Low noise small signal MOSFET
LeeVit2Dweeber
implies no BJTs (hfe isn't accurate and drifts with temperature). I have a
design that should work if the DC gate current to an enhancement mode
n-channel MOSFET is < 1 pA (and, being as greedy as anyone, < 0.1 pA would
make my life easier). I tried a BSS138 (some old PDF gave gate/source
leakage as <50 pA). Noisy! Another web posting said the problem was hot
electrons. Jolly. Most EM-MOSFET spec. sheets only give leakage i as <
100 nA. Well, 1 pA is << 100 nA, but I'd like to know what the spec IS,
not what it's BETTER THAN.
Anyone have a favorite low leakage n-channel enhancement mode MOSFET
that'll work well for iDS ~ 10 microamperes and not drown in hot electrons?
Thanks.
---------------------------------------
Posted through http://www.Electronics-Related.com
John Larkin
A clean 2N7000 has gate leakage in the ballpark of 1 fA. But mosfets
are a bit noisy, so you should use a big source resistor.
We just now need a very low-noise 50 nA current source. It's into a
summing point, so a resistor would do. If we use a low-value
resistance, like 10M, and a low voltage, the resistor Johnson noise
current is horrible. 10 volts across 200M looks OK, but the resistor
can't have shot or excess noise, and nobody makes 200M metal films. We
are testing various resistors to see how noisy they are.
10 uA looks huge to us these days.
What are you making?
John
Winfield Hill
>
> I'm trying to accurately source a ~ 10 microampere current. Accurate
> implies no BJTs (hfe isn't accurate and drifts with temperature). I have
> a design that should work if the DC gate current to an enhancement mode
> n-channel MOSFET is < 1 pA (and, being as greedy as anyone, < 0.1 pA would
> make my life easier). I tried a BSS138 (some old PDF gave gate/source
> leakage as <50 pA). Noisy! Another web posting said the problem was hot
> electrons. Jolly. Most EM-MOSFET spec. sheets only give leakage i as
> < 100 nA. Well, 1 pA is << 100 nA, but I'd like to know what the spec
> IS, not what it's BETTER THAN.
>
> Anyone have a favorite low leakage n-channel enhancement mode MOSFET
> that'll work well for iDS ~ 10 microamperes and not drown in hot
> electrons?
Well, as you know, ATE machines being what they are, testing
for very low-current leakage is expensive and slow, and isn't
done unless the manufacturer thinks there's a market for it.
I'd suggest you consider a JFET, many of these have very low
worst-case gate-current specs, because there is a market.
Alternately you can just ignore the specs.
As for noise, JFETs also have lower e_n than mosfets. BTW,
did you try a larger-value source resistor? That reduces
the effect of e_n.
--
Thanks,
- Win
Vladimir Vassilevsky
LeeVit2Dweeber wrote:
> I'm trying to accurately source a ~ 10 microampere current. Accurate
> implies no BJTs (hfe isn't accurate and drifts with temperature). I have a
> design that should work if the DC gate current to an enhancement mode
> n-channel MOSFET is < 1 pA (and, being as greedy as anyone, < 0.1 pA would
> make my life easier). I tried a BSS138 (some old PDF gave gate/source
> leakage as <50 pA). Noisy! Another web posting said the problem was hot
> electrons. Jolly. Most EM-MOSFET spec. sheets only give leakage i as <
> 100 nA. Well, 1 pA is << 100 nA, but I'd like to know what the spec IS,
> not what it's BETTER THAN.
>
> Anyone have a favorite low leakage n-channel enhancement mode MOSFET
> that'll work well for iDS ~ 10 microamperes and not drown in hot electrons?
> Thanks.
The 10uA shouldn't be any problem. Consider a ~100k resistor, voltage
source and the voltage feedback loop to keep the current constant.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
whit3rd
wrote:
> I'm trying to accurately source a ~ 10 microampere current. Accurate
> implies no BJTs (hfe isn't accurate and drifts with temperature).
That analysis is flawed. 'hfe' isn't proportional to your current,
unless you
somehow have decided to start with an accurate sub-microampere
base current, and amplify it with a grounded-emitter transistor.
The base current is a (maybe) half-percent of the emitter current, and
its
drifts are only a fraction of that.
So, what are the REAL requirements? What is your accuracy target,
what
is the output voltage range, are you OK with a calibration procedure?
And, what power is available? Iit's easy to do this kind of thing
with a
good voltage reference and sense resistor, but it takes a few volts,
and the output compliance range has limits that depend on the power
supplies.
Joerg
> I'm trying to accurately source a ~ 10 microampere current. Accurate
> implies no BJTs (hfe isn't accurate and drifts with temperature). I have a
> design that should work if the DC gate current to an enhancement mode
> n-channel MOSFET is < 1 pA (and, being as greedy as anyone, < 0.1 pA would
> make my life easier). I tried a BSS138 (some old PDF gave gate/source
> leakage as <50 pA). Noisy! Another web posting said the problem was hot
> electrons. Jolly. Most EM-MOSFET spec. sheets only give leakage i as <
> 100 nA. Well, 1 pA is << 100 nA, but I'd like to know what the spec IS,
> not what it's BETTER THAN.
>
> Anyone have a favorite low leakage n-channel enhancement mode MOSFET
> that'll work well for iDS ~ 10 microamperes and not drown in hot electrons?
> Thanks.
>
If you want something with written specs talk to this company:
http://www.aldinc.com/pdf/ALD110800.pdf
http://www.aldinc.com/pdf/ALD1108xxPerformance.pdf
The example specs 10pA typical but I believe they can provide special
parts for you.
If nothing suits your needs you could consider peltier cooling. Maybe
use one of the modules from the laserdiode guys.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Phil Hobbs
How about an actual specification? You can get BJTs that are more
stable than most resistors.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Robert Baer
The lower the current that has to be tested, the longer it takes:
lower the "limit" by an order of magnitude and (without nasty tricks
that will most likely not be used), the test time is increased by an
order of magnitude.
Too damn expensive to take more than a second to test ONE parameter.
Iffen you wants two no, than tesstem yersself.
LeeVit2Dweeber
dagmarg...@yahoo.com
<scheelin@n_o_s_p_a_m.comcast.net> wrote:
> I'm trying to accurately source a ~ 10 microampere current. Accurate
> implies no BJTs (hfe isn't accurate and drifts with temperature). I have a
> design that should work if the DC gate current to an enhancement mode
> n-channel MOSFET is < 1 pA (and, being as greedy as anyone, < 0.1 pA would
> make my life easier). I tried a BSS138 (some old PDF gave gate/source
> leakage as <50 pA). Noisy! Another web posting said the problem was hot
> electrons. Jolly. Most EM-MOSFET spec. sheets only give leakage i as <
> 100 nA. Well, 1 pA is << 100 nA, but I'd like to know what the spec IS,
> not what it's BETTER THAN.
>
> Anyone have a favorite low leakage n-channel enhancement mode MOSFET
> that'll work well for iDS ~ 10 microamperes and not drown in hot electrons?
> Thanks.
You might consider an electrometer-grade J-FET. The 2n4117a was an
old stand-by.
But, 1pA max. leakage needed to make a 10uA current implies you need 1
part in 10^7 precision. Really? Wow.
A less demanding application might just use a Howland and your
favorite op amp. Cascode a FET for high speed or compliance.
--James Arthur