How to make 1nA (nanoamp) transistor/opamp current source when transistors leak? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: May 13, 2025, 11:13:37 pm
content: Suppose I wanted to make a transistor/opamp current source that outputs 1nA +/- 20%. (and 10nA and 100nA)A common spec for a BJT transistor is to leak 50nA at cutoff.How does one get around leakage? Using a current divider? Or maybe using a output diode and a push-pull configuration? Not really sure...

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 13, 2025, 11:48:14 pm
content: Keep it cool and be prepared to select by testing ?eghttps://fscdn.rohm.com/en/products/databook/datasheet/discrete/transistor/bipolar/bc848bw.pdfgives a typical ICBO curve of 2pA at 25'C, but it has increased to 10pA at 50'Caddit: Another measurement point from goggle, this at 1VQuote

author: Someone
author_rank: Super Contributor
timestamp: May 14, 2025, 12:02:32 am
content: Staring at a corner and missing the whole picture:cutoff current is generally specified in a particular biasing, does that apply to your intended circuit?model a current source with an idealised zero leakage part (particularly zero reverse leakage) and see how stable the result is....want 1nA? why on earth pass that through a transistor? you've got an opamp right there.

author: incf
author_rank: Frequent Contributor
timestamp: May 14, 2025, 12:05:20 am
content: I suppose one other option I had failed to consider might be to replace the transistor with a 1 gigaohm resistor and a series diode. (but diodes leak too, but maybe it might not matter)A MCP6001 or similar modern op-amp has picoamps of bias current. Perhaps it could use multiple op-amps (push-pull outputs) to produce exactly 1nA across the 1GOhm resistor.Not sure able low leakage diodes. I don't think a diode exists with max rated leakage at 70C in the nanoamp/picoamp range.edit:@Someonebeat me to it1nA is for measurement purposes in a product

author: Someone
author_rank: Super Contributor
timestamp: May 14, 2025, 12:26:05 am
content: Quote from: incf on May 14, 2025, 12:05:20 amYou're still looking in the wrong direction. Explain the actual larger problem rather than the technical barrier to your imagined solution and you might get a useful answer.Using input bias current as a current source is almost certainly not a solution to any realistic problem.

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2025, 02:34:51 am
content: Quote from: incf on May 13, 2025, 11:13:37 pmThe 50 nanoamp specification, or sometimes 25 nanoamp, comes from the automatic test equipment which cannot test to a lower current without undue time, or at all. 10s of picoamps, or even lower, is typical, but you will have to qualify and test the parts yourself.QuoteThere are a couple of ways, starting with using a 2N4117 JFET, or a small signal MOSFET. A current source can also be made with a low input bias current operational amplifier without a discrete output transistor. I think there is another tricky way using a pair of matched transistors in a translinear configuration, like a log/antilog circuit, where one transistor is used to set the Vbe of the other for a specific collector current.

author: MariuszD
author_rank: Regular Contributor
timestamp: May 14, 2025, 06:37:22 pm
content: You can buy 1 gigaohm resistors and ultra low bias current opamp and make something like this

author: peter-h
author_rank: Super Contributor
timestamp: May 15, 2025, 03:15:58 pm
content: BAS116 diode has very low leakage. I use them for input protection on hi-Z inputs (one to GND, one to VCC, both reverse-biased).Are you sourcing this current (in which case you may want a P channel device) or sinking it (more device options).I looked up the common 2N3819 but they don't spec the drain current with the device fully turned off. It might be picoamps... Similar with the small RF MOSFETs like the old BF998 (dual gate).

author: incf
author_rank: Frequent Contributor
timestamp: May 15, 2025, 03:23:52 pm
content: Quote from: peter-h on May 15, 2025, 03:15:58 pmIt can be either, but I'd prefer to source if I can.That's always the trouble, lack of published specifications. And being too small to ask for a custom batch of parts. And not being able to economically sort them ourselves at the factory. (not to mention the risk of the part suddenly and permanently changing in performance between batches)I think the dual op-amp configuration with a high value output resistor plus blocking diode can work adequately (it's only a 20% tolerance spec on the measurement)

author: mawyatt
author_rank: Super Contributor
timestamp: May 15, 2025, 04:05:55 pm
content: If your DUT is floating you can use a single low bias/offset current op-amp and a single resistor. C is optional to help reduce mains and other noise.Best

author: Terry Bites
author_rank: Super Contributor
timestamp: May 15, 2025, 04:16:30 pm
content: Here's a way to make use of an ultra low bias opamp and a single resistor to create a low current.The opamp power supplies need to be floating wrt the load.But then you will likely be running the source off of batteries or an ultra low leakage DC-DC converter to keep mains CM currents out of the system.Once you get into high value resistor territory you need to use PCB guard rings and ultra clean the board after construction.