Class AB common source mosfet power amplifier

[John Larkin]

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

So, what's my BS score on this one?

John

 

[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

So, what's my BS score on this one?

John

The amplifier? No BS there. Actually there was a Motorola standard
part in the mid-60's that used a specially designed OpAmp chip plus
2N2222 and 2N2907 discrete chips to make such a power amplifier. I'm
drawing a blank on its part number right now... back to the archive
and look ;-)

...Jim Thompson

 

[Joerg]

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

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.

So what were they? Left = blue, right = red?

 

[JosephKK]

John Devereux [email protected] posted to
sci.electronics.design:

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,

From what i have read in the thread so far, i think i might suggest a
class AC amplifier. Class A around the center +/- 2V or so and class
C for larger excursions. Very difficult total design and setup
though.

 

[JosephKK]

John Devereux [email protected] posted to
sci.electronics.design:

John Devereux wrote:
[...]

Not for this - the final thing will have 8 mosfets paralleled for
each shown above, and will also use a copy driven in antiphase to
get full bridge drive. 32 mosfets in all,~10kW pulsed, > 1kW rms!

Wow! Who's playing lead guitar during that concert?

Yes - should drive a decent size sub-woofer!

Its actually for a vibrator. (Not that sort! A voice-coil actuator
for vibration testing).

They are called shakers, thank you. And not all that big of one
either. I have worked with ones in the 100kW+ rms range. I have
seen them big enough to shake a 10,000 lb satellite to 30 G. (random
or swept sine)

 

[JosephKK]

John Larkin [email protected] posted to
sci.electronics.design:

John Devereux wrote:

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.

This topology can be problematic because of the wild variation of
the gain depending on the load impedance and current. I assume you
are running in the class AB, right?
Right

Nevertheless it can be worked out however you may have to add the
heavy correction capacitors between drains and gates (and maybe
some nonlinear elements too it the goal is the high performance).
OK


But why this topology? It is unlikely that you need R-R operation
at +/-80V. Why not a trivial pnp-npn emitter follower?

I don't *need* rail to rail. As I said, it is mainly so I can bolt
the devices to the same heatsink without insulation which would
reduce the heat transfer. Power mosfets have the drain connected to
the tab, so this implies the topology above.

BTW, if you can drop the rail voltage to somewhat +/-50V, you can
probably use an audio IC like TDA7294.

Not for this - the final thing will have 8 mosfets paralleled for
each shown above, and will also use a copy driven in antiphase to
get full bridge drive. 32 mosfets in all,~10kW pulsed, > 1kW rms!

One other note: mosfets spec'd for switching use have a bad habit of
exploding when used in linear mode. We tested a lot of "300 watt"
power fets, bolted to a solid copper block, and most of them
couldn't dissipate 300 watts for 100 milliseconds when running at
higher drain voltages. Imagine loud noises. It has to do with
nonuniform heating or something. IXYS has some fets specially
designed to not explode in this mode.

So test your candidate fets to destruction to make sure you have
margin! Or go switchmode.

John

For this use it would be a Class D amplifier. Carefully linearized for
doing random at various levels.

 

[JosephKK]

John Devereux [email protected] posted to
sci.electronics.design:

John Devereux wrote:

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.

[...]

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

This topology can be problematic because of the wild variation of
the gain depending on the load impedance and current. I assume
you are running in the class AB, right?

Right

Nevertheless it can be worked out however you may have to add the
heavy correction capacitors between drains and gates (and maybe
some nonlinear elements too it the goal is the high performance).

OK


But why this topology? It is unlikely that you need R-R operation
at +/-80V. Why not a trivial pnp-npn emitter follower?

I don't *need* rail to rail. As I said, it is mainly so I can bolt
the devices to the same heatsink without insulation which would
reduce the heat transfer. Power mosfets have the drain connected to
the tab, so this implies the topology above.


BTW, if you can drop the rail voltage to somewhat +/-50V, you can
probably use an audio IC like TDA7294.

Not for this - the final thing will have 8 mosfets paralleled for
each shown above, and will also use a copy driven in antiphase to
get full bridge drive. 32 mosfets in all,~10kW pulsed, > 1kW rms!

One other note: mosfets spec'd for switching use have a bad habit
of exploding when used in linear mode. We tested a lot of "300
watt" power fets, bolted to a solid copper block, and most of them
couldn't dissipate 300 watts for 100 milliseconds when running at
higher drain voltages. Imagine loud noises. It has to do with
nonuniform heating or something. IXYS has some fets specially
designed to not explode in this mode.

The datasheets do show SOA for various duration pulses, which I will
respect.

So test your candidate fets to destruction to make sure you have
margin!

Will do.

Or go switchmode.

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

As a Class D amplifier it may be a much easier design.

 

[JosephKK]

Glen Walpert [email protected] posted to sci.electronics.design:

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.)

The tradeoffs are different at that power level.

 

[Tim Williams]

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".

Here's a hint: I've seen the inside of an Unholtz-Dickie 10kW shaker amp.
For output, this has two water-cooled heatsinks, each mounted with about 50
x stud package BJTs, rated for around 2A, 200V each I would guess. The
supply was 70V for each rail, floating so each side (composed of NPNs
only!) could drive each half of the output independently. I've never seen
this in solid state before, but I do recognize it from exotic tube
amplifier designs as the circlotron topology. On the front panel, beside
the incredibly over-the-top blur of buttons and setups and conditions and
adjustments, are meters showing output voltage up to 100Vrms and collector
current up to 200 or 400A (I forget which).

Of course, that's 10kW continuous rated.

A friend of mine enjoys the 1-2kW amps he's built. He's fond of OnSemi
parts, of which he's used (I think) TO-3 packaged BJTs for output, and
somewhere around +/-80V supply. He says it's stable down to like 1 or 2
ohms load (I forget how much peak current output), and brags that the
driver stage can deliver 7A peak to the outputs, providing full power out
to 300kHz or so. With that bandwidth, it doesn't matter that it is in fact
biased class C. Obviously, he does have a rather hefty toroidial power
transformer and lots of heatsinking.

Tim

 

[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- --------------------------------------------

[...]

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.

Sorry for not replying till now - I wanted to simulate it.

It seems to work embarassingly well for resistive and capacitive
loads, while inductive loads make it oscillate. Slowing it down,
e.g. with capacitors across the opamp current sensing resistors, can
make it stable again. I will play with it some more.

Are the diodes for supply sequencing?

 

[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- --------------------------------------------

[...]

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.

Sorry for not replying till now - I wanted to simulate it.

It seems to work embarassingly well for resistive and capacitive
loads, while inductive loads make it oscillate. Slowing it down,
e.g. with capacitors across the opamp current sensing resistors, can
make it stable again. I will play with it some more.

Are the diodes for supply sequencing?

Yup. Keeps the transistors from frying if the HV supplies aren't up. I
like to add a couple of led's in those paths, too, just for fun.

If you want to close an overall loop on output current, inductive
loads (the kind I get!) can resonate and wreck loop stability. So loop
dynamics, and maybe some load damping, have to be tweaked.

John

 

[Joerg]

John Devereux [email protected] posted to
sci.electronics.design:

John Devereux wrote: [...]

Not for this - the final thing will have 8 mosfets paralleled for
each shown above, and will also use a copy driven in antiphase to
get full bridge drive. 32 mosfets in all,~10kW pulsed, > 1kW rms!

Wow! Who's playing lead guitar during that concert?

Yes - should drive a decent size sub-woofer!

Its actually for a vibrator. (Not that sort! A voice-coil actuator
for vibration testing).

They are called shakers, thank you. And not all that big of one
either. I have worked with ones in the 100kW+ rms range. I have
seen them big enough to shake a 10,000 lb satellite to 30 G. (random
or swept sine)

I've seen some where they had to regularly pour another layer of
concrete onto the foundation and then stop and move to a new spot when
the bottom of that concrete "stalactite" was x many feet into the ground
(some legal limit for whatever reason).