One of the application of an LED is to bias a bipolar transistor, e.g. to make up a current source, using a emitter resistor to set the current.
For perfect bias stability, the LED and the BJT should have the same tempco.
If I am not wrong, the tempco of a PN junction is -2.2mV/K.
(Current Sources and Voltage References: A Design Reference for Electronics ..., Linden T. Harrison)
Opto-couplers tend to use GaAlAs LEDs at ~880nm. Their tempco is about -1.8mV/K.
The only LED I can find in literatures that has close to -2.2mV/K is GaN at 375nm.
We have done some simple experiments with a CCS made of BJT & 630nm as well as BJT & 375nm.
The latter was more stable over a temperature range of 30°C.
Wonder if anyone else has similar experience or even better solutions ?
Thanks in advance,
Patrick
For perfect bias stability, the LED and the BJT should have the same tempco.
If I am not wrong, the tempco of a PN junction is -2.2mV/K.
(Current Sources and Voltage References: A Design Reference for Electronics ..., Linden T. Harrison)
Opto-couplers tend to use GaAlAs LEDs at ~880nm. Their tempco is about -1.8mV/K.
The only LED I can find in literatures that has close to -2.2mV/K is GaN at 375nm.
We have done some simple experiments with a CCS made of BJT & 630nm as well as BJT & 375nm.
The latter was more stable over a temperature range of 30°C.
Wonder if anyone else has similar experience or even better solutions ?
Thanks in advance,
Patrick
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https://books.google.co.th/books?id...2BAn2#v=onepage&q=diode painted black&f=false
Bob Peese section of his book....
Bob Peese section of his book....
The good old two transistor CCS does not have any problems with light interference. And if you run enough current through the main pass transistor it will even glow for you like an indicator
Ditto for a 4 transitor ccs, two PNP on top of two NPN, which adds quite a bit of dB of PSRR, but at the obvious costs of double the cheap silicon. The price difference works out to nothing in the overall scheme of things.
Same thing than when you parallel two transistor VS one you win 3dB ?
(or you can think temperatures of the junction)
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Lt. Steve, you still haven't deduced I'm not exactly smart ?
(a smaller one is a matter of wear, not wind)
Hehe. No, but I'm learning.
Ah, but it is the wind that carries the grains of sand that do the wearing, Grasshopper.
se
Hehe. You need two so you can have a "smaller one."
se
At last an area where I have real experience. I have many!
Hehe. Ironically, I have none. But I do have a little sister, so I do know of the mighty wind that can issue forth from "a smaller one."
"MOOOOOOOOOMMMM! STEVE PULLED MY HAIR AGAIN!!!"
se
Do other diode types offer benefit for biasing purpose.
Principles, Applications and Selection of Receiving Diodes
THx-RNMarsh
Principles, Applications and Selection of Receiving Diodes
THx-RNMarsh
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