amplifying a sub femtoamp of current

[Arch]

I have to do broadband detection -can't go with tuned circuits.
John do you of cold JFETs which won't have freeze-out at 4 kelvin?

Sorry, I mean
"I believe this is possible to detect provided the NEEDED data acquisition
time is NOT longer than life time of the ion.":

This can be assured by detecting for along time, approx. 1 sec - the
digitizer we have goes at 1 MHz sampling rate - this makes 1 million
data points to store, not bad at all.

Also, i am looking use the transimpedance amplifier with a resistor in
feedback, described by Bob Pease in
"http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=4346".
This might help me to put a high value resistor in feedback without
compromising the bandwidth.

 

[Arch]

I have to do broadband detection -can't go with tuned circuits.
John do you know of any commercial cold JFETs which won't have
freeze-out at 4 kelvin? One which i can buy?

Sorry, I mean
"I believe this is possible to detect provided the NEEDED data acquisition
time is NOT longer than life time of the ion.":

This can be assured by detecting for along time, approx. 1 sec - the
digitizer we have goes at 1 MHz sampling rate - this makes 1 million
data points to store, not bad at all.

Also, i am looking use the transimpedance amplifier with a resistor in
feedback, described by Bob Pease in
"http://www.elecdesign.com/Articles/Index.cfm?AD=1&ArticleID=4346".
This might help me to put a high value resistor in feedback without
compromising the bandwidth.

 

[John Larkin]

I have to do broadband detection -can't go with tuned circuits.
John do you know of any commercial cold JFETs which won't have
freeze-out at 4 kelvin? One which i can buy?

It's my unofficial understanding that regular silicon jfets work cold.
I'm sure google has refs, and the Review of Scientific Instruments is
rife with stuff like that.

What do you estimate as the spindown time constant of a trapped ion? I
guess you'd want minimal resistive losses in the amp to reduce damping
of the orbit.

Really low-level charge amps use pure capacitive feedback; see Knoll's
book on radiation detectors. Or how about a jfet amp with no feedback
at all, just the floating gate on your pickup electrode?

John

 

[Arch]

It's my unofficial understanding that regular silicon jfets work cold.
I'm sure google has refs, and the Review of Scientific Instruments is
rife with stuff like that.

I have found some Ge JFETs from NASA, will try to get some. For the
time being i am thinking to use the GaAs FETs i have and measure their
leekage current. I know it has to decrease when i cool them to 4
Kelvin.

What do you estimate as the spindown time constant of a trapped ion? I
guess you'd want minimal resistive losses in the amp to reduce damping
of the orbit.

I have to check the numbers for that, if i recall correct the lmiting
factor which causes the ion orbit to decay - is collisions with other
gas molecules not due to the resistive decay of the preamp. The
detection duration is typically in the order of couple of seconds,
typically.

Really low-level charge amps use pure capacitive feedback; see Knoll's
book on radiation detectors. Or how about a jfet amp with no feedback
at all, just the floating gate on your pickup electrode?

How would the FET will be biased in this case? In my system there are 2
detection electrodes for which i am planning some sort of differential
configuration.

 

[John Perry]

I have to check the numbers for that, if i recall correct the lmiting
factor which causes the ion orbit to decay - is collisions with other
gas molecules not due to the resistive decay of the preamp. The
detection duration is typically in the order of couple of seconds,
typically.

I'm not well versed in this stuff, so please, guys, be gentle in your
flames if I'm completely out of order.

After following all the good advice on your analog signal conditioning,
it's my impression from this thread that you'll still be pretty far down
in the noise.

If that's not correct, you could simply acquire broadband data at, say,
5Mhz for several seconds and run an FFT on it. This would give you at
one shot all the ion species present in the experiment.

If you are indeed buried in noise, you could run correlations on the
same acquired data with offsets equal to the period of each expected ion
species, which should dig any ion signals out of a lot of noise.

John Perry

 

[John Larkin]

I have found some Ge JFETs from NASA, will try to get some. For the
time being i am thinking to use the GaAs FETs i have and measure their
leekage current. I know it has to decrease when i cool them to 4
Kelvin.

As I mentioned, GaAs fets are nasty dudes. The defect densities and
trapping states are awful. I'd excpect the Ge's to be leakier than
silicon, too.

I have to check the numbers for that, if i recall correct the lmiting
factor which causes the ion orbit to decay - is collisions with other
gas molecules not due to the resistive decay of the preamp. The
detection duration is typically in the order of couple of seconds,
typically.

Cool, pretty high Q once you get up to a MHz. Even so, a resistor will
add Johnson noise.

How would the FET will be biased in this case? In my system there are 2
detection electrodes for which i am planning some sort of differential
configuration.

Just use it common-source with floating gate. Gate voltage will be
close to zero, so drain current will be close to Idss, gain will be
high, and you'll boil a little bit of helium. I did an unbiased jfet
infrasonic microphone preamp once, using a GR ceramic mic element, for
listening to static tests of the Saturn V main engines, with the mics
scattered around southern Mississippi. They accused my amp design of
intermittent motorboating but eventually figured out they were picking
up the subsonic mating calls of alligators.

Who is that specialty fet company... Interfet? I think they may have
some cryo-rated jfets.

John

 

[Robert Baer]

I'm not well versed in this stuff, so please, guys, be gentle in your
flames if I'm completely out of order.

After following all the good advice on your analog signal conditioning,
it's my impression from this thread that you'll still be pretty far down
in the noise.

If that's not correct, you could simply acquire broadband data at, say,
5Mhz for several seconds and run an FFT on it. This would give you at
one shot all the ion species present in the experiment.

If you are indeed buried in noise, you could run correlations on the
same acquired data with offsets equal to the period of each expected ion
species, which should dig any ion signals out of a lot of noise.

John Perry

I am thinking he would be doing dam well to squeek out 100KHz
bandwidth...

 

[Haude Daniel]

As I mentioned, GaAs fets are nasty dudes. The defect densities and
trapping states are awful. I'd excpect the Ge's to be leakier than
silicon, too.

However, GaAs works down to 4K. Si doesn't. Si JFETs perform beautifully
at 77K though.

Just use it common-source with floating gate. Gate voltage will be
close to zero, so drain current will be close to Idss, gain will be
high, and you'll boil a little bit of helium.

Make that nitrogen.

Who is that specialty fet company... Interfet? I think they may have
some cryo-rated jfets.

Yeah, but not for LHe temperatures. I once did some literature research on
the subject, and the upshot is: Depletion-mode Si JFETs don't work at 4K
due to carrier freeze-out. Enhancement-mode Si MOSFETs work if you're
lucky. GaAs works in general.

In my drawer I have a dual JFET that is rated at 4K. It is actually
mounted on a thermally insulating stud inside a TO78 metal can package
together with a small heating resistor. It was bought 15 years ago for
$500 from a company that doesn't exist any more. Talk about "Not
recommended for new designs" ;-)

--Daniel

 

[John Larkin]

However, GaAs works down to 4K. Si doesn't. Si JFETs perform beautifully
at 77K though.


Make that nitrogen.


Yeah, but not for LHe temperatures. I once did some literature research on
the subject, and the upshot is: Depletion-mode Si JFETs don't work at 4K
due to carrier freeze-out. Enhancement-mode Si MOSFETs work if you're
lucky. GaAs works in general.

In my drawer I have a dual JFET that is rated at 4K. It is actually
mounted on a thermally insulating stud inside a TO78 metal can package
together with a small heating resistor. It was bought 15 years ago for
$500 from a company that doesn't exist any more. Talk about "Not
recommended for new designs" ;-)

--Daniel

Thanks for the update. The application would benefit from minimum
capacitance, so plumbing the signal out to 77K would be an issue.
Maybe a driven guard back from a jfet amp at LN temperature would
work.

John

 

[Arch]

Hmm, i have an option of using a 70K heat shield to mount the JFETs,
but then you add the capacitance of a meter long wire carrying the
signal from the detector to preamp. I have tried and have still not
found JFETs which can be used for liquid He temp. So, am i right in
thinking that there are no other commercial devices suited from my
applications (low noise and 4 Kelvin) other than GaAs FETs.
I have some low noise JFETs from interfet - i can make a simple preamp
and cool it to see how far it can go with reasonable performance -
though i am sure its not meant for cryogenic operation.

 

[Arch]

Hmm, i have an option of using a 70K heat shield to mount the JFETs,
but then you add the capacitance of a meter long wire carrying the
signal from the detector to preamp. I have tried and have still not
found JFETs which can be used for liquid He temp. So, am i right in
thinking that there are no other commercial devices suited from my
applications (low noise and 4 Kelvin) other than GaAs FETs.
I have some low noise JFETs from interfet - i can make a preamp with
floating gate (common source) and cool it to see how far it can go with
reasonable performance - though i am sure its not meant for cryogenic
operation.

 

[Phil Hobbs]

Hmm, i have an option of using a 70K heat shield to mount the JFETs,
but then you add the capacitance of a meter long wire carrying the
signal from the detector to preamp. I have tried and have still not
found JFETs which can be used for liquid He temp. So, am i right in
thinking that there are no other commercial devices suited from my
applications (low noise and 4 Kelvin) other than GaAs FETs.
I have some low noise JFETs from interfet - i can make a simple preamp
and cool it to see how far it can go with reasonable performance -
though i am sure its not meant for cryogenic operation.

One approach would be to attach the JFET with high thermal resistance
leads, such as fine brass wire--the thermal resistance is very high at
low temperature, so you might be able to run a normal JFET at 77K in a
4K environment without adding a significantly increased heat load. You
can set the dissipation remotely by changing VDD.

The thermal conductivity integral of stainless steel at 77K is about
200W/m, so if you could stand 50 mW dissipation, you could get 77K with
three 1-mm stainless steel wires, 1 cm long--with lots of room to reduce
these numbers if 50 mW is too much. You can't go too low, of course,
since radiation will eventually dominate.

Of course, you'd either have to apply the heat externally, or make sure
the JFET was turned on before you transfer the helium.

Cheers,

Phil Hobbs

 

[Sjouke Burry]

Hmm, i have an option of using a 70K heat shield to mount the JFETs,
but then you add the capacitance of a meter long wire carrying the
signal from the detector to preamp. I have tried and have still not
found JFETs which can be used for liquid He temp. So, am i right in
thinking that there are no other commercial devices suited from my
applications (low noise and 4 Kelvin) other than GaAs FETs.
I have some low noise JFETs from interfet - i can make a simple preamp
and cool it to see how far it can go with reasonable performance -
though i am sure its not meant for cryogenic operation.

I remember a very old article talking about a
quad CMOS NAND IC (like 74hc00) of the first
generation,used at liquid helium temperature
as an analog pre-amplifier for a scanning
detector in space.
I remember they were very suprised about the
quality.

 

[John Larkin]

One approach would be to attach the JFET with high thermal resistance
leads, such as fine brass wire--the thermal resistance is very high at
low temperature, so you might be able to run a normal JFET at 77K in a
4K environment without adding a significantly increased heat load. You
can set the dissipation remotely by changing VDD.

The thermal conductivity integral of stainless steel at 77K is about
200W/m, so if you could stand 50 mW dissipation, you could get 77K with
three 1-mm stainless steel wires, 1 cm long--with lots of room to reduce
these numbers if 50 mW is too much. You can't go too low, of course,
since radiation will eventually dominate.

Of course, you'd either have to apply the heat externally, or make sure
the JFET was turned on before you transfer the helium.

Cheers,

Phil Hobbs

People use fine manganin wire for this, too. And somebody makes fine
cryo-coax.

How about a silicon diode as a temperature sensor, and a resistor
heater, with the control circuit at 77K or room temp? Adds 2 more
wires. Below about 20K, a silicon diode's voltage drop gets huge, and
gives tons of signal vs temperature around 4K, typically biased at 10
uA.

Or maybe just monitor the jfet current as the temperature indication,
and drive the heater resistor as needed. Wow, that's a new jfet bias
scheme! Who is it that claims there are no new circuits?

I guess there's still the fundamental problem... is there enough
signal? If the thing is to be untuned, I guess it'll need an FFT to
find the orbital frequency. How long does it take to do a 2M point FFT
these days?

John

 

[Ken Smith]

Who is that specialty fet company... Interfet? I think they may have
some cryo-rated jfets.

I'd expect they do. If you show up with a bag full of money wanting
special testing of an existing part, they are happy to do it for you.

 

[vasile]

Ok, so you have a sort of mass spectrometer ?
Except the 20pF parasitical capacitance of the plate, which is the
parasitical capacitance of the iron-glass pass through assembly from
the vacuum chamber to the outside world ?
The time constant of any current to voltage converter is about RC (3RC
if you want to be more accurate). Converting say 1pA on a precise glass
isolated 1 Gohm resistor (and a very accurate electrometric OP AMP,
you'll get after the first stage about 1mV ( a reasonable value for the
future magnification) if I didn't mistake with the magnitude orders.
The parasitic capacitance will give you a time constant of about
1Gohmx20pF = 20mS.

So how you'll be able to get a bandwith of 10Khz to 1Mhz with such a
plate capacitance?


greetings,
Vasile surducan
senior engineer
National Institute for Isotopic and Molecular Technology,
Cluj-Napoca
Romania

 

[Arch]

One approach would be to attach the JFET with high thermal resistance

leads, such as fine brass wire--the thermal resistance is very high at
low temperature, so you might be able to run a normal JFET at 77K in a
4K environment without adding a significantly increased heat load. You
can set the dissipation remotely by changing VDD.

I like Phil's idea, i have Phosphor bronze wire which i w'ld use.
http://www.lakeshore.com/temp/acc/am_wirets.html
The Thermal conductivity of this wire is merely 1.6 W/m.K around 4
Kelvin.

The thermal conductivity integral of stainless steel at 77K is about
200W/m, so if you could stand 50 mW dissipation, you could get 77K with
three 1-mm stainless steel wires, 1 cm long--with lots of room to reduce
these numbers if 50 mW is too much. You can't go too low, of course,
since radiation will eventually dominate.

Of course, you'd either have to apply the heat externally, or make sure
the JFET was turned on before you transfer the helium.

I have to heat it externally as the amplifier will be inside the
cryostat well before we start to detect. For this as John said we can
use a resisitive heater and monitor the JFET current. How about that?

 

[Arch]

One approach would be to attach the JFET with high thermal resistance

leads, such as fine brass wire--the thermal resistance is very high at
low temperature, so you might be able to run a normal JFET at 77K in a
4K environment without adding a significantly increased heat load. You
can set the dissipation remotely by changing VDD.

I like Phil's idea, to run the JFETs at around 70 K in 4K environment.
i have Phosphor bronze wire which i w'ld use.
http://www.lakeshore.com/temp/acc/am_wirets.html
The Thermal conductivity of this wire is merely 1.6 W/m.K around 4
Kelvin.

The thermal conductivity integral of stainless steel at 77K is about
200W/m, so if you could stand 50 mW dissipation, you could get 77K with
three 1-mm stainless steel wires, 1 cm long--with lots of room to reduce
these numbers if 50 mW is too much. You can't go too low, of course,
since radiation will eventually dominate.

Of course, you'd either have to apply the heat externally, or make sure
the JFET was turned on before you transfer the helium.

I have to heat it externally as the amplifier will be inside the
cryostat well before we start to detect. For this as John said we can
use a resisitive heater and monitor the JFET current. How about that?

 

[Arch]

One approach would be to attach the JFET with high thermal resistance

leads, such as fine brass wire--the thermal resistance is very high at
low temperature, so you might be able to run a normal JFET at 77K in a
4K environment without adding a significantly increased heat load. You
can set the dissipation remotely by changing VDD.

I like Phil's idea, to run the JFETs at around 70 K in 4K environment.
i have Phosphor bronze wire which i w'ld use.
http://www.lakeshore.com/temp/acc/am_wirets.html
The Thermal conductivity of this wire is merely 1.6 W/m.K around 4
Kelvin.

The thermal conductivity integral of stainless steel at 77K is about
200W/m, so if you could stand 50 mW dissipation, you could get 77K with
three 1-mm stainless steel wires, 1 cm long--with lots of room to reduce
these numbers if 50 mW is too much. You can't go too low, of course,
since radiation will eventually dominate.

Of course, you'd either have to apply the heat externally, or make sure
the JFET was turned on before you transfer the helium.

I have to heat it externally as the amplifier will be inside the
cryostat well before we start to detect. For this as John said we can
use a resisitive heater and monitor the JFET current.
Doing 2M point FFT is not a big deal which yes is done to find the
frequency of the ion. Its typically done on the fly, you won't even
notice on a good PC.

 

[Phil Hobbs]

I like Phil's idea, to run the JFETs at around 70 K in 4K environment.
i have Phosphor bronze wire which i w'ld use.
http://www.lakeshore.com/temp/acc/am_wirets.html
The Thermal conductivity of this wire is merely 1.6 W/m.K around 4
Kelvin.



I have to heat it externally as the amplifier will be inside the
cryostat well before we start to detect. For this as John said we can
use a resisitive heater and monitor the JFET current. How about that?

Make sure you're using the thermal conductivity integral, and not just
the value at 4K times the temperature drop--you'll significantly
underestimate the thermal loading otherwise. Flip to the second plot in
that Lakeshore app note and make sure you understand the distinction.

Cheers,

Phil Hobbs