High speed photodiode amp

This circuit does a good job of isolating the photodiode capacitance from the rest of the circuit. It may help to put a resistor in parallel with the photodiode to keep some current flowing through the transistor

-- this keeps the speed up when going back to the no-light condition, but gives you some work to do with the offset voltages.

High R = slow & sensitive.

Low R = fast & insensitive.

Do really mean 10us? That has a bandwidth in the ballpark of all of 35kHz, and you'd be hard-pressed to find an op-amp that doesn't have at least 10x that.

If you really don't need sensitivity, just connect a photodiode directly to a scope set to 50 ohms input impedance. No probe, just photodiode -> coax cable -> BNC connector on scope.

You don't need an amp at all. If you got a cheap silicon photodiode (Optek, maybe) and dump it into, say, a 50 ohm resistor, 2 mw of optical power would give about 50 mv of signal, plenty for a scope. Most pd's give 0.5 amps/watt or so.

Figure a few pF for the pd, x50 ohms, is a tau way below 1 ns. Use a coax from the pd to the scope and terminate at the scope in 50 ohms.

Back-bias the pd 5-10 volts, of course.

John

Roger,

OK, the rule of thumb for BW is roughly 2pi/tr. From your spec of rise time, I would advise you to oshoot for a BW of 630kHz. Definately do-able.

I' m not sure I understand this statement, I am assuming that you do NOT want your amplifier to oscillate, correct? What then is the intent of this spec?

to

op-amp

to

about

Possibly, but these 'exotic op-amps' are pretty cheap, and are already biased and to the most part compensated for you. You might have an easier time with the op-amp. And yes, I've designed photodiode front-ends with both transistors and op-amps.

You need some more specs though. - what gain do you need? (what's the incident power, roughly) - what is the capacitance of your photodiode? - what is its responsivity at the wavelength of your laser?

-darin

No need to back bias the diode.

Eh? The general rule-of-thumb is 0.35/tr

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Errr...

From risetime, an "optimal step response" amplifier's bandwidth is ~ 0.35/tr. For a single time-constant circuit, -3dB bw= 1/(2pi*tau). The detector bandwidth only needs to be "comfortably faster" than 35kHz (or less, if dominated by a single time constant). That should be trivially simple at these power levels (see John Larkin's posting, and note that Tim's posting probably has a reversed diode polarity).

HTH--

-f

dominated

Riiiight. I read 'risetime' to read 'reproduce step input at fundamental 1/tau plus enough harmonics to suppress Gibbs'.

Not the same ;-)

I stand corrected.

P.S. Sorry for munging your name *Robert*

It will be faster biased, but that may not matter here, in the microseconds. I'm not sure if the sensitivity is the same, pv versus biased mode, but then he has a lot of light.

John

will

Thanks, that is useful to know.

Robert Scott Ypsilanti, Michigan

Robert, contrary to what others tell, you'll have aproblem. The speed is not high speed, rather low speed, at least what we're usually used to. No, your problem is the source. CO2 at 10um requires a sensor that is sensitive in that range. Silicon isn't. Germanium isn't. A thermopile is, but has nowwhere the required bandwidth. Possibly a LN2 cooled CdHg something.

Rene

How about some sort of secondary emission, like flourescence? Remember, I have a lot of excess optical power to play with. The laser is part of an engraving system.

Robert Scott Ypsilanti, Michigan

will

John

Robert,

Rene

Indeed. I've see an antenna array with recifiers, a nice photolitho job. The recifier being a quantum dot between 2 crossed Lambda-half. At liquid helium I believe. That device would be able to be as fast as Robert wishes, but the support equipment tends to the unwieldy.

Rene

have

engraving

ah so if its an engraving system how about a spinning disc ? the amount the spot elongates with speed will give you the pulse width and you might be able to tell the risetime from the edge sharpness. ok so i havnt done the maths and it probably has to go at some rdiculous RPM lol, but lets see, 100mm disc at 1mm for 10us ... = ~20krpm , ... a cd player ?

Colin =^.^=

That is exactly the problem that I want to diagnose. We are indeed seeing "tails" on the engraving spots that should be small. So it would be nice to have a direct way of verifying the faulty fall time so we could go the laser vendor and say "Look, your laser isn't turning off fast enough".

Robert Scott Ypsilanti, Michigan

Even the light from a standard led is more than enough at a distance of