John Larkin wrote...
Right, it's more fun to do the various calculations than to run a
boring simulation. But next comes the point where one has to either
get out the drill and soldering iron, or procrastinate by firing up
Spice to test the design. I assume that's Jim's position, because
he wrote, "I'll throw it into PSpice and see how it works out, then
I might build one up." Well, that's when Spice becomes interesting.
If it can be believed, that is. Hmm, does the tool test the design,
or does the design test the tool?
Howinthehell are you gonna get the oxygen out of the lamp cord?Jim said:Jim Thompson wrote:
[snip]
[snip]I see so much trash audio here, that, in my "spare time", I decided I
would design an audio amplifier... class AB, but no bias pot
required... and, contrary to my thinking BJTs are better, it will be
power MOSFET output... the little device is for bias... all done just
to prove I can do it better ;-)
...Jim Thompson
Or are you just going to make some lame claim that it's better 'cause
you actually know what you're doing with the materials at hand?
"Sounds" good to me ;-)
I may even use lamp cord to connect the speakers.
...Jim Thompson
Howinthehell are you gonna get the oxygen out of the lamp cord?Jim said:Jim Thompson wrote:
[snip]
I see so much trash audio here, that, in my "spare time", I decided I
would design an audio amplifier... class AB, but no bias pot
required... and, contrary to my thinking BJTs are better, it will be
power MOSFET output... the little device is for bias... all done just
to prove I can do it better ;-)
...Jim Thompson
[snip]
Or are you just going to make some lame claim that it's better 'cause
you actually know what you're doing with the materials at hand?
"Sounds" good to me ;-)
I may even use lamp cord to connect the speakers.
...Jim Thompson
[snip]If you can do it better, will you kindly post your results? I've been
dicking around (excuse me, heuristically engineering) with FET amplifiers to
replace my thirty year old comp-symm bipolar designs and as yet, I haven't
been able to make anything better. Equal, yes, but FETS are a bit more
pricey than their bipolar equivalents, and unless I can justify going to the
cost of new devices, the old 5-bipolar current amplifier fed by an opamp
with feedback around the whole mess will continue to be the standard.
My constraint is a 12 volt supply (13.6 when running) but I'm allowed 5%
distortion before anybody even bats an eye. Both speaker (8 ohms) and
headphone (32 ohms) amplifiers.
Jim
John Larkin wrote...
Right, it's more fun to do the various calculations than to run a
boring simulation. But next comes the point where one has to either
get out the drill and soldering iron, or procrastinate by firing up
Spice to test the design. I assume that's Jim's position, because
he wrote, "I'll throw it into PSpice and see how it works out, then
I might build one up." Well, that's when Spice becomes interesting.
If it can be believed, that is. Hmm, does the tool test the design,
or does the design test the tool?
[snip]If you can do it better, will you kindly post your results? I've been
dicking around (excuse me, heuristically engineering) with FET amplifiers to
replace my thirty year old comp-symm bipolar designs and as yet, I haven't
been able to make anything better. Equal, yes, but FETS are a bit more
pricey than their bipolar equivalents, and unless I can justify going to the
cost of new devices, the old 5-bipolar current amplifier fed by an opamp
with feedback around the whole mess will continue to be the standard.
My constraint is a 12 volt supply (13.6 when running) but I'm allowed 5%
distortion before anybody even bats an eye. Both speaker (8 ohms) and
headphone (32 ohms) amplifiers.
Jim
I propose only to do a MOSFET amplifier design better than that done
by the proponents, NOT to show that MOSFET amplifiers are better,
because they're NOT.
On Fri, 31 Dec 2004 16:35:43 -0700, Jim Thompson
[snip]I propose only to do a MOSFET amplifier design better than that done
by the proponents, NOT to show that MOSFET amplifiers are better,
because they're NOT.
I like mosfets in power amps because they're so rugged - no second
breakdown - not to mention fast and easy to drive. I like to assign
one opamp to drive each fet,
making all sorts of drive, sharing, and
thermal problems just go away. In some cases, especially high-power
pulsed stuff, the fets wind up being cheaper than bipolars because
they don't mind being pushed hard.
Haven't used a bipolar power stage in years. One of my guys designed a
simple zener-emitter follower preregulator using bipolars. It kept
blowing up. He quit and I replaced the bips with fets, and it just
works now.
John
On Fri, 31 Dec 2004 16:35:43 -0700, Jim Thompson
[snip]I propose only to do a MOSFET amplifier design better than that done
by the proponents, NOT to show that MOSFET amplifiers are better,
because they're NOT.
I like mosfets in power amps because they're so rugged - no second
breakdown - not to mention fast and easy to drive. I like to assign
one opamp to drive each fet,
I'm curious about that configuration. Can you show me.
Sounds like load-line and/or inductive loads. I've not lost a BJT in
the field (and I used to design ignitions); not to say I haven't
killed many in the lab while designing snubbers ;-)
On Fri, 31 Dec 2004 16:35:43 -0700, Jim Thompson
[snip]
I propose only to do a MOSFET amplifier design better than that done
by the proponents, NOT to show that MOSFET amplifiers are better,
because they're NOT.
I like mosfets in power amps because they're so rugged - no second
breakdown - not to mention fast and easy to drive. I like to assign
one opamp to drive each fet,
I'm curious about that configuration. Can you show me.
Simple...
----+------
|
------------+ d
out---+------g
+--- - | s
| c |
+-------------+--r1 --+
|
r2 or whatever
|
------+------
which makes sort of a perfect follower: high Zin, millivolts of
offset, and you don't give a damn about fet matching. The C-R1 thing
can sometmes be deleted, depending. LM8261 is a nice driver amp.
Of course, when the fet blows, the opamp goes too, but opamps are
cheap.
Sounds like load-line and/or inductive loads. I've not lost a BJT in
the field (and I used to design ignitions); not to say I haven't
killed many in the lab while designing snubbers ;-)
I thing turnon surges got it sometimes, but more likely the base
zenered at turnoff. As long as you don't overdo the gate voltage, fets
are very tough.
John
Sounds like load-line and/or inductive loads. I've not lost a
BJT in the field (and I used to design ignitions); not to say
I haven't killed many in the lab while designing snubbers ;-)
That's barely a switch... show me a push-pull version ;-)
...Jim Thompson
Jim Thompson said:I see so much trash audio here, that, in my "spare time", I
decided I would design an audio amplifier... class AB, but no
bias pot required... and, contrary to my thinking BJTs are
better, it will be power MOSFET output... the little device is
for bias... all done just to prove I can do it better ;-)
Winfield Hill said:Jim, I should warn you that spice models for these VMOS FETs are
woefully inadequate, failing to correctly model the gate
spreading resistance, the important sub-threshold region, or
even the cascode JFET drain-capacitance scene for that matter.
They also lack any reliable thermal model. Modest analytical
calculations can often better predict a MOSFET's performance
under unusual circumstances.
Win. Are you following/understanding Cyril Bateman's articles
on modelling power MOSFETs in EW, and what are your opinions
on his approach?
I'm of the simulation-tests-the-design camp. "Design" is done by
math, art and experience, NOT with a simulator. I'm troubled by the
current vintage of designers, who seem to have no hands-on experience
whatsoever.
Don't universities have lab courses anymore?
Its been over three years, and I'd guess 90% of my class have never held
a soldering iron.
They do, but the impression I get (in my experience anyway) is that
universities don't really teach design very well at all. They focus more
on analysis and theoretical mathematics.
My university threw some circuits at me and said "given this, calculate
X,Y and Z". Very few lecturers seem interested in saying WHY certain
components are chosen, and what compromises are made. So when students
are faced with designing something, they often blindly fiddle with it
hoping to make it work.
Then again, I'm probably just bitter. I think its stupid that even after
2 telecomms subjects, no one has explained how to actually get a signal
from an antenna. We've done a fair amount of mathematical theory on
modulation etc but there has been no 'applied' teaching. Even a
qualitive explanation would be nice.
Tony Williams wrote...
I have the first three of his articles in a stack and expect the
fourth final article to arrive soon. I've glanced at them, but
clearly some concentrated study is in order. Hopefully he has
addressed two of my four issues above (the two most important to
audio amplifier designers). I have recommended his articles to
others (hoping they'll check them out). Such as yourself, Tony?
BJT power amplifiers are so easily destroyed by reactive loads.
Engineering education used to be very practical.
After WWII, various
forces conspired to make it more "academically respectable"
at some cost
to true engineering, which is to design.
