thanks MrAl, I am not sure how in LTspice you change the Vbe voltage of any of the NPN models......i suppose you just open the .sub file in notepad, find where it says "vbe=" and change it to whatever you want?
The circuit, as you say, appears to offer brilliant cross regulation of two (or more) strings of leds, even though only one string is actually directly regulated. There simply is no need for a separate opamp negative feedback loop around the emitter resistor of each led string.
Or am I discounting the effect of transistor heating, which ltspice doesn't do too well?...as described here, the transistor with only two leds will heat up much more than the one with three leds, and so its vbe will decrease significantly, and I reckon you could get one transistor with vbe = 0.5v, and the other with vbe = 0.9v
...this would give 44mA in one string and 56mA in the other string.....not too great. But is it so that the higher current would tend to occur in the regulated string (which only has two leds, so has a hotter transistor), and so at least the unregulated string would run *less* current than the regulated string, so therefore nothing would overheat so much?
I'd say this LED circuit is an excellent candidate for a rear/brake/indicator LED light on a car.....multiple strings could be used and they wouldn't even need regulation, because only one string needs regulation, the others are regulated by proxy, and also by the magic of the "emitter degeneration"?
I don't even think it matters too much if the pass transistors won't be well thermally coupled?
All those linear regulator IC's specifically for car rear led lights (on sale by semico's).....they seem to be a waste of money......as we see here, a couple of npn's and a single regulated string, and you're done......the regulated string isn't even that well regulated....its just 2.5V = Vbe + V(emitter resistor)...........and then I(led) = V(emitter resistor)/R(emitter)
beautifully simple, beautifully accurate, beautifully cheap.
Does anybody see a disadvantage?