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Low current source
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arivalagan13:
Hi all,
I want to test a 10^9 transimpedance gain transimpedance amplifier. both for AC and DC.
The range of currents I would like to test is between 100 pA to 0.1 nA.
Any suggestions for doing the same, please?

Regards
ArM
graybeard:
Making current sources down to 1nA is easy.  My typical go to circuit is this:



I use a JFET for low currents <1µA and a BJT for larger.  The issue you will have is leakage currents.  You will need to use a FET input op-amp, but even so the input leakage currents from the ESD protection diodes may bite you since they are typically on the order of 10s to 100s of pA.   You also need to be careful for the gate leakage current on the JFET.

You may need to use your transimpedance amplifier to measure the input leakage current of op-amps.

BTW how do you account for ESD protection diode leakage or input bias currents on your transimpedance amp?

Did you know that 0.1nA = 100pA???
Benta:

--- Quote from: graybeard on June 10, 2020, 04:59:06 pm ---Making current sources down to 1nA is easy.  My typical go to circuit is this:

I use a MOSFET or JFET for low currents <1µA and a BJT for larger.  The issue you will have is leakage currents.


--- End quote ---

Did you ever try this in practice? The drain-source leakage current is in the uA range for a MOSFET, rising to almost the mA range when hot.

BJTs are better, but still in the nA range.

Unless of course you have access to some super-duper magic MOSFETs, JFETs and BJTs.

SiliconWizard:
Yes, I can't see how you could get this into the nA territory...
TimFox:
The Keithley "picoamp current sources", such as the 261  http://web.mit.edu/8.13/8.13d/manuals/keithly-261-picoampere-source.pdf  are nothing but an accurate voltage source and a collection of high-value resistors, equivalent to those used in the Keithley electrometers that a 261 would be used to calibrate or check.  So long as the transimpedance amplifier has a negligible input voltage compared with the 261 voltage source, this is a simple solution.  If there is a substantial series resistance in the amplifier's input protection network, it will change the calibration.
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