Fast, efficient IR LEDs?

How about a 40V shunt regulator that varies less than 2V from 25C down to -70C (or less)? Is that COOL or what?

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Windows? Yuck. I use gcc.

No. It should be easy to hold that to around 0.25V without even trying. ...Jim Thompson

If Phil has to switch constantly, yes. Otherwise there could be a "spiking circuit" that swings the C with gusto.

It's inside a feedback loop that needs to have >1 MHz bandwidth, so it's more a loop stability vs bandwidth issue. There are alternatives, but they're all more complicated than what we've got. I'd probably prefer to trade off efficiency rather than speed and capacitance.

Thanks

Phil

Then you'll probably be in the market for the good stuff. This is an example of one I've used in an optical feedback situation. I only needed a little under 100MHz BW but it could have given me a lot more:

Hopefully not--that's megahertz, not gigahertz. But I do want to be able to work down at 500 nA or 1 uA.

Cheers

Phil Hobbs

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By "spike", Joerg is referring to dynamic biasing. A common technique is micropower designs. You juice up the current under conditions that need the bandwidth, then relax to the low bandwidth state. This can lead to instability, so you need to be careful. [Poles will be dynamic too.]

Thinking aloud here, I would guess the only way you can get this circuit to be high speed and low power is to use optical feedback on the led itself. If you have a photodiode that monitors the led and you see it lagging, the feedback can provide the overdrive. I guess voltage feedback at the led could do that to, but that would be tricky since the led is a nonlinear load. The optical feedback scheme isn't really low power, but it can lead to the lowest power solution. That is, you can't fight physics, but you can design with efficiency, only using high current when you need it.

Obviously. So start your own VDA! ;-)

Tim

TL431, 12V zener to soak up the 10V past Vka(max), and two resistors.

TL431 isn't usually spec'd as low as -70C though.

Tim

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Sorry; miss-typed; should have said 400V shunt regulator. Typical worst-case variation is 1V total; absolute max tot variation is 2V in a sample of 20.

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You may wish to consider a laser diode operating below critical current.

Thanks, I know that trick. Thing is, I need a 5000:1 output power range, or thereabouts--i.e. 3 uW - 15 mW. The bandwidth is going to be way more than enough at the high end, and the problem is to keep the feedback poles from crossing at a frequency where there's over-unity gain.

There are other approaches possible that require different approaches, but they require more tweaking--e.g. two ranges with two LEDs using different optical coupling fractions.

Cheers

Phil Hobbs

Or have an offset in there where the LED (or LD below lasing threshold as Joseph suggested) runs at a regulated base power level. BTDT, but in my case that was in order to remain above lasing threshold.

This gizmo is an advanced photoreceiver that maintains shot-noise-limited performance (2 dB above shot noise) from ~10 nA to

The ones uses two photodiodes wired in series (!) to get a sub-Poissonian photocurrent to null out the primary photocurrent. That's a trick I've never seen before, so I might have invented it. It obviously requires some careful feedback to keep the currents in balance, but the result is a nice linear photoreceiver with almost no additional input capacitance.

Two photodiodes in series have the same photocurrent but *half the shot noise*, so the cancellation current is actually quieter than the photocurrent, without needing resistive degeneration. (I also manage to keep all 300-kelvin resistors out of the signal path, which is key.)

The optical feedback is sort of a poor-man's photomultiplier: most of the LED light goes to another photodiode, driving an ordinary TIA which produces the output. It's a really sweet solution overall, with the one disadvantage that it needs two tweaks.

Cheers

Phil Hobbs

Luckily I never had to do that. BW was always tens of MHz but they gave me plenty of amplitude to work with. However, up there on that pedestal it had to be super low noise because we had to extract modulation.

Neat! But now you've spilled the beans and can't patent it :-(

Patents aren't worth much anyhow these days. Seems like most of what they do is trigger patent trolls who then bog down whole businesses.

I assume you mean the balancing of the two PDs in series. Is there no way to servo that? Maybe by occasionally interrupting the optical path?

Sounds fascinating! More info please ;-) ...Jim Thompson

I can patent it for the next year, at least in the USA. I might do that, we'll see.

There's a bias feedback loop that looks after that. It doesn't have to be that accurate since the PDs run at 14V of reverse bias--keeping the junction of the two PDs reasonably still is all that's required.

The tweaks are for making sure that the two photocurrents are reasonably close to begin with, and to govern the poorly specified efficiency of the LEDs. (IR LEDs have output power specs that are almost as loose as the V_T spec of your average JFET.)

You should be able to buy them in a couple of months, if all goes well. (No home should be without one, after all.) ;)

Cheers

Phil Hobbs