current source design

[artem]

This is challenge question to me , so may be you guys can help.

I am in need of current source with following characteristics :

- fixed current 5 - 10 mA
- power supply range 2 -3 V
- keep working when minimal voltage drop is (across current source) down to 200 mV ( this requirement is most important)
- simple, constituting up to 2 transistors and passives
- current accuracy no worser than 20 % over power supply and temperature (-25 +75) ranges
- does not need tuning at production time
- does not need components selection( let say CMOS transistors for strict V
thgs)
- price no more 5 cent


This current source is needed to keep LED current permanent regardless of power supply and temperature to get stable connection over the optical fiber loop. That is why i need low dropout when voltage on supply is about 2 V and closed to LED's forward voltage.

I proposed solution in another way(using feedback through fiber optical loop) and dont think that it is an elegant, other variants are component consuming or do not comply with some of the requirements .

Thanks in advance

P.S. Yet it shoud also be economic and does not consume more than 50 percent of current going to load

 

[sunking]

delta Vbe/R1 + Vbe/R2
output use cascode

 

[artem]

cascode ?? But what is minimum voltage drop (i need as low as 200 mV) if there will be 2 transistors in chain ?
May b1e i talk about different thing . Please link to reference or explanation.
Thanks