Composite amps

Photodiodes from Russia for similar applications -

$300 for a garden variety PN photodiode? Nice work if you can get it. You can get an MPPC that size with a gain of a million for about the same money.

Cheers

Phil Hobbs

Seems spendy, I think the big PD's from osi-optoelectronics are ~$50.

The extra noise from x-rays is easy to understand. Each x-ray is going to come in and make a number of e-h pairs. (let's say on average there are ten electrons made per photon.) So the 'pieces' of charge causing the noise are ten times bigger and you get ten times the noise density.

George H.

Years back I bought PD's from advanced-photonix (I think they had a different name to start.. bought out or something.) Then the prices more than doubled and I went elsewhere. They still seem to be in business.

George H.

Russian photodiode for 13 nm, is expensive. Example: 4keV photon, 4000/3.6 ~1100e, square root = 33 quantum noise 33 times greater than dark current

Demo version e-PHEMT 40-140 mA (or SiGe BFU910F 2mA) boosted TIA :

Gate current ATF-53189 ~85nA

That's sort of silly, because it relies on the OPA140 to complete the loop, and that grossly limits the speed. You can do a _lot_ better than

It works a lot better if you run the pHEMT as a follower and return the anode of the PD to the pHEMT source via a capacitor. You'll need a good ferrite bead in the gate to prevent oscillation--try a BLM18BA100SN1.

The ATF53189 is obsolete, like all the 3-GHz HP/Avago/Broadcom pHEMTs.

A gizmo of mine that works that way is at . The detector capacitance is between 350 and 1400 pF, so the comparison is reasonable.

If you used a PIN diode and reverse biased it, you'd be around 100 pF rather than 8 nF for that 1 cm**2 device. That would get you a noise reduction of about 38 dB essentially free. That sort of improvement matters to some of us. ;)

Of course it might or might not work with X-rays.

Cheers

Phil Hobbs

IMHO, the follower or my wiring diagram does not make any difference - only the location of the symbol GND ! From the point of view of the photodiode, one circuit pulls on the lower terminal, the other circuit operates on the upper pin. All properties are similar. My JFET BF862 is installed more conveniently - single-supply compatible. It is very interesting to observe the voltage at the input of the amplifier.

Why no one broke the taboo - disconnect the non-inverting input Op Amp from GND ?

OK sure... just run with no window?

Yeah, I know little of x-rays. Is the depth of the depletion region such that the x-ray loses all it's energy there? You could use the noise to measure the QE of the detector. (If you knew the energy.) (maybe you already do that.) (or measure x-ray energy from the noise.. OK that's probably a silly idea.. easier to measure x-ray energy from a pulse height or total current or something.)

OK that's fun.

Dmitriy, (SED's own Russian troll* :^) 2 us rise time with 1 meg and 2200pF is nice. With a smaller PD and both bootstrap and cascode I think I got about a

If not that you should consider setting aside an afternoon to try biasing the PD. Have you read the TIA section in AoE3? (Win's book)

George H.

OK the layout of the circuit is of utmost importance. You could post a pic or something. You do realize that you've biased the photodiode by 1/2 your supply voltage. (Doesn't that also bias your DC output?.. checks trace, right ~1.3 volts below ground.) I've seen other schematics that bias the opamp rather than the photodiode. I've never liked them because I like DC =0 at zero light. Better for me is a lop-sided supply for the opamp. (and bias the PD more. At least 10V unless the manufacturer says not to.)

George H.

Because you've got a 10 MHz op amp inside the feedback loop of a 12-GHz f_max pHEMT, which is just plain silly.

There's nothing special about ground anyway.

Cheers

Phil Hobbs

The capacitance of my detectors is below 300 pF and i use negative bias. cap 2200 pF for picture only))

Very th>That's sort of silly, because it relies on the OPA140 to complete the

From bootstrapping photodiode to bootsrapping non-inverting input Op Amp. The task of the bootstrap circuit is to keep the voltage at the photodiode constant by directing the photocurrent to the amplifier. STOP ! Error detected)) The current should not go to the amplifier, but to the feedback resistor Rf ! Now you guess what needs to be changed in the circuit so that the amplifier OPA140 turns from a snail into a cheetah ? Slew Rate OPA140 20 V/?s

Hi Phil, I downloaded the above and was chewing through it last night. Great stuff!

George H.

Re: noise BW Yeah, it's like 1.22x for two noninteracting RC poles, so 1.11 for Butterworth sounds roughly right.

Re: shunt resistance

For bias voltages

Yeah well the 1.11 number is in AoE so I figured everyone knew it. :^)

Huh OK... I did this measurement on bpw34 today. At modest voltages (6 to 21V I got ~10 mV from a TIA with 100 meg FB ~0.1 nA.. at 10V rev. that's ~100 G ohm..!? OK I was also seeing ~10 mVrms of

Oh, probably simple stuff you know, but it resonated with me.

Thanks, I saw the opa111 was the last time buy on DK.. for ~$50 you can relive past. :^)

George H. Re opa197: at least spec wise doesn't the opa192 knock it away? (maybe I'm missing something?) (5uV offset and 0.2 uV/C drift)

I don't use that as a low noise high-performance amp, but as a general-purpose gumdrop. It's stable with a big output cap, 3.3u film or 100u polymer.

OK Thanks. Is there some nice opamp like this (opa197 or 192) that has a little more GBW? I'm looking at table 4X.2 (AoEx) High spped VFB's... there are a lot to choose from. 50-100 MHz would be nice.

opa1611 looks OK

George H.

At 4 keV, I'd want a proportional counter with (Xenon?) gas, or a fairly large volume ion chamber (leakage current in biased parallel-plate capacitor). Neither is a tiny solid detector.

A phosphor, of course, can generate light flashes when hit by X-rays, and photodiodes can be efficient at detecting the secondary radiation.

For gas sensor :

years ago I did X-ray fluorescence spectrometry with an Fe55 source (5.9keV) and a CCD as detector, image processed to find all pixels with empty neighbours