LED question Vf rise vs. light output

If the OP will simply lower his load resistance, he will quickly understand that his idea that load R only affects one of the transitions is incorrect. The impedance that drives the photodiode capacitance is essentially the load resistance, regardless of rise or fall. The photodiode junction acts like a current source with consequently high output impedance. Anything more complex is a waste of time. Paul Mathews

Ok, I had some nuisance work like things to do today, but I was able to do a quick test just now. ;-) With a 1M resistor under the photodiode, I adjusted the spacing between the IR LED and the photodiode so that the pulses were peaking at 5V indicating that I was fully saturating the photodiode, IIUC. Rise time on the scope was about 14uS. Changing the resistor to 100K, and readjusting the spacing so that the pulses were peaking at 5V, the rise time was still ~14uS. Changing to 10K made the gain so low that it was impossible to obtain a 5V peak, but at 500mV (the most I could get out of it) the rise time was ~1.4uS meaning that the slope (dV/dT) doesn't seem to change regardless of the load resistor I use. What say ye now? :-) Seriously though, if current has to flow thru the resistor regardless, then the capacitance of the reverse biased photodiode junction must be quite small (as in 1pF or so?).

I am going to do the current shunt thing that miso suggested, so that I can see if that affects the rise times I'm seeing.

Thanks for the advice. :-)

Ok, I'm stupid. I tried some more resistor values and with more careful adjustment of the LED and photodiode spacing, I do see changes that do seem to correspond to the resistance changes. How on earth can you get usable output from a photodiode without major amplification or using a nuclear light source?

You feed the diode to a transimpedance amp. If you keep an eye out, you can find these off the shelf since you aren't the only person on planet earth in need of amplifying a photodiode. ;-) The transimpedance on these boxes are switch selectable, often reading

10^9. I have one of these:

and also one from EG&G PAR. More gain, less bandwidth. I got the UDT for $5 at a swap meet. The EG&G was more like $35 since i bought it online.

1. You're not 'saturating the photodiode'...you're using up your source compliance. Don't.
2. If the energy you're trying to sense is extended over an area larger than your photodiode, you can use a lens to converge it.
3. Transimpedance amplifiers are fairly simple to implement. Paul Mathews

I think I understand what you mean. I was illuminating it to the point that the current it "wished" to source could not flow thru the load resistor with only 5V pushing it and actually added to the voltage at the junction. If I increased the reverse bias to 12V, I wouldn't automatically get 12V peaks on my measurements. IOW, I could get some more dynamic range out of it by using a higher reverse bias voltage, without losing sensitivity. Am I close to getting it yet?

That would certainly help. :-) I'm really not worried about range right now (I can aim the LED straight into the photodiode), I just want the fastest response that I can get right now.

I've been looking at this some, but won't I end up compromising my slew rate again? I've got some MCP6024 10MHz op-amps, but the specs claim on 7V/uS slew. I understand that is probably better than what I'm looking at with the big load resistor right now, but if I understand, it's still not going to be able to show me say a 10nS rise time (assuming that's how fast an IR LED can ramp up its brightness).

I'll play with the op-amp when I get a chance later.

Thanks for your help. :-)

1. Why do you need so much amplitude at the output? 100 mV signals are easy to see on most fast scopes.
2. High slew rate opamps are easily found.
3. The usual way to get fast response is to use a very small photodiode. Use a lens if necessary. This makes they system more directional, which is often a highly desirable characteristic. For example, it can reduce the amount of current from extended (not point) ambient sources. a 1mm x 1mm photodiode has 1% of the capacitance of a similar 10mm x 10mm photodiode.
4. Phototransistors are surprisingly linear over a few decades of photocurrent and easier to use than transimpedance amps. However, you would need to choose one with an accessible base terminal and load the base collector junction to get decent speed. This degrades sensitivity. In any case, do not approach the saturation region (small Vce) of the transistor characteristic, since it will slow down drastically if you do. Paul Mathews Paul Mathews

I should have mentioned this earlier: Do not expect your average IR emitter to have anything like 10 ns risetimes, although there are a few that can approach that figure. To get into the low nanoseconds, you generally need to go to a laser emitter. Some LEDs actually have risetimes that are nearer to 1 microsecond. Paul Mathews

I do suspect that high instantaneous current helps. One thing is that LEDs have more capacitance than many would suspect, so the usual

30-nanosecond or whatever lag has some part from getting the voltage across the capacitance to change. Keep in mind that the junction is quite thin, much thinner than the chip as a whole, and dielectric constants of LED chip materials can be high. I am under the impression that capacitance of LEDs can get well into the double digits of picofarads. I have heard of people knocking down the usual 10's-of-nanoseconds figure by using high instantaneous current and low source impedance. I also suspect active sink of charge from the LED when trying to make it turn off can shave off some nanoseconds. Also keep in mind capacitance and inductance of wire/cable/whatever leading to the LED since this is occaisionally significant.

White LEDs are likely slower due to response of the phosphor.

- Don Klipstein ( snipped-for-privacy@misty.com)

No doubt some LED driver circuits themselves have slow risetimes. All my fast LED driver experience is with current sources with lots of voltage compliance, and we still see significant rise and fall time limitations.. The point is this: many high output LEDs are actually not very fast. If you really need to modulate at more than 1 MHz or so, you will need either special emitters designed for speed or (better yet) diode lasers. We're now working with VCSEL clusters that act much like IR LEDs optically, have about 1W/A output, and can be modulated at 100 MHz and higher. Paul Mathews

I just got around to trying (PSpice) a 1N4001 rectifier diode pulsed thru 1K, 5V with 10ns risetime.

Takes 50ns to get up to 400mV... at 5us the voltage is 621mV and is still rising at the rate of 7mV/us

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
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