Use od LED for CCS and cascode stages

[bocka]

I don't think it's a problem with volume production, if you're only using LEDs from one manufacturer. You can change them. I've made some designs with LEDs as voltage reference, tolerances of a bjt are higher. I'm astonished how small the tolerances of a LED are. Maybe because they are selected to several intensity groups...

Parts with no second source is a nightmare.

Yes, it is. Today many comlex parts like communication controller, FPGA, memory, ADC etc. have no second or third source. Give me fits more and more

[mikek]

Originally posted by bocka


No, I highly disagree with you. If you're not using bulk LEDs but only from one manufacturer the tolerances are very small, about +/-1% I measured in a lot of 200. The TL431 is not better.

A LED provides very low noise, too. I can't see any reason why using a TL431 instead of a LED. Use a brand like a TLLR3360.

I agree completely...thanks for your input bocka..

[AndyM]

Not to forget that LEDs have a steeper I-V-characteristic than low voltage zeners or a double normal diode.

[L Daniel Rosa]

I like the complementary BJT CCS because it allows for a two wire inline regulator. I know full well that it requires tweaking, but the simplicity is it's prime appeal. There was some discussion recently at http://headwize2.powerpill.org/ubb/s...=4179&fdays=20 and there are some illustrations as well.

The '431 is an excellent component but does have some liabilities in this application. The current to the '431 would have to be regulated as well and is prone to oscillation under some conditions.

[keyne]

that is a neat circuit!
thanks!
unfortunately a bit limited in max current, because the LEDs each take about half of it.
my simulator shows impedance of about 3 meg-ohm, very nice for this simple ckt. I will keep it in mind.