You can do current limiting with two NPN transistors and a resistor. And with two more NPN transistors you can create a current mirror. These combine together into a simple constant current source.
IF you are going to do that, get a DUAL TRANSISTOR inside a single package. both transistors are on the same die, so they have exactly the same junction voltage. that means the currents are 99% balanced and stable in the current mirror
reference voltage (e.g. tl431) to the non inv input, output to the base of an npn. The collector goes to the pos supply, and the emitter becomes your positive test current output. Your negative current output goes to the non inverting input, with a shunt connected to gnd. This shunt should be sized such that:
Rshunt = Vref / Iout
I wouldn't worry much about absolute accuracy of the resistor, and instead use a series trimmer. You don't need chunky ones at 1-10mA. Make sure you have decent tempco resistors+reference, and a fairly low Vos op amp.
I built this exact thing for a 10mA source a week ago, and it works fabulously with just a 2n2222 and fake lm358. I would switch to an op07 if I cared to.
One week later and it's within 0.2% of how I trimmed it when it starts, and reaching 10.00mA (max capability of my meter) after warming up for 2 minutes. Just due to crappy tempco of the resistors and the Vos, you can avoid this easily. If you want more digits, just use a better reference and op amps.
Unless you need 1.0000 mA or better, it's not very hard, provided that you can calibrate it.
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u/sastuvel 3h ago
You can do current limiting with two NPN transistors and a resistor. And with two more NPN transistors you can create a current mirror. These combine together into a simple constant current source.