Lowest noise amps

In article <de79sc0283e@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:
[....]
The 2sk170 with its under-1nV spot voltage-noise level isn't best
employed in a follower circuit, because this low noise level would
force one to follow the JFET follower with an unusually-low-noise
BJT amplifier, such as one using Rohm's 0.55nV 2sd786 transistors.
That's not appealing, compared to a properly-designed common-source
JFET amplifier that provides all the advantages without the pain.
Click to expand...
Yes, an amplifier where the feedback to the source is taken from much
later in the circuit is a better way to go. You usually want to follow
the cascoded pair of JFETs with a low noise PNP stage.

--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote...
Winfield Hill wrote:
[....]
The 2sk170 with its under-1nV spot voltage-noise level isn't best
employed in a follower circuit, because this low noise level would
force one to follow the JFET follower with an unusually-low-noise
BJT amplifier, such as one using Rohm's 0.55nV 2sd786 transistors.
That's not appealing, compared to a properly-designed common-source
JFET amplifier that provides all the advantages without the pain.

Yes, an amplifier where the feedback to the source is taken from
much later in the circuit is a better way to go. You usually want
to follow the cascoded pair of JFETs with a low noise PNP stage.
Click to expand...
Actually that's a good place to switch to IC technology, such as a
fast opamp. Plus, a BJT often fits better than another JFET for the
input JFET's cascode stage.


--
Thanks,
- Win
 
Adrian Tuddenham wrote:
Ken Smith <kensmith@green.rahul.net> wrote:


In article <de4qhj01mnq@drn.newsguy.com>,
Winfield Hill <Winfield_member@newsguy.com> wrote:

Ken Smith wrote...

[...]

Watch out in the capacitive micro-pre-amp. The gate capacitance of
the very low noise JFETs is enough to effect the linearity.

At 2mA, I think I'd look at the LSK170 for my low noise FET.

I just purchased 100 Toshiba 2sk170, and measured samples at 1 and
3mA: under 1nV. I like that their input capacitance is lower than
many others with similar or lower e_n. I purchased mine from MCM.

It's easy to devise circuits that eliminate any linearity effects
from the JFET's gate capacitance changing with drain voltage.

This is not the distortion I was refering to. In a capacitive microphone,
the sound moves one plate of a capacitor by the amount I'll call X. For
small spacings between plates, the capacitance of the capacitor varies as
1/X. If the charge on the capacitor is fixed, the voltage varies as 1/C.

You end up with 1/(1/X) or simply a voltage that varies with X.

If the gate capacitance is high, you end up with 1/(Cgs + 1/X) and hence
distortion.


Although in a 'follower' circuit, the effect of the gate-to-source
capacitance is considerably reduced. (Gate-to-ground capacitance is not
affected unless you bootstrap the surrounding metalwork)

Errr...
Click to expand...
Methinks you are incorrect in saying that if the capacitor plate
moves by amount X, that the capacitor varies as 1/X.
That is not possible; moving the plate by 0.1 micrometer cannot give
a capacitance change a thousand t1mes more than a movement of 0.1
millimeter.
Your statement implies that a zero movement gives an infinite change
of capacitance (this naturally is in the limit).

So, start with the formula for *total* capaciance, where everything
is kept constant except the dielectric thickness X:
C=K/X where C is capacitance in some reasonable units like
picofarads, and K is a constant that includes fixed things like
dielectric constant, plate area, etc.
HERE AND ONLY HERE does the *total* plate seperation X change the
*total* capacitance by 1/X (aka "by definition".
*BUT*
The sound *changes* that seperation, and one wished to know how the
capacitance changes.
Hmmmm.....
Sounds like high math...maybe, hummmm, differential calculus, maybe????
So, d(1/X) = -(1/X*X)dX .
My, oh my!
Sounds like it is LINEAR - that is to say, the capacitance changes
the same way that X changes!
What a massive surprise!
.
.
.
.
.
.
.
.
.
*NOT*
 
Winfield Hill wrote:

Ken Smith wrote...

Winfield Hill wrote:
[....]

The 2sk170 with its under-1nV spot voltage-noise level isn't best
employed in a follower circuit, because this low noise level would
force one to follow the JFET follower with an unusually-low-noise
BJT amplifier, such as one using Rohm's 0.55nV 2sd786 transistors.
That's not appealing, compared to a properly-designed common-source
JFET amplifier that provides all the advantages without the pain.

Yes, an amplifier where the feedback to the source is taken from
much later in the circuit is a better way to go. You usually want
to follow the cascoded pair of JFETs with a low noise PNP stage.


Actually that's a good place to switch to IC technology, such as a
fast opamp. Plus, a BJT often fits better than another JFET for the
input JFET's cascode stage.


You mean, as an example, the lower device on the totem is the
Click to expand...
low-noise JFET, and the upper device is a bipolar transistor.
 
I read in sci.electronics.design that Robert Baer
<robertbaer@earthlink.net> wrote (in
<PcZNe.8494$Wi6.8147@newsread2.news.pas.earthlink.net>) about 'Lowest
noise amps', on Sun, 21 Aug 2005:

Methinks you are incorrect in saying that if the capacitor plate
moves by amount X, that the capacitor varies as 1/X.
That is not possible; moving the plate by 0.1 micrometer cannot give
a capacitance change a thousand t1mes more than a movement of 0.1
millimeter.
Your statement implies that a zero movement gives an infinite change
of capacitance (this naturally is in the limit).
Click to expand...
Does seem a bit unlikely.
So, start with the formula for *total* capaciance, where everything
is kept constant except the dielectric thickness X:
C=K/X where C is capacitance in some reasonable units like
picofarads, and K is a constant that includes fixed things like
dielectric constant, plate area, etc.
HERE AND ONLY HERE does the *total* plate seperation X change the
*total* capacitance by 1/X (aka "by definition".
*BUT*
The sound *changes* that seperation, and one wished to know how the
capacitance changes.
Hmmmm.....
Sounds like high math...maybe, hummmm, differential calculus, maybe????
So, d(1/X) = -(1/X*X)dX .
My, oh my!
Sounds like it is LINEAR - that is to say, the capacitance changes
the same way that X changes!
What a massive surprise!
Click to expand...
Harrumph! dC/dX = d/dX(K/X) = (K/X^2). Inverse square law! However, the
excursion is minute, even compared with the small X in a miniature
electret capsule, so the even-harmonic distortion is negligible up to,
typically, 120 dB SPL, beyond which distortion is not the greatest of
your concerns.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On 20 Aug 2005 06:06:20 -0700,
Winfield Hill <Winfield_member@newsguy.com> wrote
in Msg. <de79sc0283e@drn.newsguy.com>

A cascode configuration reduces the Crss gate-drain capacitance, and
bootstrapping the cascode off the source
Click to expand...
What does 'bootstrapping the cascode off the source' mean?

robert
 
Robert Latest wrote...
Winfield Hill wrote

A cascode configuration reduces the Crss gate-drain capacitance,
and bootstrapping the cascode off the source

What does 'bootstrapping the cascode off the source' mean?
Click to expand...
Just that. Control the BJT cascode voltage from the JFET source.


--
Thanks,
- Win
 
In article <slrndggubp.1n9.boblatest@localhost.localdomain>,
Robert Latest <boblatest@yahoo.com> wrote:
[...]
What does 'bootstrapping the cascode off the source' mean?
Click to expand...
Vcc
!
/
\
/
\
! !
\! !
!--+---/\/\/-- GND
e/! !
! !
! ---
! ---
! !
!- !
---->! !
!-+----
s !


Asssuming there is lots of feedback brought into the source of the JFET,
all three terminals of the JFET have exactly the same AC signal on them so
the input impedance is infinite.

The highest input impedance I've ever been able to do at a few KHz was
200G. To do this, absolutely everything near the first stage has to have
the same signal on it.

--
--
kensmith@rahul.net forging knowledge
 
In article <PcZNe.8494$Wi6.8147@newsread2.news.pas.earthlink.net>,
Robert Baer <robertbaer@earthlink.net> wrote:
[...]
Errr...
Methinks you are incorrect in saying that if the capacitor plate
moves by amount X, that the capacitor varies as 1/X.
That is not possible; moving the plate by 0.1 micrometer cannot give
a capacitance change a thousand t1mes more than a movement of 0.1
millimeter.
Your statement implies that a zero movement gives an infinite change
of capacitance (this naturally is in the limit).
Click to expand...
I think perhaps I was unclear in the statement. The sound changes X from
some initial value. At rest the two plates are at some distance.

This is how all of the high end microphones are in fact made.

--
--
kensmith@rahul.net forging knowledge
 
John Woodgate wrote:
I read in sci.electronics.design that Robert Baer
robertbaer@earthlink.net> wrote (in
PcZNe.8494$Wi6.8147@newsread2.news.pas.earthlink.net>) about 'Lowest
noise amps', on Sun, 21 Aug 2005:

Methinks you are incorrect in saying that if the capacitor plate
moves by amount X, that the capacitor varies as 1/X.
That is not possible; moving the plate by 0.1 micrometer cannot give
a capacitance change a thousand t1mes more than a movement of 0.1
millimeter.
Your statement implies that a zero movement gives an infinite change
of capacitance (this naturally is in the limit).


Does seem a bit unlikely.


So, start with the formula for *total* capaciance, where everything
is kept constant except the dielectric thickness X:
C=K/X where C is capacitance in some reasonable units like
picofarads, and K is a constant that includes fixed things like
dielectric constant, plate area, etc.
HERE AND ONLY HERE does the *total* plate seperation X change the
*total* capacitance by 1/X (aka "by definition".
*BUT*
The sound *changes* that seperation, and one wished to know how the
capacitance changes.
Hmmmm.....
Sounds like high math...maybe, hummmm, differential calculus, maybe????
So, d(1/X) = -(1/X*X)dX .
My, oh my!
Sounds like it is LINEAR - that is to say, the capacitance changes
the same way that X changes!
What a massive surprise!


Harrumph! dC/dX = d/dX(K/X) = (K/X^2). Inverse square law! However, the
excursion is minute, even compared with the small X in a miniature
electret capsule, so the even-harmonic distortion is negligible up to,
typically, 120 dB SPL, beyond which distortion is not the greatest of
your concerns.
I suggest you go back to college.
Click to expand...
 
Ken Smith wrote:

In article <PcZNe.8494$Wi6.8147@newsread2.news.pas.earthlink.net>,
Robert Baer <robertbaer@earthlink.net> wrote:
[...]

Errr...
Methinks you are incorrect in saying that if the capacitor plate
moves by amount X, that the capacitor varies as 1/X.
That is not possible; moving the plate by 0.1 micrometer cannot give
a capacitance change a thousand t1mes more than a movement of 0.1
millimeter.
Your statement implies that a zero movement gives an infinite change
of capacitance (this naturally is in the limit).


I think perhaps I was unclear in the statement. The sound changes X from
some initial value. At rest the two plates are at some distance.

This is how all of the high end microphones are in fact made.

Yes; what i have seen is a solid "front" with holes or slots, and a
Click to expand...
metallized mylar diaphram is stretched over the "back" of that piece.
The plastic is the insulator, and a conductive ring makes the
electrical contact to the metallized surface *and* stretches the mylar
uniformly.
It is rare that the metallized mylar is placed between two solid (but
perforated) plates.
 
I read in sci.electronics.design that Robert Baer
<robertbaer@earthlink.net> wrote (in
<BrfOe.3$5B4.0@newsread2.news.pas.earthlink.net>) about 'Lowest noise
amps', on Mon, 22 Aug 2005:

I suggest you go back to college.
Click to expand...
I suggest you explain instead of just distributing your opinions.
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
On Mon, 22 Aug 2005 07:55:24 GMT, Robert Baer
<robertbaer@earthlink.net> wrote:

Ken Smith wrote:

In article <PcZNe.8494$Wi6.8147@newsread2.news.pas.earthlink.net>,
Robert Baer <robertbaer@earthlink.net> wrote:
[...]

Errr...
Methinks you are incorrect in saying that if the capacitor plate
moves by amount X, that the capacitor varies as 1/X.
That is not possible; moving the plate by 0.1 micrometer cannot give
a capacitance change a thousand t1mes more than a movement of 0.1
millimeter.
Your statement implies that a zero movement gives an infinite change
of capacitance (this naturally is in the limit).


I think perhaps I was unclear in the statement. The sound changes X from
some initial value. At rest the two plates are at some distance.

This is how all of the high end microphones are in fact made.

Yes; what i have seen is a solid "front" with holes or slots, and a
metallized mylar diaphram is stretched over the "back" of that piece.
The plastic is the insulator, and a conductive ring makes the
electrical contact to the metallized surface *and* stretches the mylar
uniformly.
It is rare that the metallized mylar is placed between two solid (but
perforated) plates.
Click to expand...
Back in the 60's when I worked at the University of Rochester
Accoustics Lab part of my job was replacing the aluminized Mylar
diaphragms in a custom microphone designed for -3 dB @ 2 MHz (for
shock wave measurements). This was the only Mylar diaphragm
microphone in the lab, and it was a real pain because of the pathetic
stability of Mylar; the diaphragms needed to be changed once a month
or so to maintain reasonable performance. Real measurement
microphones (Bruel & Kjaer, etc.) designed for reproducible response
over a long term always use solid metal foil diaphragms. The slotted
front cover is always removed for any precision measurements as it
severely degrades the performance of the microphone; measuement
microphone are always provided with response curves with and without
the cover, the curve without being the good one, the curve with being
for harsh field applications where the cover must be left on to
protect the diaphragm.

Regardless of diaphragm material there is always an air gap between
the diaphragm and the rear electrode which changes thickness with
diaphragm movement, exactly as JW and KS have described. Perhaps some
low end mics use the Mylar as the insulator between the capacitor
plates (at the edge support), but I have not seen this design; usually
the stationary electrode is insulated from the mic body and the
aluninized Mylar or metal diaphragm (moving electrode) is connected to
the mic body. The Mylar serves only to support the aluminum moving
electrode. Parallel plate capacitor theory is a decent 1st
approximation even though the diaphragm deflects into a shape closer
to spherical (with a very large radius) than planar (the edges do not
move).

I will attempt to post a description of condenser microphones with
construction diagrams from Acoustical Engineering by Harry F. Olson
(1957) to ABSE. Construction details have changed a bit since then
but the principle of operation has not.
 
John Woodgate wrote:

I read in sci.electronics.design that Robert Baer
robertbaer@earthlink.net> wrote (in
BrfOe.3$5B4.0@newsread2.news.pas.earthlink.net>) about 'Lowest noise
amps', on Mon, 22 Aug 2005:

I suggest you go back to college.


I suggest you explain instead of just distributing your opinions.
Not an opinion.
Click to expand...
No explaination needed for anyone that can read and knows a little math.
 
Robert Baer wrote...
John Woodgate wrote:

Robert Baer wrote

I suggest you go back to college.

I suggest you explain instead of just distributing your opinions.

Not an opinion. No explaination needed for anyone that can
read and knows a little math.
Click to expand...
How about getting off your horse and joining the discussion?


--
Thanks,
- Win
 
On 23 Aug 2005 04:02:56 -0700, Winfield Hill
<Winfield_member@newsguy.com> wrote:

Robert Baer wrote...

John Woodgate wrote:

Robert Baer wrote

I suggest you go back to college.

I suggest you explain instead of just distributing your opinions.

Not an opinion. No explaination needed for anyone that can
read and knows a little math.

How about getting off your horse and joining the discussion?
Click to expand...
Methinks he is moaning about a missing minus. Perhaps picky in a
discussion of amplitude, where use of absolute value could reasonably
be presumed.

Did my PDF ever show up on ABSE?
 
Winfield Hill wrote:

Robert Baer wrote...

John Woodgate wrote:

Robert Baer wrote

I suggest you go back to college.

I suggest you explain instead of just distributing your opinions.

Not an opinion. No explaination needed for anyone that can
read and knows a little math.

How about getting off your horse and joining the discussion?

--
Thanks,
- Win
Click to expand...
My sentiments entirely !

Graham
 
I read in sci.electronics.design that Glen Walpert <gwalpert@notaxs.com>
wrote (in <ufcmg1tkmr33vivu8e7lgq3m3sb96fd3mh@4ax.com>) about 'Lowest
noise amps', on Tue, 23 Aug 2005:

Methinks he is moaning about a missing minus. Perhaps picky in a
discussion of amplitude, where use of absolute value could reasonably
be presumed.
Click to expand...
Oh, my goodness. I shall have to leave s.e.d immediately, in disgrace!
Oh, the SHAME. A missing MINUS!
--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
Glen Walpert wrote:

On 23 Aug 2005 04:02:56 -0700, Winfield Hill
Winfield_member@newsguy.com> wrote:


Robert Baer wrote...

John Woodgate wrote:


Robert Baer wrote

I suggest you go back to college.

I suggest you explain instead of just distributing your opinions.

Not an opinion. No explaination needed for anyone that can
read and knows a little math.

How about getting off your horse and joining the discussion?


Methinks he is moaning about a missing minus. Perhaps picky in a
discussion of amplitude, where use of absolute value could reasonably
be presumed.

Did my PDF ever show up on ABSE?
It is a simple differential of the equation for capacitance as a
Click to expand...
finction of the plate seperation.
The capacitance change has a simple linear relation to the change in
seperation (leaving out constant multiplier).
And someone is tying to imply that it is 1/(X*X)...
 
On Tue, 23 Aug 2005 21:36:23 +0100, John Woodgate
<jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Glen Walpert <gwalpert@notaxs.com
wrote (in <ufcmg1tkmr33vivu8e7lgq3m3sb96fd3mh@4ax.com>) about 'Lowest
noise amps', on Tue, 23 Aug 2005:

Methinks he is moaning about a missing minus. Perhaps picky in a
discussion of amplitude, where use of absolute value could reasonably
be presumed.

Oh, my goodness. I shall have to leave s.e.d immediately, in disgrace!
Oh, the SHAME. A missing MINUS!
Click to expand...
No need for such sarcasm, John. I'm sure we can all think of many
much better reasons why you should leave SED in disgrace!
 
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