http://webpages.charter.net/dawill/tmoranwms/Elec_Compound.html
Rak(on) ~= 100 ohms on that 38HE7.

The "use a switcher" solution is not new to me - I *did* look into
it, and it looked quite hard at these power levels, (compared to a
simple "common drain"/complementary pair follower linear). Also the
manufacturer of the actuator recommends against usage with class D
amplifiers, for reasons which are not totally clear to me. There is a
concern about magnetic field noise. Etc.

You *can* buy amplifiers for this application, and they are class AB,
linear. I know I should just buy one, but they are not cheap and I got
interested and wanted to see if I could make one. It doesn't look too
difficult, a conventional audio amplifier "scaled up". Then I got
curious about a whether common-source version was practical. That
option is looking like a bit too much effort at present, for a
one-off, but still interesting to think about.

 

[Nobody]

That seemed even more complicated (and I do not need to save power).

A switching amplifier will dissipate far less heat.

Efficiency is usually less about saving energy and more about not
dissipating it in inconvenient places.

 

[Robert Latest]

You *can* buy amplifiers for this application, and they are class AB,
linear. I know I should just buy one, but they are not cheap

Have you looked at big-ass audio amps for PA (stage) systems? Not consumer
stuff. They're powerful, nearly indestructible, and comparatively cheap
because even pro audio gear is mass-produced.

Here's something that will produce quite a bit of vibration. The PL340 will
put 4000 continuous watts into a 4 ohm bridge. The class-D PL380 puts 5000
watts into 8 ohms.

http://www.qsc.com/products/amps/powerlight3/powerlight3.htm

robert

 

[John Devereux]

Have you looked at big-ass audio amps for PA (stage) systems? Not consumer
stuff. They're powerful, nearly indestructible, and comparatively cheap
because even pro audio gear is mass-produced.

Here's something that will produce quite a bit of vibration. The PL340 will
put 4000 continuous watts into a 4 ohm bridge. The class-D PL380 puts 5000
watts into 8 ohms.

http://www.qsc.com/products/amps/powerlight3/powerlight3.htm

Thanks, I think these could be an option too.

 

[Fred Bloggs]

OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------

Any ideas?

This is not for audio, so don't need super low distortion - but I
don't want any cross-over distortion to be *too* obvious either.

Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

TI has a line of CMOS OAs with zero crossover distortion using just this
type of complementary AB output stage biased with matched current
mirrors and driven by injecting the drive into a high impedance node in
such a way to disrupt its equilibrium. I'm sure you an duplicate that
with discretes and additional OAs to substitute for the various matching
requirements.

 

[Jim Thompson]

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC
[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

 

[Glen Walpert]

The "use a switcher" solution is not new to me - I *did* look into
it, and it looked quite hard at these power levels, (compared to a
simple "common drain"/complementary pair follower linear). Also the
manufacturer of the actuator recommends against usage with class D
amplifiers, for reasons which are not totally clear to me. There is a
concern about magnetic field noise. Etc.

The shaker mfgr recommendation against class D amplifiers may be quite
old, say from the time of the 16 kHz switching frequency servo
amplifier (a short lived early design which was a big pain in the
ear). Modern class D amplifiers leak a lot less of the switching
frequency, which is now high enough to easily filter to very low
levels. There is probably no longer any valid reason for not using a
good class D amp, at least if you decide to buy one. (Your linear
approach is certainly easier to design and make work for a one off.)

 

[John Larkin]

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC
[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

Everybody knows that electricity is heavy, so likes to fall from the
top of the sheet down.

John

 

[John Devereux]

OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------

Any ideas?

This is not for audio, so don't need super low distortion - but I
don't want any cross-over distortion to be *too* obvious either.

Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

TI has a line of CMOS OAs with zero crossover distortion using just
this type of complementary AB output stage biased with matched current
mirrors and driven by injecting the drive into a high impedance node
in such a way to disrupt its equilibrium. I'm sure you an duplicate
that with discretes and additional OAs to substitute for the various
matching requirements.

Yes, I am sure I could easily do that. Ahem.

I did look at lots of rail-to-rail output stages - with my level of
expertise, they were difficult to interpret. It looked likely they
would be relying on characteristics that are well defined with on-chip
parts, but difficult with discretes. E.g. ratios of emitter areas,
thermal and gain matching.

 

[Jim Thompson]

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC
[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

Everybody knows that electricity is heavy, so likes to fall from the
top of the sheet down.

John

Yep. My recent consultation in Rancho Bernardo had "port order"
requirements for left and right edges of the schematics.

...Jim Thompson

 

[Spehro Pefhany]

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC
[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

And I wish nothing less for whoever came up with the LM324 pinout.

Best regards,
Spehro Pefhany

 

[John Larkin]

OK, I have been simulating away but am now all spiced out!

I am trying to come up with a linear (i.e. not switched) power mosfet
amplifier that works with the output device drains tied together. This
is mainly so they can be bolted to the same heatsink without using
insulators, although an extended voltage swing would also be good.

I.e. generally like this:

. V+ --------------------------------------------
. |
. [R] (optional)
. |
. |<
. ------|| P-Channel
. |-
. Drive |--------------------[LOAD]--->GND
. |-
. ------|| N-Channel
. |>
. |
. |
. [R] (optional)
. |
. V- --------------------------------------------

Any ideas?

This is not for audio, so don't need super low distortion - but I
don't want any cross-over distortion to be *too* obvious either.

Frequency range is up to a few kHz, voltage rails will be +/- 80V max.

TI has a line of CMOS OAs with zero crossover distortion using just
this type of complementary AB output stage biased with matched current
mirrors and driven by injecting the drive into a high impedance node
in such a way to disrupt its equilibrium. I'm sure you an duplicate
that with discretes and additional OAs to substitute for the various
matching requirements.

Yes, I am sure I could easily do that. Ahem.

I did look at lots of rail-to-rail output stages - with my level of
expertise, they were difficult to interpret. It looked likely they
would be relying on characteristics that are well defined with on-chip
parts, but difficult with discretes. E.g. ratios of emitter areas,
thermal and gain matching.

Here's the basic idea of a "perfect" class AB signal splitter, similar
to what Win posted, and sort of like what I use in my ultralinear NMR
gradient drivers. It really has zero crossover distortion, and
quiescent bias is independently adjustable from splitter gain.

http://img85.imageshack.us/my.php?image=psplitay8.jpg


The idea of taking signals from an opamp's power supply pins has been
around for decades.

John

 

[Jim Thompson]

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC
[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

And I wish nothing less for whoever came up with the LM324 pinout.

Best regards,
Spehro Pefhany

Isn't the LM324 the device that can be plugged into a socket, rotated
off by 180°, and NOT fry?

...Jim Thompson

 

[Jim Thompson]

On Thu, 08 Nov 2007 08:59:19 -0800, John Larkin

[snip]

Here's the basic idea of a "perfect" class AB signal splitter, similar
to what Win posted, and sort of like what I use in my ultralinear NMR
gradient drivers. It really has zero crossover distortion, and
quiescent bias is independently adjustable from splitter gain.

http://img85.imageshack.us/my.php?image=psplitay8.jpg


The idea of taking signals from an opamp's power supply pins has been
around for decades.

John

Yep. I was doing it in 1971 in regulators for the Hughes TOW Missile.

...Jim Thompson

 

[Fred Bartoli]

Jim Thompson a écrit :

On Wed, 07 Nov 2007 20:06:55 -0800, D from BC

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC

[snip]
Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC
Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC
He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

And I wish nothing less for whoever came up with the LM324 pinout.

Best regards,
Spehro Pefhany

Isn't the LM324 the device that can be plugged into a socket, rotated
off by 180°, and NOT fry?

Supplies?

 

[Spehro Pefhany]

On Wed, 07 Nov 2007 20:06:55 -0800, D from BC

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC

[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

And I wish nothing less for whoever came up with the LM324 pinout.

Best regards,
Spehro Pefhany

Isn't the LM324 the device that can be plugged into a socket, rotated
off by 180°, and NOT fry?

...Jim Thompson

Can't say as I've actually tried it (that I can remember anyway, I'm
sure it could have happened) but it's a 14-pin DIP with power pins in
the middle on both sides, plus on the "bottom", GND on the "top". I
don't think it would survive unless the supply current was limited.

Best regards,
Spehro Pefhany

 

[Jim Thompson]

On Thu, 08 Nov 2007 07:24:06 -0700, Jim Thompson

On Wed, 07 Nov 2007 20:06:55 -0800, D from BC

On Wed, 07 Nov 2007 19:57:05 -0800, D from BC

[snip]

Ooops wrong link and did some schematic errors too...
Please stand by...
D from BC

Ok...try now..
http://www.members.shaw.ca/chainsaw/SED/somoldamp.jpg


D from BC

He who placeth positive supply in lower half of schematic and vice
versa shall have perpetual insomnia ;-)

...Jim Thompson

And I wish nothing less for whoever came up with the LM324 pinout.

Best regards,
Spehro Pefhany

Isn't the LM324 the device that can be plugged into a socket, rotated
off by 180°, and NOT fry?

...Jim Thompson

Can't say as I've actually tried it (that I can remember anyway, I'm
sure it could have happened) but it's a 14-pin DIP with power pins in
the middle on both sides, plus on the "bottom", GND on the "top". I
don't think it would survive unless the supply current was limited.

Best regards,
Spehro Pefhany

Maybe it was the "guaranteed fry" that I was trying to suppress from
memory ?

...Jim Thompson