LED on Photodiode step response

Crank up the bias till the diode is fully depleted, and see.

Cheers

Phil Hobbs

t- Hide quoted text -

Thanks Phil,

I can try more bias. Is there any way to know it's fully depleted? (Or do I just take it to the maximum as specified by the maker. (30 volts in this case.)

The white LED showed more of the long tail... but less of a sharp turn- on, I wonder if it's just the LED time response that I'm seeing.

George H.

Hide quoted text -

The blue from the LED will mostly be getting absorbed in the epi, so you won't see much of it. Diffusion out of the epi may be what you're seeing on the other LEDs too. Try a 900 or 940 nm LED and see if that makes a difference.

Re full depletion: I'd crank it right up to 30V, with an RC to protect it from breakdown. You can probably go significantly higher than that. Most diodes speed up amazingly when they're fully depleted--you may see the bandwidth double between 27 and 30V. Donati has a curve somewhere. (Great book, Donati, but a bit unevenly edited. Still, a sterling effort for an E2L writer.)

Cheers

Phil Hobbs

Yes, some photodiodes have response times on the order of 1-2 microseconds. One I use, a physically large, high capacitance one (~400pF under bias) is quoted as 2.5us by its manufacturers, who refer to the charge carrier transit time phenomenon you mention.

I have no idea if the charge carrier transit delay is dependent on current levels, but it probably isn't as photodiodes are such perfect transducers. I mention it to stimulate musing from someone else.

Nemo

top

ure

The

e a

am

ect

if

.net-Hide quoted text -

k,

t- Hide quoted text -

OK 30 Volt bias made no difference in the step response. (I added in two 9V batteries, I put 100 ohms in series, 0.1uF) The lack of a change in the fast response time at 30 V may not be surprising, there's a lot of excess stray capacitance in the circuit. (7 pf or so.) So I may not have noticed.

I'm going to try an IR LED. (I thought those were too slow though. ... I'll find out.)

George H.

Thanks Nemo, I sent an email to OSI (the manufacturer). They may be able to tell me something. (If I get sent to the right person!)

George H.

Hide quoted text -

You could also switch the LED on/off by shorting the LED current to ground, thereby discharging the LED capacitance. As for the photo diode, I would advise 0.5 times max, 15 volts. The factsheet of the photo diode should have a plot of capacitance versus voltage.

top

ure

The

e a

am

ect

if

.net-Hide quoted text -

Ohh, maybe that long tail is just the series 100 ohms and C of the LED? (20nF that can't be it.) I looked at the current through the LED and that was nice and square.

If OSI has that information they are keeping it well hidden on their web site. Perhaps the email will shake something out of them... (More likely I'll be ignored.)

George H.

- Hide quoted text -

net

Actually, any factsheet will do, to show the relation between capacity and voltage. Google:

shows the relation between voltage and capacity.

net

Oh, and I forgot to tell you, NEVER USE A WHITE LED!!! They use a fluorescent coating to convert blue light to white light, and that fluorescent keeps on emitting light long after the LED switches of!!!!!! Use a blue one, if you need high energy photons, besides, IR diodes have one bad property, you cant see them working.... (with your own eyes that is.)

op

Your circuit is slow.

While ago I measured the pulse response of a cheap general purpose LED=20 vs moderately good transimpedance photodetector (Fc ~ 500 MHz). The=20 timings were in 50ns range.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Would a white LED give an afterglow from the phosphor? Blue be better to try?

Grant.

.)

e top

get

cture

he

ti,

=A0The

hey

ve

ave a

I am

xpect

e if

al.net-Hidequoted text -

n-

Goes as the square root of the bias voltage?

That's Great! Is it yours? Can I give a link to it?

George H.

.)

e top

get

cture

he

ti,

=A0The

hey

ve

ave a

I am

xpect

e if

al.net-Hidequoted text -

n-

OK, I'll have to order some blue LEDs.

Thanks,

George H.

Hide quoted text -

A white LED has a phosphor that is likely slow.

Try a laser diode. They are (usually) fast.

I did encounter some laser diodes that had a weird slow step response when used at low pulse rates. They behaved, electrically and optically, as if there were an inductor in series with the diode. At high rep-rates, 1 MHz maybe, the effect disappeared.

Try looking at the voltage across the LED, and see if it's a clean step.

What's the signal path from the photodiode to the scope? Since all the LEDs seem to have similar waveforms, maybe the detector is the problem.

John

net

I can see an 850 nm laser diode!

John

ng

he top

ure

Yeah, I've got swithces, terminal blocks and IC sockets hanging on the inputs, about 7pF over the opamp(~1-3pF) and photodiode (~12pF).

Was there any long tail at a few micro-seconds? Perhaps too long for you to have looked?

text -

Increasing the pd bias voltage will sweep the carriers out faster. He could try that.

John

Yikes! Clean it up!

John