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BJT/mosfet/jfet minimal usable current?
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Kleinstein:
The Tl072 is relatively high drift and noisy at low frequency. So it's not a really good choice.
exe:
Thank you guys very much for detailed answers.

The reasons I don't want current pump are: 1) my load is floating, so low-side sensing is fine 2) it doesn't let setting compliance voltage. As for output impedance I don't really need precise output because I don't have precise equipment to measure it. I'm also not sure if Howland circuit is easy to modify the gain (= multiple ranges). Otherwise I like it.

> The below circuit at Analog Devices looks more practical

Saw it, thank you. Doesn't let setting compliance voltage either. The performance they claim is very impressive to me. It is also bipolar, which is cool, but not required for me :). My understanding is it's not that different from a typical current sink, apart from the differential amplifier and buffer to provide ground-level reference.

I probably don't really compliance voltage to be adjustable, but for fun why not. I'll try to build a circuit with output directly tied to opamp for the sake of simplicity. May be not that great because of self-heating, but at least it will give some taste of what it feels like working with nanoamps :).
exe:
Wow, I just hooked LMC662AIM opamp, ~1M resistor, made a typical current sink and it worked. At least it shows 0.09-0.11uA when measured by my DMMs. I didn't even clean the rosin flux.

I also measured current by connecting DMM with ~10M impedance in voltage mode, shows 0.98V for ~0.099 Vset (tried to dial-in 0.1V on my analog power supply). So, there is some error, but I'm more than happy, considering there is no decoupling, long leads, and thin ground wire is shared between Vsupply and Vset. Power supply was unipolar 12V.

Opamp seems to be noisy in specs, but I don't know if I should worry about 22nV*Hz^(1/2). I mostly play with bjts, leds, etc. So I don't think noise is a problem. Do you guys have a suggestion for a better opamp that wouldn't break the bank?

Next step gonna be hook it to stm32f373 with integrated DAC and ADC, add a display and it will be a beast.

UP: ah, I know why I have LMC662 on my shelf: I tried to build this circuit, but wasn't able to get 1G resistor: http://www.vk2zay.net/article/251 . So, if it's good for picoamps, then it's good for nano-amps.
Kleinstein:
As so often its a balance between current noise and voltage noise. So the best OP depends on the impedance / current range.
A few good candidates, about with increasing price tag:
TLC272, TLC277,  LMC6482, OPA197, max4238, LTC2050, OPA145

The AZ OPs (max4238, LTC2050) may be interesting for DC / low frequency. They do however have a little more bias and current noise and also higher frequency voltage noise.

For the noise, my feeling is that the normal higher frequency noise is the lesser problem. The more critical part could be 1/f noise and thus higher noise at a low frequency (e.g. 0.1 Hz). Low current experiments tend to be more on the slow side.

For the configuration, I would also consider having a TIA at the low side and just drive the high side with an OP.
David Hess:

--- Quote from: T3sl4co1l on July 19, 2019, 12:44:47 pm ---Otherwise, there are small JFETs, like PN4116 series, which would actually have trouble reaching 10mA so you might pick something not quite as tiny, but that are guaranteed for very low leakage.
--- End quote ---

PN4116s could be used in parallel or one could drive a bipolar transistor.


--- Quote ---Small BJTs have good performance, but aren't often rated for it; for example a typical 2N4401 leaks nanoamps at room temperature, but you'll never find a datasheet that agrees.
--- End quote ---

Typically common bipolar parts like the 2N5087/2N5089 would be used at low currents but you are right, the datasheets will never give guaranteed leakage specifications that low simply because testing at low currents costs too much.  The only common low current tested part I am aware of is the 2N4117 JFET and even it lacks a cutoff current specification.

The BC550/BC560 may also be suitable.  Look for bipolar parts which have specified and high hfe at low collector currents.  Also look for superbeta parts and ones intended for logging applications.  It is actually pretty amazing how low you can go.

For what it is worth, common small signal Darlington transistors can work down to at least 1 microamp.


The Howland current pump is not the only way to use an operational amplifier to make a current source.  It just applies for a bilateral current source.

Oh, and for such a large output range, I would use a bipolar anti-log converter to get rid of the resistor and range switching.  Forget trying to go from 10 milliamps to 10 nanoamps with resistors unless you want a lot of current shunt switching.  Or look up how multimeter ohms converters work if you must use that method.
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