What exactly do you need this current buffer to manage - what current range and what precision ?
Truthfully, I don't have an application for this circuit yet. It caught my eye because it's simple, uses all discrete parts, and operates with the accuracy/range that I normally worry about.
Usually, my needs for any kind of amplifier or current source are in the range of hundreds of microamps to hundred of milliamps. Probably a precision of 5 or 10%. I'm not building a cutting-edge amplifier or anything; as far as I'm concerned, nanoamps don't exist haha.
Here's a good example: one time the circuit I was designing had an expected input voltage range of 5-10V, and we were driving an LED directly off that rail. I thought, "you know, it would be nice if that LED always shined at a consistent brightness regardless of input voltage", so I decided to add a couple components to keep the LED current constant-ish (4-6mA instead of 5-10mA). My final design accomplished this using a RRIO op-amp, but it kind of felt like overkill.
I like the simplicity of a solution using discrete components, but if I design something like that, I know there's going to be 0 effort put towards matching the transistors: the board house is just going to throw on the parts as it gets them. So, if I want to be confident in using a solution like this, I need to be able to figure out the worst-case error, so I can determine if that's acceptable.
These days, dual/double transistors are cheaper to mount than two singles and take up less PCB space.
Thermal tracking helps, but you also get both parts from the same vendor, and some vendors do hint at attempts to match the nominal specs. So your production spread is expected to be less.
Absolutely, I agree there's almost never a reason not to use a dual transistor package. I'm just curious how big a difference that makes, and/or how to calculate the worst-case scenario where the Vbe are on opposite extremes.
eg https://assets.nexperia.com/documents/data-sheet/BC847BPN.pdf
shows 655mV(typ) for both PNP an NPN VBE at 2mA , and the BC847BPNH-Q data gives the same typical HFE of 300 for the pairs.
Of course, the curves have differing shapes, for NPN vs PNP, so the match is not a perfect tracking match.
LRC give the same typical HFE of 290 for their LBC846BPDW
Transistors from the same lot typically match to within 10s of millivolts.
...
Having the transistors in the same package yields better thermal tracking. Some designs use separate manually matched transistors and then mount them together on the same heat sink.
When it comes to BJT amplifiers, I've frequently heard about the importance of matching transistors to each other, and how to accomplish that and keep them the same temperature. What I don't really understand yet is "how much better does your circuit become when you carefully match transistors to each other?" Or, in other words, "if I don't match anything, and the transistors have a 10 or 20C difference between them, what's the worst that could happen?"
I'm trying to improve my intuition about these kinds of circuits so I can feel more comfortable using them in the future.
That is how I do it. I take a bunch of the transistors and measure the Vbe voltages at a current which is representative of the application circuit. Matching to better than 100 microvolts is not too difficult.
A lot of analog circuit design feels like black magic to me, so it's nice to hear that at least 1 part is pretty straightforward. If you need to know the Vbe of transistors, measuring them and matching them makes total sense.