Driver to drive?

"wolti_At" <wolti@sil.at> wrote ...
Hello,

I am currently working on a design where I need to amplify the signal
generated from a glass electrode and a reference electrode. Right now
I have a basic input amplifier with an instrumentation amplifier from
analog devices working. The challenging task on these designs is the
very high output resistance of the glass electrodes (up to 200Meg) and
the noise immunity. The input stage is very simple and all input
traces have a guard ring which is biased to the common mode voltage
(to reduce leakage). The inamp currents are handled by a connection to
the media which the probe is placed in.
Since this is my first design with electrodes I heard a bit around on
the net and some people seem to have a different input stage which is
based on a capacitor and an integrator. I wonder if someone has seen
such other circuits because it would be interesting for me to evaluate
other options.

Thanks for any input,
Christian Walter
Click to expand...
Apart from all the good advice already given on input amplifiers: also be
very careful with the connection cable. Normally a screened cable is used.
Most 'normal' screened cables generate electrical charge when moved or bend.
For this very high impedance use special cables are available.

And of course mounting method, connections, etc. - all must be very clean
and low leakage.

Arie de Muynck
 
Eeyore wrote:
Jorden Verwer wrote:
Eeyore wrote:
Noise is AC
Offset is DC

End of story.
Not at all. You're oversimplifying. If offset were truly DC, it wouldn't
drift.

Drift is a different phenomenon unrelated to noise. Usually thermally
caused.
Click to expand...
I meant drift of the offset. Of course drift is different from noise.
But offset really does have a lot in common with noise, when you think about
it.

Also, noise is often thermally caused as well. ;)
 
Eeyore wrote:
Jorden Verwer wrote:
Eeyore wrote:
Jorden Verwer wrote:
Eeyore wrote:
Noise is AC
Offset is DC

End of story.
Not at all. You're oversimplifying. If offset were truly DC, it
wouldn't
drift.

Drift is a different phenomenon unrelated to noise. Usually thermally
caused.
I meant drift of the offset. Of course drift is different from noise.
But offset really does have a lot in common with noise, when you think
about
it.

Not at all in any remote way shape or form.
Don't you at least agree there are many similarities between 1/f noise and
Click to expand...
offset?
 
"Vladimir Vassilevsky"

However there are few special cases when a FET output stage has an
advantage:

1) With FETs, it is simpler to control bias current, because of the
negative dependency from the temperature.
Click to expand...

** Completely wrong.

The tempco of typical (ie switching) fets is strongly positive - the gate
threshold voltage drops by about 6mV per degree C.

Bias compensation is non trivial and crucial to get right or thermal runaway
is likely.



...... Phil
 
"Jamie Morken" <jmorken@shaw.ca> wrote in message
news:cjBAk.11843$PK.3081@newsfe04.iad...
Hi,

I have some fets that I am heatsinking, and using a common aluminum bar to
press all the fets on the heatsink rather than screwing them to the
heatsink with shoulder washers individually, is there a "silpad" or
similar roll or strip of insulating material for this purpose with no
holes, that is sold in lengths?

cheers,
Jamie
Click to expand...
Certainly.

www.bergquistcompany.com

Bob
--
== All google group posts are automatically deleted due to spam ==
 
"Arny Krueger" <arnyk@hotpop.com> wrote in message
news:VtudnV-SiYK520_VnZ2dnUVZ_oTinZ2d@comcast.com...
Good
power amps sound the same and they sound like a piece of wire with gain.

There are tons of power amps that can't be distinguished from a piece of
wire with gain, while driving well-designed speakers.
Click to expand...
Ah, but since audiophools argue endlessly about cable and wire
characteristics, you need to specify exactly what TYPE of wire with gain.
:) :)

MrT.
 
"Fred Bartoli" <" "> wrote in message
news:48d22b99$0$18754$426a34cc@news.free.fr...
Please explain how you obtain even distortion products from a
symmetrical (suppose paired mosfets) push-pull.

Any symmetrical function produces odd (and no even) harmonics.
Click to expand...
But isn't that part of the problem, producing Mosfets with identically
symmetrical characteristics in N-ch and P-ch ?
Of course good circuit design can compensate for many such factors, so the
final result is what is important, not some evangelical belief, IMO.

MrT.
 
"Mr.T"
"Fred Bartoli"
Eeyore

Please explain how you obtain even distortion products from a
symmetrical (suppose paired mosfets) push-pull.

Any symmetrical function produces odd (and no even) harmonics.

But isn't that part of the problem, producing Mosfets with identically
symmetrical characteristics in N-ch and P-ch ?
Click to expand...

** For switching MOSFETs ( ie Hexfets etc ) there are no true
complimentary P and N pairs - not even close. The P ch types all have
much higher "on" resistances and lower max drain current ratings compared to
their Nch counterparts.

OTOH - lateral MOSFETs ( ie 2SK176 and 2SJ56 etc ) are near perfect
Nch and Pch compliments - varying slightly in gate capacitance.

More symmetrical in an audio output stage than nearly any BJT compliments
around.




...... Phil
 
"Kevin McMurtrie"

Time to check the specs for this decade. Visit IRF. You can get a
surface mount MOSFET in package similar to TO-220 that has 0.0008 Ohms
on resistance, 300W dissipation, 24V rating, and a 1600A surge rating.
Click to expand...

** So you can use it to replace the engine starter relay in a car.

Yawnnnnnnn ....




....... Phil
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:48D26873.19BC70AA@hotmail.com...
Arny Krueger wrote:

hahn.alan@gmail.com> wrote
RichD wrote:
Who do MOSFET sound better than bipolar, as an audio amp
output driver?

MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
odd harmonic distortin. on and on.

Not in any relevant way for audio power amps.

The first two are highly relevevant in ANY circuit using NFB. Basic
stability criteria.
Click to expand...
Only if the bandwidth and the phase margins are small relative to the target
audio bandwidth, which is unlikely.
 
"RichD" <r_delaney2001@yahoo.com> wrote
news:bbebdd9d-6f40-4db6-a49f-b84c61f8e094@w39g2000prb.googlegroups.com...
Are longitudinal and transverse the
total variety of waves? What is the
definition of a wave, anyhow?
Click to expand...
The longitudinal and transverse separately are only in math. In real media
are oscillations around almost fixed locations in ALL DIRECTIONS. Like in
water: "
Ripples on the surface of a pond are actually a combination of transverse
and longitudinal waves; therefore, the points on the surface follow orbital
paths."
In math media are ideal (eg. incompressible). In reality no such.
S*
 
"Spehro Pefhany"

Anything approaching 1600A would blow the leadwires off a TO-220.
Click to expand...


** Not if it is only for a few microseconds.

That is how the things are usually rated.



...... Phil
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in
message news:48D26A28.54C8D508@hotmail.com

Arny Krueger wrote:

This whole "amp sounds better" stuff was fully debunked
30 years ago. Good power amps sound the same and they
sound like a piece of wire with gain.

There are tons of power amps that can't be distinguished
from a piece of wire with gain, while driving
well-designed speakers.

There are quite a few amps that meet the same criteria
while driving even the weirdest speaker load.

A really good power amp will destroy a poorly-designed
speaker before it starts sounding bad, and really good
power amps aren't all that unique.

The speaker is a large part of it. Highly reactive ( i.e.
lots of inductance or capacitance ) speakers can
prematurely trigger device protection.
Click to expand...
Agreed. Speakers that require heroic amplifiers (heroic amps definitely
exist and are easy enough to obtain) are IMO not well designed.

We discovered that
EV's SX500 was particularly bad in this respect in PA for
example. Now that is certainly audible.
Click to expand...
Agreed. However we have only part of the equation - the name of the
speaker. What was the amp?

The SX 500i EDS suggests that it might be a handful for weak-kneed amps. Its
impedance curve does go down to 4 ohms at several points below 1 KHz.

http://www.electrovoice.com/documents/ev/sx500-eds.pdf

Using pairs paralleled can be a short road to amp problems.

In contrast, the sequel ZX-5 stays above 8 ohms over the same range:

http://www.electrovoice.com/documents/ev/ZX560PI_EDS.pdf

Needless to say, I have 4 ZX-5s. ;-)
 
"Kevin McMurtrie" <kevinmcm@sonic.net> wrote in message
news:kevinmcm-21B447.20064318092008@C-61-68-245-199.per.connect.net.au
In article <bN2dnYxR6YNE3k_VnZ2dnUVZ_tCdnZ2d@comcast.com>,
"Arny Krueger" <arnyk@hotpop.com> wrote:

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote
in
message news:48D22443.DDA0AA02@hotmail.com
Kevin McMurtrie wrote:

In article
Xns9B1CC001D5128damon161attbicom@127.0.0.1>, Damon
Hill <damon1SIX1@comcast.netnet> wrote:

RichD <r_delaney2001@yahoo.com> wrote in
news:b78fc9c2-fe9c-444c-8ac5-
606090de2aba@p31g2000prf.googlegroups.com:

Who do MOSFET sound better than bipolar, as an audio
amp output driver?

They do? Seems like it's possible to design good
amplifiers either way.

--Damon

Exactly - zero difference in quality capabilities.
It's usually a matter of impedance matching. Silicon
transistors have a fixed loss around 0.5 volts.
MOSFETs have a resistive loss inversely proportional
to their voltage rating. That usually makes MOSFETs
less expensive for low impedances and transistors less
expensive for high impedances.

TOTAL AND COMPLETE GARBAGE.

Agreed. Actually, what Kevin said is the exact reverse
of generally accepted practical knowledge. Bipolar is
generally more efficient when the impedances get really
low. MOSFETs were trendy for a while, but most new
power amp designs seem to have bipolar outputs.

Time to check the specs for this decade.
Click to expand...
Been there done that.

Visit IRF. You
can get a surface mount MOSFET in package similar to
TO-220 that has 0.0008 Ohms on resistance, 300W
dissipation, 24V rating, and a 1600A surge rating.
Click to expand...
The low voltage rating takes this device out of consideration for serious
audio amps for home or professional use. It may or may not be up to snuff
for automotive use.

Try coming up with a relevant example - something that can take at least 100
volts.

Now I remember why I left this newsgroup.
Click to expand...
I lack the time or bad judgment required to feel any remorse at all over
that. If you can't stand the heat - get out of the kitchen!
 
"Phil Allison" <philallison@tpg.com.au> wrote in message
news:6jgjvaF38qo5U1@mid.individual.net
"Kevin McMurtrie"


Time to check the specs for this decade. Visit IRF. You can get a
surface mount MOSFET in package similar to
TO-220 that has 0.0008 Ohms on resistance, 300W
dissipation, 24V rating, and a 1600A surge rating.


** So you can use it to replace the engine starter relay
in a car.
Yawnnnnnnn ....
Click to expand...
Actually, it is marginal even for that. Most automotive guys like devices
that are safe at higher voltages. However, the solar panel inverter guys
might love it to death.
 
"MooseFET"

Rumors aside MOSFETs are not really easier to operate in parallel.
You still need emitter / source resistors.


** Which MOSFETS ????

Amazing how so many MOOSE like idiots have no idea there are TWO kinds
!!!!!!!!!!!!!!

The kind known as " lateral " share current just fine when in parallel in
LINEAR applications with no source ballast resistors.

Septic Tank Imbecile.





...... Phil
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in
message news:48D26873.19BC70AA@hotmail.com
Arny Krueger wrote:

hahn.alan@gmail.com> wrote
RichD wrote:
Who do MOSFET sound better than bipolar, as an audio
amp output driver?

MOSFETS HAVE WIDER BANDWIDTH ,less phase shift , lower
odd harmonic distortin. on and on.

Not in any relevant way for audio power amps.

The first two are highly relevevant in ANY circuit using
NFB. Basic stability criteria.
Click to expand...
Yes, but we're talking about modern devices, not legacy devices. A ton of
very acceptable power amps were built with power devices with FT on the
order of a MHz. Every modern bipolar device designed for power amps that I
see has FT above 10 MHz.

I'm not saying that 100 KHz devices are optimal for audio, I'm saying that
in 2008, higher bandwidth than commonly available devices for audio power
amps is irrelevant.
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in
message news:48D267C0.5D34D275@hotmail.com
Arny Krueger wrote:

"RichD" <r_delaney2001@yahoo.com> wrote

Who do MOSFET sound better than bipolar, as an audio amp
output driver?

Who do? That's voodoo!

Only to be expected from Arny "Any amp with less than
0.1% THD at full power sounds the same as all other amps."
Click to expand...
As usual, you're misrepresenting me.

Nonlinearity corresponding to 0.1% THD can be audible given choice of
musical program material. So, that number is way too high. Measurements
made at just one power level, like full power, are obviously bogus.

Obviously, the amp has to be able to perform within spec while driving a
real-world tough (but not badly designed) speaker load.

That all said, the world is full of good power amps. There are some bad
ones, too.
 
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in
message news:48D3AE09.7779D150@hotmail.com
Arny Krueger wrote:

"Kevin McMurtrie" <kevinmcm@sonic.net> wrote in message

Visit IRF. You
can get a surface mount MOSFET in package similar to
TO-220 that has 0.0008 Ohms on resistance, 300W
dissipation, 24V rating, and a 1600A surge rating.

The low voltage rating takes this device out of
consideration for serious audio amps for home or
professional use. It may or may not be up to snuff for
automotive use.

Try coming up with a relevant example - something that
can take at least 100 volts.

More like 230-250V !
Click to expand...
Well, yes, now that they are so easy to obtain.

I was trying to make things easy for him.

In fact we built a lot of pretty good power amps with 60-90 volt parts, back
in the day.
 
it.
Not at all in any remote way shape or form.
Don't you at least agree there are many similarities between 1/f
noise and offset?
Click to expand...

Actually, I do. By and large, they amount to the same thing. Its all low
frequency variations. For example, if one designs a chopper amp to get low
offset, it also kills/corrects for 1/f noise as well. If one has 1/f
problems in an system, one immediately thinks about using a chopper..well I
do any way...


Kevin Aylward
www.anasoft.co.uk
SuperSpice
 
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