MOSFET output stage

[Kevin Aylward]

Seems to be a few that miss that though. Esp those Chinese copies of
copies.



Why stop at £200 ? Oh !
http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm

Kevin, you'd be just the guy to do a harmonic analysis of the
distortion spectrum of a properly biased bipolar vs lateral mosfet
amp wouldn't you ? Any chance ?

It has just occurred to me, that you probably live quite close to me. I
live in Stevenage now, just a tad away from the old Luton hunting grounds. I
would say a joint visit to the pub for some Guinness might not go amiss.

Oh.. check out my current venture. Available for
bookings...www.blonddee.co.uk


Kevin Aylward
www.anasoft.co.uk
SuperSpice

 

[Kevin Aylward]

Kevin Aylward wrote:

Why stop at £200 ? Oh !
http://www.asa.org.uk/asa/adjudications/Public/TF_ADJ_44177.htm

This link seems to be bad. I did find this though

http://divinecables.co.uk/mains-power-cable/19/kemp-hi-power-cable#

All I can say, is that I am still stunned..and shocked...shocked and
stunned...all I said was that I was taller than Jesus, not that I was bigger
than Jesus...


Kevin Aylward

 

[Eeyore]

It has just occurred to me, that you probably live quite close to me. I
live in Stevenage now, just a tad away from the old Luton hunting grounds. I
would say a joint visit to the pub for some Guinness might not go amiss.

Sounds ok to me. I'm in St Albans. Will contact you off group.

Oh.. check out my current venture. Available for
bookings...www.blonddee.co.uk

OK will do. I know someone who might even book you.

Graham

 

[Eeyore]

This link seems to be bad. I did find this though

Works here.

http://divinecables.co.uk/mains-power-cable/19/kemp-hi-power-cable#

All I can say, is that I am still stunned..and shocked...shocked and
stunned...all I said was that I was taller than Jesus, not that I was bigger
than Jesus...

I'd like to start a movement to have all these liars and fraudsters shut down.

Graham

 

[Kevin Aylward]

Sounds ok to me. I'm in St Albans.

Yeah, just down the road...16.7M according to the AA. Although with a
suggested time of 20mins, obviously, the AA don't know how to drive...

Will contact you off group.

Look foward to it. My phone # is on my websites.

OK will do. I know someone who might even book you.

Oh.. nice...

Kevin Aylward

 

[Kevin Aylward]

Works here.

worked now.

I'd like to start a movement to have all these liars and fraudsters
shut down.

Indeed. If I had actually said that I was bigger than Jesus, I would have
said so...

As you may have noticed from visiting churches, the average height a while
back, was a lot smaller...

Kevin Aylward

 

[Eeyore]

worked now.


Indeed. If I had actually said that I was bigger than Jesus, I would have
said so...

As you may have noticed from visiting churches, the average height a while
back, was a lot smaller...

By pure chance I just received some info from Russ Andrews (several booklets)
on behalf of somone who didn't want to give their identity away.

Cream of the bunch I think was a 'special' phono lead optimised for subwoofer
connections in multi-speaker systems for deeper and richer bass or whatever
nutcase story they were making up.

I really would like to see that company go down along with all the fraudulent
hi-fi rags too.

Graham

 

[Eeyore]

Eeyore wrote:

Learn something about LATERAL mosfets that were designed for audio. I've already
given part number and links to data sheets.

That doesn't really matter. The transfer function only needs to be
continuous so that you can close a loop around it, and the fet needs
to be able to stand the peak power dissipation. That can easily be
done with vertical "switching" type fets. A modern FLOOD architecture
[1] works great with most any kind of fet. Lots of things have changed
in the last few decades.

John

[1] Of course you've never heard the term before. I just invented it.

Fine. Can you elaborate some more on it ? Laterals have some truly lovely features
for audio. The only downside being a slightly highish Ron. Not really a problem when
(as I have) used as many as 6 in parallel (12 mosfets per channel / 24 per amp). They
also match beautifully with no need for source balance resistors (so some of the Ron
loss 'goes away').

An opamp per fet, closing a local loop, feedback from the fet source,
makes each fet look like a perfect unity-gain, fast, zero-offset
device.

Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
output overshoot.

Opamps are cheap, but fets and heat sinks are expensive. Power
all those gate-drive opamps from a DC-DC converter floating on the
output node; DC/DC bricks are cheap nowadays, too. Do a simple output
current limit for fast overloads and back that up with a digital fet
power dissipation simulation that provides the real protections.

I've toyed in the past with doing device protection using an analogue multiplier actually
funnily enough.

That
will optimize the hell out of the power supply, fets, and heat sink,
giving a lot more safe power for the buck, especially in
complex-signal non-sinusoidal apps like audio and NMR gradient
drivers.

Use a bunch of smaller fets rather than a few big ones; that speeds
things up and spreads the heat out across the heat sinks better.

For the audio version, use two "hot" heat sinks, with no insulators
under the fets.

Something I am very much in favour of. I like that junction to see cool aluminium as fast
as possible.

Include full BIST. It's worth it for the savings in production test
alone.

The output stage will be so quiet and linear that no overall feedback
is needed or helpful.

The audiophools will hate this.

All the better !

Graham

 

[Jan Panteltje]

Interesting idea. I'll have to chew that one over. I can see possible problems fron op-amp
output overshoot.

If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2
you will see this is exactly what is done with the lower output MOSFET, combined with opamp
makes unity gain.
The top is already a source followwr.
The over current protection same thing, opamps.
It is likely this what makes the Boucherot network not needed.

You could do the same thing with the top MOSFET in a discrete design, if you must.

I've toyed in the past with doing device protection using an analogue multiplier actually
funnily enough.

I only see over current protection in a lot of amps, plus thermal,
the real thing was a bridge that caused foldback current limit,
couple of resistors and diodes, makes it any load proof.

 

[Kevin Aylward]

John Larkin wrote:
Eeyore wrote:

Learn something about LATERAL mosfets that were designed for
audio. I've already given part number and links to data sheets.

That doesn't really matter. The transfer function only needs to be
continuous so that you can close a loop around it, and the fet
needs to be able to stand the peak power dissipation. That can
easily be done with vertical "switching" type fets. A modern FLOOD
architecture [1] works great with most any kind of fet. Lots of
things have changed in the last few decades.

John

[1] Of course you've never heard the term before. I just invented
it.

Fine. Can you elaborate some more on it ? Laterals have some truly
lovely features for audio. The only downside being a slightly
highish Ron. Not really a problem when (as I have) used as many as
6 in parallel (12 mosfets per channel / 24 per amp). They also
match beautifully with no need for source balance resistors (so
some of the Ron loss 'goes away').

An opamp per fet, closing a local loop, feedback from the fet source,
makes each fet look like a perfect unity-gain, fast, zero-offset
device.

Interesting idea. I'll have to chew that one over. I can see possible
problems fron op-amp output overshoot.

I have a simple embodiment of that concept here, done a while ago, in
virtual land;-)

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

Its a push/pull gain loop around the output devices, forcing them to be
unity gain followers.

You can get lower distortion, at the expense of speed, because you have to
compensate earlier.

Common mode feedback at the second stage, allows for enormous dc/lf gain.
cascodes to allow the use of fast small transistors to do all the main work.
Emitter follower buffer to reduce the current swing in the input pair, as
per doug self. Spice says it should be in the < 0.0001% , 20Khz range,
maybe...

Kevin Aylward

 

[Kevin Aylward]

If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the
blockdiagram on page 2 you will see this is exactly what is done with
the lower output MOSFET, combined with opamp makes unity gain.
The top is already a source followwr.
The over current protection same thing, opamps.
It is likely this what makes the Boucherot network not needed.

I doubt it. I haven't seen the data sheet, but it is not usual to be able to
stabilise an amp with what I believe you are describing here.

Consider one amp feeding the other, both running open loop, with overall
feedback, to the 1st. Now consider the case where the second amp is
configured with local feedback, to make it a unity buffer, following the 1st
amp. Naively , one might argue that the 2nd stage now has a wider bandwidth,
due to feedback, such that the "new " system might be stable, i.e. one main
rolloff due to the 1st amp. However, in realty, topologically, nothing
changes. The determine the stability of the system, one needs to break the
loop at a point that breaks *all* feedback paths at once. This point will be
the point directly at the output of the 2nd amp. when this is done, it is
clear that the stabiliy is still due to the total loop gain of both amops
cascaded.

Excepting for the special cases, e.g. , where feedback is used to neutralise
r.f amps, feedback in general, cannot be used to widen bandwidth, if the
purpose of that wider bandwidth is to achieve stability, in this type of
arrangement

To wit, There is no such thing as a free lunch...

Kevin Aylward

www.superspice.co.uk

 

[Kevin Aylward]

Eeyore wrote:
John Larkin wrote:
Eeyore wrote:

Learn something about LATERAL mosfets that were designed for
audio. I've already given part number and links to data sheets.

That doesn't really matter. The transfer function only needs to be
continuous so that you can close a loop around it, and the fet
needs to be able to stand the peak power dissipation. That can
easily be done with vertical "switching" type fets. A modern
FLOOD architecture [1] works great with most any kind of fet.
Lots of things have changed in the last few decades.

John

[1] Of course you've never heard the term before. I just invented
it.

Fine. Can you elaborate some more on it ? Laterals have some truly
lovely features for audio. The only downside being a slightly
highish Ron. Not really a problem when (as I have) used as many as
6 in parallel (12 mosfets per channel / 24 per amp). They also
match beautifully with no need for source balance resistors (so
some of the Ron loss 'goes away').


An opamp per fet, closing a local loop, feedback from the fet
source, makes each fet look like a perfect unity-gain, fast,
zero-offset device.

Interesting idea. I'll have to chew that one over. I can see
possible problems fron op-amp output overshoot.

It needs a little loop tweaking, roughly....


V+
|\ |
-----------| + |
| d
| out-------+------Rg--------g
| | s
+------| - | |
| |/ Cf |
| | |
+--------------------+------Rf---------+
|
|
Rs
|
|
+------------- output rail


and an opamp that can slam the gate hard enough, not a big problem
nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
can be small, and the quiescent bias voltage can be small, because the
opamp offset voltage can be tiny. The fets share the load exactly, and
the standing bias current can be designed in, exactly, with no
adjustments.

The driver stage sees only opamps, so doesn't have to work very hard.

However....there are some issues with using whole op-amps, rather than
discrete transistors as I have in the noted circuit in my other post. You
may need 200V+ ratings, and a very fast one at that!!!


Kevin Aylward
[email protected]
www.kevinaylward.co.uk

 

[Jan Panteltje]

I doubt it. I haven't seen the data sheet, but it is not usual to be able to
stabilise an amp with what I believe you are describing here.

It is always a good idea to lookuop what we are talking about.

Consider one amp feeding the other, both running open loop, with overall
feedback, to the 1st. Now consider the case where the second amp is
configured with local feedback, to make it a unity buffer, following the 1st
amp. Naively , one might argue that the 2nd stage now has a wider bandwidth,
due to feedback, such that the "new " system might be stable, i.e. one main
rolloff due to the 1st amp. However, in realty, topologically, nothing
changes.

Some people are really good at that stuff, sort of reasonin gI mean.

But if you look at the combination MOSFET - opamp, as John Larkin is describing
in an other post in this thread, then you can treat that as one 'perfect MOSFET'.
Of course it is not really perfect, but you can make that as stable or unstable as you want.
Then basically what you do is chaining stable blocks together.
If you then apply feedback, you have to use the phase characteristic of all those,
and, as long as you prevent too high frequencies from circulating, it should be stable,
and largely independent of the load.
I note the TDA9274 has one capacitor to roll of in the driver...
This is normal, at least in the amps I designed.

The determine the stability of the system, one needs to break the
loop at a point that breaks *all* feedback paths at once. This point will be
the point directly at the output of the 2nd amp. when this is done, it is
clear that the stabiliy is still due to the total loop gain of both amops
cascaded.

You mean 'open loop gain?'
Yes, but he second amp would have gain 1.

Excepting for the special cases, e.g. , where feedback is used to neutralise
r.f amps, feedback in general, cannot be used to widen bandwidth, if the
purpose of that wider bandwidth is to achieve stability, in this type of
arrangement

I was not suggesting to widen bandwidth, although strong local feedback would of course
widen the bandwidth of a stage,

Honestly, I have to think about this a bit, maybe run it in spice.
Fact remains that the TDA9274 is the only amp I know that needs no Boucherot circuit

To wit, There is no such thing as a free lunch...

It seems to exist for US bankers ATM.

 

[RichD]

Damn right there is.

I'm thinking of the bits that attach to the
copper thingy which loops around the
magnets which make the air move.

-
Rich

 

[Eeyore]

If you loo kat the TDA9274 datahseet (the ST DMOS chip), then in the blockdiagram on page 2
you will see this is exactly what is done with the lower output MOSFET, combined with opamp
makes unity gain.
The top is already a source followwr.
The over current protection same thing, opamps.
It is likely this what makes the Boucherot network not needed.

You could do the same thing with the top MOSFET in a discrete design, if you must.

Thanks for the tip.

I only see over current protection in a lot of amps, plus thermal,
the real thing was a bridge that caused foldback current limit,
couple of resistors and diodes, makes it any load proof.

Except you don't want an audio amp to foldback, just shut down when it sees an 'impossible'
load.

Graham

 

[Eeyore]

Eeyore wrote:
John Larkin wrote:
Eeyore wrote:

Learn something about LATERAL mosfets that were designed for
audio. I've already given part number and links to data sheets.

That doesn't really matter. The transfer function only needs to be
continuous so that you can close a loop around it, and the fet
needs to be able to stand the peak power dissipation. That can
easily be done with vertical "switching" type fets. A modern FLOOD
architecture [1] works great with most any kind of fet. Lots of
things have changed in the last few decades.

John

[1] Of course you've never heard the term before. I just invented
it.

Fine. Can you elaborate some more on it ? Laterals have some truly
lovely features for audio. The only downside being a slightly
highish Ron. Not really a problem when (as I have) used as many as
6 in parallel (12 mosfets per channel / 24 per amp). They also
match beautifully with no need for source balance resistors (so
some of the Ron loss 'goes away').


An opamp per fet, closing a local loop, feedback from the fet source,
makes each fet look like a perfect unity-gain, fast, zero-offset
device.

Interesting idea. I'll have to chew that one over. I can see possible
problems fron op-amp output overshoot.

I have a simple embodiment of that concept here, done a while ago, in
virtual land;-)

http://www.kevinaylward.co.uk/ee/circuits/VeryLowDistortionAmp2.jpg

Its a push/pull gain loop around the output devices, forcing them to be
unity gain followers.

You can get lower distortion, at the expense of speed, because you have to
compensate earlier.

Common mode feedback at the second stage, allows for enormous dc/lf gain.
cascodes to allow the use of fast small transistors to do all the main work.
Emitter follower buffer to reduce the current swing in the input pair, as
per doug self. Spice says it should be in the < 0.0001% , 20Khz range,
maybe...

Not exactly short of current mirrors ! ;~)

What gave you the idea ?

Graham

 

[Eeyore]

Their selection tables show this as a 280 amp TO-220:

https://ec.irf.com/v6/en/US/adirect/ir?cmd=eneNavigation&N=0+4294841672+4294852589+4294852430

but the datasheet qualifies this as merely 75:

http://www.irf.com/product-info/datasheets/data/irf2804.pdf

They do rate it for 330 watts!

Pulsed I hope !

Graham

 

[Eeyore]

It needs a little loop tweaking, roughly....

V+
|\ |
-----------| + |
| d
| out-------+------Rg--------g
| | s
+------| - | |
| |/ Cf |
| | |
+--------------------+------Rf---------+
|
|
Rs
|
|
+------------- output rail

and an opamp that can slam the gate hard enough, not a big problem
nowadays. That whole thing becomes one ideal pseudo-fet of many. Rs
can be small, and the quiescent bias voltage can be small, because the
opamp offset voltage can be tiny. The fets share the load exactly, and
the standing bias current can be designed in, exactly, with no
adjustments.

The driver stage sees only opamps, so doesn't have to work very hard.

It's certainly interesting.

I'm wondering what the transition from one side to the other would be like i.e the crossover
point. I'm wondering if one might get a brief dead band. What would be ideal would be if the
power device never fully turned off and left say 10mA Iq.

funnily enough.

I digitize everything - heatsink temp, supply currents, load voltage -
and run a realtime simulation of fet power dissipation and resulting
junction temperature, with shutdown at, say, 140 C.

Nice if you have the budget. I found a cheap NJR / JRC multiplier I had in mind.

An adaptive fan speed would be a nice touch... no fan until it's
really needed. Maybe next time.

Oh that's dead easy, transistor on the heatsink, measure Vbe for the constant blow rate plus
add a rectified sniff of the HT line ripple. The fans powers up before the heatsink gets hot.
Ordinary DC fans work fine off a variable voltage.

(c) me. LOL.

Graham

 

[Eeyore]

However....there are some issues with using whole op-amps, rather than
discrete transistors as I have in the noted circuit in my other post. You
may need 200V+ ratings, and a very fast one at that!!!

If you could clamp the input, would that do it ?

Graham

 

[Eeyore]

I'm thinking of the bits that attach to the
copper thingy which loops around the
magnets which make the air move.

An unusual design but the copper thingy is very likely a heatsink, in
which case you're referring to the actual output devices. They in turn
usually have devices called 'drivers' which precede them, although it's
less necessary with mosfets, only for ultimate performance..

Graham