Basic advance question - design of photodiode amplifiers

Hi All, Just posing a little question on the design of photodiode amplifiers. It is interesting how, so often, when you look into doing something you haven't done before, you run into interesting bits of 'common knowledge' that you don't know, and have a hard time finding!

In this case, I am designing a simple color sensor. I originally was going to cook book it with a pre-made sensor IC that I2C communicated, but ran into two months of programming difficulties, non-documented features and bugs, and have finally decided to 'simplify' things, and just roll my own.

Now, National has Sensor Webench, that will basically let you take a sensor, and they will build the amplifiers you need for you. Interesting, but you don't LEARN anything. Also, if you need a sensor not on their 'list,' you are pretty muich SOL. I need a visible light photodiode, and I didn't find one that I liked on their list.

So, I decided to try rolling my own, and looking at both their examples, and in AoE, I see one interesting thing - a negative bias on the photodiode. So, my first question - how is this negative bias usually generated? And at what voltage? It is just 'assumed' that you know these things already! I am trying to build a very portable device, and wanted to use a single 3.3V supply for everything.

Thanks for any advice and good sources for more research.

Charlie

Reply to
Charlie E.
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Well, part common knowledge, part can be inferred. For instance, from the physical phenomena at work: photons generate electron-hole pairs. They're just kind of loose in the silicon, wandering around randomly. Some may recombine, generating a photon (in suitable semiconductors, e.g. GaAs, which is to say, LEDs and photodiodes are the same damn thing, which is true to a degree) or heat (silicon doesn't produce photons very well), but most are retained in their respective region (N or P). This action generates a photocurrent (at Vf =3D 0V), with some maximum voltage behind it (If =3D 0 at whatever Vf, maybe 0.5V, the current being returned through recombination or whatever).

Now, even if the material is nonlinear (which it is, being a diode junction), it stands to reason that you'll get more current at 0V than at 0.5V. It then stands to reason that, if you put a back bias on the thing, you can get even more current out. Physically, you can imagine applying an electric field, it pulls the electrons and holes apart, minimizing recombination and therefore maximising photocurrent. There is also a secondary effect, when the electric field is strong enough, electrons or holes will be accelerated to ionization potential, resulting in inelastic collisions, knocking out more electrons and holes, resulting in even more current. This is how very high gain photodiodes are operated.

There is one more advantage to back bias: junctions capacitance drops dramatically. Even if you aren't doing it to maximize photocurrent or generate avalanche currents, you can still improve speed noticably for this reason.

3.3V might be enough (although you'll probably manage more like 1.5V, needing some headroom for op-amps?), but the best way to tell is to read datasheets. If nothing else, you can put in a charge pump (minding that it may be noisy!).

Tim

Reply to
Tim Williams

I have never tried to bias for more sensitivity. I should. If the bias voltage has noise at the recorded frequency, your liable to run into more problems. I also don't know how biasing affects overall s/n ration assuming the bias is noise free.

greg

Reply to
GregS

The negative bias is to reduce the capacitance of the photodiode. You only need it if you are trying for a high speed circuit. You may well be able to use a single supply configuration. It depends on the photodiode and the speed required.

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John Devereux
Reply to
John Devereux

...and most photodiode circuits tend to be in the "high speed" category. Otherwise a phototransistor is often a better choice - except where a very non-rohs compliant CdS resistive sensor is better...photodiodes are usually only chosen when speed is of the essence (in my limited experience - we used them for looking at laser pulse risetimes...)

--
Cats, coffee, chocolate...vices to live by
Reply to
Ecnerwal

The quantum efficiency is actually pretty well independent of reverse bias.

The main reason for reverse biasing is that in a PIN device it reduces the diode capacitance by a factor of 5 to 8, which gets you the same factor improvement in bandwidth (or equivalently in high frequency SNR).

The big trick is that the bias supply has to be quieter than the input amp, because the two noise sources enter exactly the same way--basically the amp jiggles one end of the capacitor and the supply jiggles the other, so the effective voltage noise is the RMS sum of the two. Use a capacitance multiplier on the bias supply and you'll be all set.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Phototransistors...eeeeeewwwwwwwww. Friends don't let friends, and all that.

They're horrible. Okay for night lights and slow IRDA, maybe, but always a very poor choice for anything requiring decent SNR, due to (a) small active area, (b) crummy packaging so you can't concentrate the light on the small active area, (c) poor linearity, (d) poorly controlled gain, (e) excessive noise.

Other than that, they're wonderful. ;)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

I've been searching for a packaged optocoupler that's an led and a fast pin diode, without much luck. Seems like there used to be more of these. Most of the things I find are phototransistors or digital thingies.

John

Reply to
John Larkin

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Download the presenttion.

Bob

Reply to
<castlebravo242

Some OCs, e.g. 4N25, pin out the base of the phototransistor so that you can use it as a photodiode (the PD is the BC junction, but you knew that already). They're faster that way, but of course the CTR is lower by a factor of beta--it's probably 0.1-1%.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Full of misinformation, unfortunately. They perpetuate the silly notion that there's nothing you can do about the eN*Cd*Rf noise peak, whereas changing the circuit topology can help a lot. See

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Cheers,

Phil Hobbs

Reply to
Phil Hobbs

Oh, and they also perpetuate the zero bias heresy--claiming that the noise is lower at zero bias, whereas in real life this is never, ever true--the small shot noise of the very small leakage current is always dwarfed by the eN*Cd noise current due to the amplifier input noise, so reducing Cd by applying reverse bias is always a win.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Hi Bob, Thanks! I found that one this morning. Nice general data, but no math...

Charlie

Reply to
Charlie E.

Hi Phil, Thanks, looks good, but is overkill for my application. Only going to need to do a few ADCs (maybe 6-8) in 500ms, so don't need it fast. Need it cheap and accurate! ;-)

Charlie

Reply to
Charlie E.

I remember that paper ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

3V is enough bias to hugely improve the capacitance of the photodiode. You didn't say a lot about the speed the sensor needs to work at.

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makes some giant JFETs for making photopreamps on large photodiodes. Unfortunately you need a couple of car batteries to provide the Idss of them

You may only need a fairly simple design if the bandwidth is narrow. You pick the op-amp on the basis of its input noise voltage and input noise current. Match the v(noise)/I(noise) to the Xc of the photodiode at your working frequency.

Reply to
MooseFET

I disagree here, there are plenty of slower applications where you need photodiodes rather than phototransistors. In fact I don't think I've ever used a phototransistor in a real design, do people still use them? :) Last one I had was ~30 years ago as a kid.

Photodiodes can give you large active areas when needed and are extremely linear. They can still be pretty fast - even unbiased - if run into e.g. an inverting opamp circuit to cancel the capacitance.

--

John Devereux
Reply to
John Devereux

The (old) Fairchild FPT100 series were great. In those daze, most phototransistors were 2N2222s with top chopped off (metal can) and a glass lens. The FPT100 series were *designed* to BE a phototransistor; the whole die are was the base; the emitter was a dinky dot in one corner. Orders of magnitude more sensitive. One version had a clear moulded plastic lens and the other version was flat.

Reply to
Robert Baer

Photo transistiors are actually quite nice. You will find them in optocouplers for example. It al depends on the requirements. Manufacturers optocouplers seem to have no problem making the light coupling, and, as the world is digital now, who cares about linearity. And noise ;-) Lots of gain is what we need, saturation.. so who cares if it varies a bit. :-)

Reply to
panteltje

Okay, if you're building night lights or the moral equivalent thereof, go for it. Confusing high output with high sensitivity is unfortunately one of the most common mistakes in electro-optics. Assuming that noise floors are flat is another.

Cheers,

Phil Hobbs

--
Dr Philip C D Hobbs  (Just for you, Jan)
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Reply to
Phil Hobbs

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