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
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
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
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
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.
If you want something with written specs talk to this company:
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.
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
have a
would
electrons?
Very helpful. Thanks.
--------------------------------------- Posted through
ve a
ds <
s?
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
The data sheet spec tels you what the TESTing parameter is. 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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.