Re: Pete Lefferts LED current source

Tim Wescott wrote:
Walt Jung wrote:
Jim Thompson wrote:
On Tue, 21 Oct 2008 16:32:42 GMT, Walt Jung <tester3@xxxxxxxxxxxx>
wrote:

John Larkin wrote:
On Tue, 21 Oct 2008 08:25:18 -0700 (PDT), miso@xxxxxxxxx wrote:

On Oct 21, 7:51 am, Walt Jung <test...@xxxxxxxxxxxx> wrote:
I am trying to establish who and when first used an LED (RED or GRN) as
a voltage reference to bias the base of a bipolar xstr as a current
source/sink. To my best knowledge, this was Pete Lefferts of NSC, back
in 1975.

1. Peter A. Lefferts, “LED Used as Voltage Reference Provides
Self-Compensating Temp Coefficient,” Electronic Design, Feb. 15, 1975,
pp. 92.

Comments welcome.

Walt Jung
LEDs were used in some audio power amps for biasing if you want a
commercial application.

An interesting side note, when the high efficiency LEDs hit the
market, some chips had difficultly driving them due to the higher "on-
voltage" (forward bias). That is, you drove them with some chips, and
the light was dimmer due to the current source running out of
compliance. When I did the MAX7219, I put in more headroom to handle
these higher voltage devices.
I had the opposite problem. We use blue led's on module front panels
as VMEbus activity indicators. The early Cree SiC parts neeeded 50 mA,
so I drove them with a couple of 74F38's, 5 volts, 27 ohms. As the
blue led's kept getting better, my customers started complaining about
being blinded by them. We're down to about 4 mA now.

Which brings up another issue... for later.

Gotta go.

John

There is some misunderstanding here. I wasn't asking about an LED *driver*, but using an LED as a Vref. Like D1 in this ckt:

+Vs >------+--------+
| |
D1 --- .-.
\ /~~> | | R1
----- | | 1K
| '-'
| |
| |
| |<' Q1
+------|
| |\
| |
| |
| |
.-. \ /
R2 | | | Iout
10K | | \ /
'-' |
| |
Gnd ------+--------+

So you're saying the LED has a -2mV/°C TC, but a larger forward?

...Jim Thompson

Yes, that is exactly what Lefferts said in the reference cited. So the xstr Vbe then gets subtracted from the LED's Vf of 1.6-1.8V, leaving a low TC drop of ~1V across the emitter R.

Walt Jung

That may depend on the particular LED used -- I'd check if I were going to use anything other than 33 year old red LEDs.


It doesn't depend much on anything except the temperature and doping level--the current dependence is really weak. (IIRC you can get up to about -3 mV/K at picoamp bias currents.)

I've used the LED/BJT follower trick often--I first saw it in probably 1980, in an app note describing a really quiet battery-powered mic preamp. Noise is usually much more important to me than small amounts of drift, so I like using forward-biased LEDs for voltage references. You get a free pilot light out of the deal, too.

Forward-biased diodes have a noise temperature of 150K at room temperature, so if you drive it reasonably hard, you can make a really quiet voltage reference this way. (I'm sure there'll be some raised eyebrows in this group over a 'voltage reference' with 150 uV/K drift. Take it up with Maxwell's Demon.)

Cheers,

Phil Hobbs
.

Relevant Pages

  • Re: Pete Lefferts LED current source
    ... a voltage reference to bias the base of a bipolar xstr as a current source/sink. ... To my best knowledge, this was Pete Lefferts of NSC, back in 1975. ... using an LED to bias a transistor ...
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  • Re: Pete Lefferts LED current source
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  • Re: Pete Lefferts LED current source
    ... a voltage reference to bias the base of a bipolar xstr as a current ... source/sink. ... “LED Used as Voltage Reference Provides ... when the high efficiency LEDs hit the ...
    (sci.electronics.design)