"John Larkin"


One problem here is current sharing. Linear-mode paralleled mosfets
don't do it very well.


** Not all power mosfets are the same. One needs to be careful to
distinguish between the usual fast switching types and the lateral mosfets
( aka audio fets) as made by Hitachi and Semelab.



One of my earlier gradient amps was done with
paralleled fets (with source resistors, at least) and we wound up
making a fixture so we could match sets of fets for production... a
real pain in the sternparts.


** Switching fets, when used in linear mode, need to be carefully matched
and ballasted - while the lateral kind have very different
characteristics and do not need ballasting or careful matching.


The advantage of an opamp per fet is that
the gate drive becomes very simple and sharing is forced to be
perfect.



** Agreed.




................ Phil

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole of
rout with the fets, plus all the other op-amp parasitics. Its the old
frequency verses accuracy trade off. Anyway, its cheating to use an
op-amp:-)


But Kevin, inside every opamp are lots of little transistors! That
should make you happy.

You don't climb mountains or sky dive then? Its the personal challenge
dude. Its just not the same.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole of
rout with the fets, plus all the other op-amp parasitics. Its the old
frequency verses accuracy trade off. Anyway, its cheating to use an
op-amp:-)

No, it's making best use of available technology. Anything else isn't
engineering, it's whimsical indulgence.

But Kevin, inside every opamp are lots of little transistors! That
should make you happy.

You don't climb mountains or sky dive then? Its the personal challenge
dude. Its just not the same.

Ah, so you're going to use only valves to make a DAC? That should keep
you challenged for a while! :-)
--

Stewart Pinkerton | Music is Art - Audio is Engineering

Stewart Pinkerton wrote:
On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole
of rout with the fets, plus all the other op-amp parasitics. Its
the old frequency verses accuracy trade off. Anyway, its cheating
to use an op-amp:-)

No, it's making best use of available technology. Anything else isn't
engineering, it's whimsical indulgence.

Oh dear...you have obviously completely missed the tongue firmly in
cheek bit in this thread. Dont try and teach your granpa to suck eggs
dude.

Secondly, in this particlar example, using two transistors, as in my
referenced circuit (http://www.anasoft.co.uk/Mospoweramp.jpg) is almost
certainly the most effective way to achieve its function. Using an op
amp, as I indicated, has all sorts of issues. For instance, it would be
way more complicated to set it up to run on +/-100V supplies.

That's pretty much the fundamental reason behind my architecture. A very
simply way of putting feedback around the output devices. If you have a
simpler way, that achieves the particular characteristics I require, I
would certainly be interested in it.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole of
rout with the fets, plus all the other op-amp parasitics. Its the old
frequency verses accuracy trade off. Anyway, its cheating to use an
op-amp:-)


But Kevin, inside every opamp are lots of little transistors! That
should make you happy.

You don't climb mountains or sky dive then? Its the personal challenge
dude. Its just not the same.



But you can't keep designing the same things, over and over. As time
goes on, things that used to be serious designs become routine, and
then some spoilsport IC designer makes it into a chip, and it's time
to move up the abstraction stack, and build more complex stuff. I once
designed a 16-bit DAC that took up a big PC board and had a couple
hundred parts. Now Maxim sells better 16-bit dacs for $3.50, and I'm
using 300 of them in the waveform generator I'm designing now. We're
doing things on one board that used to cover a wall with racks.

About the only place discretes are still really compelling, and really
fun, is at the extremes of voltage, power, or speed.

John

On Sat, 20 Nov 2004 11:54:43 GMT, "Kevin Aylward"
wrote:

Stewart Pinkerton wrote:
On Sat, 20 Nov 2004 07:16:10 GMT, "Kevin Aylward"
wrote:

John Larkin wrote:
On Fri, 19 Nov 2004 19:02:29 GMT, "Kevin Aylward"
wrote:


Yes, but as I noted, there is the pole of the opamp, and the pole
of rout with the fets, plus all the other op-amp parasitics. Its
the old frequency verses accuracy trade off. Anyway, its cheating
to use an op-amp:-)

No, it's making best use of available technology. Anything else isn't
engineering, it's whimsical indulgence.

Oh dear...you have obviously completely missed the tongue firmly in
cheek bit in this thread. Dont try and teach your granpa to suck eggs
dude.

If you were old enough to be my grandpa, I'd be seriously impressed
that you were still posting! :-)

Besides, egg-sucking was grandma's job................

Secondly, in this particlar example, using two transistors, as in my
referenced circuit (http://www.anasoft.co.uk/Mospoweramp.jpg) is almost
certainly the most effective way to achieve its function. Using an op
amp, as I indicated, has all sorts of issues. For instance, it would be
way more complicated to set it up to run on +/-100V supplies.

That would be about the only real issue.

That's pretty much the fundamental reason behind my architecture. A very
simply way of putting feedback around the output devices. If you have a
simpler way, that achieves the particular characteristics I require, I
would certainly be interested in it.

No, with these voltages, it's probably close to optimum. It's not as
if PA design isn't a mature technology these days...........
--

Stewart Pinkerton | Music is Art - Audio is Engineering