Composite amps

On Mon, 24 Feb 2020 15:46:09 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-24 15:13, John Larkin wrote:
On Mon, 24 Feb 2020 14:47:33 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf

Crappy PN photodiodes and solar cells don't respond well to large
reverse bias either.

For just about anything else, zero bias is a complete crock.

With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster.
Applying reverse bias to a PIN diode can reduce its capacitance by a
factor of 7 or so, which reduces the high frequency noise by the same
factor.

Cheers

Phil Hobbs

I know of a large organization that has wasted about a million dollars
a year, since 2002, by running a lot of very expensive Hamamatsu
photodiodes at zero bias.

https://www.dropbox.com/s/wm3a3cpxa8tcarg/S8551_1.JPG?raw=1

I suspect I know the organization. ;)
They have some very good folks though.

Cheers

Phil Hobbs
Click to expand...

Price's law. They have a few brilliant people and an army of duds.

But if the brilliant people retire, or otherwise go away, it might
take a while for the world to notice.

IBM, Xerox, Kodak, Polaroid, Boeing, Motorola, RCA, DEC, Nokia.




--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
Photodiodes from Russia for similar applications -
http://www.technoexan.ru/pdf/silicon_detector/spd100uv.pdf
https://www.researchgate.net/publication/266227378_Silicon_precision_detectors_for_near_IR_visible_UV_XUV_and_soft_X-ray_spectral_range
price less than $300
http://ixbt.photo/?id=photo:1183963
Quantum noise from fatty X-ray photons is much larger than the shot noise of the dark current at the same value))
 
On 2020-02-25 01:46, plastcontrol.ru@gmail.com wrote:
Photodiodes from Russia for similar applications -
http://www.technoexan.ru/pdf/silicon_detector/spd100uv.pdf
https://www.researchgate.net/publication/266227378_Silicon_precision_detectors_for_near_IR_visible_UV_XUV_and_soft_X-ray_spectral_range
price less than $300
http://ixbt.photo/?id=photo:1183963
Quantum noise from fatty X-ray photons is much larger than the shot noise of the dark current at the same value))
Click to expand...
$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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Tuesday, February 25, 2020 at 1:46:20 AM UTC-5, plastco...@gmail.com wrote:
Photodiodes from Russia for similar applications -
http://www.technoexan.ru/pdf/silicon_detector/spd100uv.pdf
https://www.researchgate.net/publication/266227378_Silicon_precision_detectors_for_near_IR_visible_UV_XUV_and_soft_X-ray_spectral_range
price less than $300
http://ixbt.photo/?id=photo:1183963
Quantum noise from fatty X-ray photons is much larger than the shot noise of the dark current at the same value))
Click to expand...

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.
 
On Tuesday, February 25, 2020 at 7:40:26 AM UTC-5, Phil Hobbs wrote:
On 2020-02-25 01:46, plastcontrol.ru@gmail.com wrote:
Photodiodes from Russia for similar applications -
http://www.technoexan.ru/pdf/silicon_detector/spd100uv.pdf
https://www.researchgate.net/publication/266227378_Silicon_precision_detectors_for_near_IR_visible_UV_XUV_and_soft_X-ray_spectral_range
price less than $300
http://ixbt.photo/?id=photo:1183963
Quantum noise from fatty X-ray photons is much larger than the shot noise of the dark current at the same value))

$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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...

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.

https://www.digikey.com/product-detail/en/advanced-photonix/290-12-22-241/209-290-12-22-241-ND/1012520

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 :
http://ixbt.photo/?id=photo:1329848

Gate current ATF-53189 ~85nA
 
On 2020-02-25 14:13, plastcontrol.ru@gmail.com wrote:
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 :
http://ixbt.photo/?id=photo:1329848

Gate current ATF-53189 ~85nA
Click to expand...

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
2 us rise time (massively underdamped) with 2200 pF.

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
<https://electrooptical.net/News/silicon-photomultiplier-sipm-mppc-system-for-cathodoluminescence/>.
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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
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.
Click to expand...

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.

http://ixbt.photo/?id=album:30781

Why no one broke the taboo - disconnect the non-inverting input Op Amp from GND ?
 
On Tuesday, February 25, 2020 at 4:06:42 PM UTC-5, plastco...@gmail.com wrote:
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.


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.

http://ixbt.photo/?id=album:30781

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

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.
 
On Tuesday, February 25, 2020 at 2:13:52 PM UTC-5, plastco...@gmail.com wrote:
Russian photodiode for 13 nm, is expensive.
OK sure... just run with no window?
Example: 4keV photon, 4000/3.6 ~1100e, square root = 33
quantum noise 33 times greater than dark current
Click to expand...
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.)


Demo version e-PHEMT 40-140 mA (or SiGe BFU910F 2mA) boosted TIA :
http://ixbt.photo/?id=photo:1329848

Gate current ATF-53189 ~85nA
Click to expand...

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
1 us rise time. (Phil and Win do better.)
(I had slower opamp and transitors.. )
Is the yellow trace, chan. 1, the output? That looks mostly like a
single pole. Which means (to me) that there is a single capacitance slowing
things down. It could be C in parallel with the 1 meg FB resistor.
(what does step response with 100k ohms look like?)

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.
*I say that lovingly.
 
On 2020-02-25 16:06, plastcontrol.ru@gmail.com wrote:
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.


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.

http://ixbt.photo/?id=album:30781

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

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

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
The capacitance of my detectors is below 300 pF and i use negative bias.
cap 2200 pF for picture only))

Very thin dead layer is important for photons 13 nm.
Responsivity of the detector for photons 1keV - 5keV is always the same = 0.27 A/W
Very funny detector this is a CCD camera without a window.
Only you need to connect it not to the TV, but to the oscilloscope and you can immediately analyze the materials by XRF.

Phil Hobbs wrote:
That's sort of silly, because it relies on the OPA140 to complete the
loop, and that grossly limits the speed.

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.
Click to expand...

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/Îźs

http://ixbt.photo/?id=photo:1329866
 
On Monday, February 24, 2020 at 2:47:45 PM UTC-5, Phil Hobbs wrote:
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf
Click to expand...
Hi Phil, I downloaded the above and was chewing through it last night.
Great stuff!
1.) Rs (PD shunt resistance... I've always just treated this as
infinite. Can I measure leakage current and get Rs?
2.) pg 3094 has a nice discussion of 1/f noise.
3.) Are there even better low current opamps these days?
4.) Nice effective BW calcs in App A.
I would add to that, the ENBW for a two pole filter, f_3dB and Q,
is
ENBW = f_3dB * Q *pi/2 = ~1.11 f_3dB (Q=0.707.. Butterworth)

George H.

Crappy PN photodiodes and solar cells don't respond well to large
reverse bias either.

For just about anything else, zero bias is a complete crock.

With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster.
Applying reverse bias to a PIN diode can reduce its capacitance by a
factor of 7 or so, which reduces the high frequency noise by the same
factor.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...
 
On 2020-02-26 13:36, George Herold wrote:
On Monday, February 24, 2020 at 2:47:45 PM UTC-5, Phil Hobbs wrote:
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf
Hi Phil, I downloaded the above and was chewing through it last night.
Great stuff!
1.) Rs (PD shunt resistance... I've always just treated this as
infinite. Can I measure leakage current and get Rs?
2.) pg 3094 has a nice discussion of 1/f noise.
3.) Are there even better low current opamps these days?
4.) Nice effective BW calcs in App A.
I would add to that, the ENBW for a two pole filter, f_3dB and Q,
is
ENBW = f_3dB * Q *pi/2 = ~1.11 f_3dB (Q=0.707.. Butterworth)
Click to expand...

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 << kT/e, both the forward and reverse diffusion
currents are contributing to the conductance--it's just dI/dV, and so is
fairly far from zero for a large-area diode run at zero bias.

Ideally the effective shunt resistance goes up by a factor of 2 or so
with 50 mV of reverse bias, because you shut off the reverse diffusion
current without introducing significant additional leakage. That's a
super useful trick with InGaAs diodes in dim light.

re: 1/f noise
Haven't read it recently, but in photodiodes you actually can get
significant 1/f noise at zero bias, unlike in the case of resistors.

re: low current op amps

BITD I used to really like the OPA111. Its performance was easy to
remember: 1 MHz bandwidth, 1 mV offset, 1 uV/K drift, 1 pA input bias.
(See? I haven't used one in 30 years and I still remember.) ;)

It was one of the primo op amps used in early tunnelling and atomic
force microscopy. Of course it's noisy, but not nearly as bad as the
other popular super-high-Z op amp of the day, namely the LM11.

Nowadays there are much better choices, e.g. JL's fave OPA197.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Wednesday, February 26, 2020 at 4:57:32 PM UTC-5, Phil Hobbs wrote:
On 2020-02-26 13:36, George Herold wrote:
On Monday, February 24, 2020 at 2:47:45 PM UTC-5, Phil Hobbs wrote:
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf
Hi Phil, I downloaded the above and was chewing through it last night.
Great stuff!
1.) Rs (PD shunt resistance... I've always just treated this as
infinite. Can I measure leakage current and get Rs?
2.) pg 3094 has a nice discussion of 1/f noise.
3.) Are there even better low current opamps these days?
4.) Nice effective BW calcs in App A.
I would add to that, the ENBW for a two pole filter, f_3dB and Q,
is
ENBW = f_3dB * Q *pi/2 = ~1.11 f_3dB (Q=0.707.. Butterworth)


Re: noise BW
Yeah, it's like 1.22x for two noninteracting RC poles, so 1.11 for
Butterworth sounds roughly right.
Yeah well the 1.11 number is in AoE so I figured everyone knew it. :^)

Re: shunt resistance

For bias voltages << kT/e, both the forward and reverse diffusion
currents are contributing to the conductance--it's just dI/dV, and so is
fairly far from zero for a large-area diode run at zero bias.
Click to expand...
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
60 Hz crude.. so I'm not sure of these numbers.
Ideally the effective shunt resistance goes up by a factor of 2 or so
with 50 mV of reverse bias, because you shut off the reverse diffusion
current without introducing significant additional leakage. That's a
super useful trick with InGaAs diodes in dim light.

re: 1/f noise
Haven't read it recently, but in photodiodes you actually can get
significant 1/f noise at zero bias, unlike in the case of resistors.
Click to expand...
Oh, probably simple stuff you know, but it resonated with me.
1/f noise density (V^2) ~ Log(f_high/f_low)
And then if you are recording a number (P) of separate measurements.
The low freq BW ~1/P and the noise density goes as Log(P) the number
of measurements...
"Oh", I said to myself.
re: low current op amps

BITD I used to really like the OPA111. Its performance was easy to
remember: 1 MHz bandwidth, 1 mV offset, 1 uV/K drift, 1 pA input bias.
(See? I haven't used one in 30 years and I still remember.) ;)

It was one of the primo op amps used in early tunnelling and atomic
force microscopy. Of course it's noisy, but not nearly as bad as the
other popular super-high-Z op amp of the day, namely the LM11.

Nowadays there are much better choices, e.g. JL's fave OPA197.
Click to expand...

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)
Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
Click to expand...
 
On Wed, 26 Feb 2020 16:57:25 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-26 13:36, George Herold wrote:
On Monday, February 24, 2020 at 2:47:45 PM UTC-5, Phil Hobbs wrote:
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf
Hi Phil, I downloaded the above and was chewing through it last night.
Great stuff!
1.) Rs (PD shunt resistance... I've always just treated this as
infinite. Can I measure leakage current and get Rs?
2.) pg 3094 has a nice discussion of 1/f noise.
3.) Are there even better low current opamps these days?
4.) Nice effective BW calcs in App A.
I would add to that, the ENBW for a two pole filter, f_3dB and Q,
is
ENBW = f_3dB * Q *pi/2 = ~1.11 f_3dB (Q=0.707.. Butterworth)


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 << kT/e, both the forward and reverse diffusion
currents are contributing to the conductance--it's just dI/dV, and so is
fairly far from zero for a large-area diode run at zero bias.

Ideally the effective shunt resistance goes up by a factor of 2 or so
with 50 mV of reverse bias, because you shut off the reverse diffusion
current without introducing significant additional leakage. That's a
super useful trick with InGaAs diodes in dim light.

re: 1/f noise
Haven't read it recently, but in photodiodes you actually can get
significant 1/f noise at zero bias, unlike in the case of resistors.

re: low current op amps

BITD I used to really like the OPA111. Its performance was easy to
remember: 1 MHz bandwidth, 1 mV offset, 1 uV/K drift, 1 pA input bias.
(See? I haven't used one in 30 years and I still remember.) ;)

It was one of the primo op amps used in early tunnelling and atomic
force microscopy. Of course it's noisy, but not nearly as bad as the
other popular super-high-Z op amp of the day, namely the LM11.

Nowadays there are much better choices, e.g. JL's fave OPA197.

Cheers

Phil Hobbs
Click to expand...

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.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
On Wednesday, February 26, 2020 at 9:18:30 PM UTC-5, jla...@highlandsniptechnology.com wrote:
On Wed, 26 Feb 2020 16:57:25 -0500, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-02-26 13:36, George Herold wrote:
On Monday, February 24, 2020 at 2:47:45 PM UTC-5, Phil Hobbs wrote:
On 2020-02-24 11:47, George Herold wrote:
On Monday, February 24, 2020 at 6:55:54 AM UTC-5, plastco...@gmail.com wrote:
1) grounded detector, grounded source/emitter HF transistor
2) true zero-bias operation of detector
3) my circuit is simpler
I like to read books))

OK, what do you find better about zero bias operation?
I should admit that for many years I ran all my PD's at
zero bias. I thought this gave me better 'zero' light detection.
(No DC offset with no light... but the dark current from
PDs is generally pretty low.)
Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the
electrons build up in the junction and it saturates.. more light
gives no more electrons.

George H.
(who is addicted to reading... I need to find a few new fiction writers)


Zero bias is better in one respect: you can get zero leakage current.
For jobs such as very wide range, very slow photometers, that's a win.
Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders
of magnitude in photocurrent, if you don't mind being stuck with
millihertz bandwidths:

https://electrooptical.net/www/optics/eppeldauer14decadephotocurrent.pdf
Hi Phil, I downloaded the above and was chewing through it last night.
Great stuff!
1.) Rs (PD shunt resistance... I've always just treated this as
infinite. Can I measure leakage current and get Rs?
2.) pg 3094 has a nice discussion of 1/f noise.
3.) Are there even better low current opamps these days?
4.) Nice effective BW calcs in App A.
I would add to that, the ENBW for a two pole filter, f_3dB and Q,
is
ENBW = f_3dB * Q *pi/2 = ~1.11 f_3dB (Q=0.707.. Butterworth)


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 << kT/e, both the forward and reverse diffusion
currents are contributing to the conductance--it's just dI/dV, and so is
fairly far from zero for a large-area diode run at zero bias.

Ideally the effective shunt resistance goes up by a factor of 2 or so
with 50 mV of reverse bias, because you shut off the reverse diffusion
current without introducing significant additional leakage. That's a
super useful trick with InGaAs diodes in dim light.

re: 1/f noise
Haven't read it recently, but in photodiodes you actually can get
significant 1/f noise at zero bias, unlike in the case of resistors.

re: low current op amps

BITD I used to really like the OPA111. Its performance was easy to
remember: 1 MHz bandwidth, 1 mV offset, 1 uV/K drift, 1 pA input bias.
(See? I haven't used one in 30 years and I still remember.) ;)

It was one of the primo op amps used in early tunnelling and atomic
force microscopy. Of course it's noisy, but not nearly as bad as the
other popular super-high-Z op amp of the day, namely the LM11.

Nowadays there are much better choices, e.g. JL's fave OPA197.

Cheers

Phil Hobbs

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.
Click to expand...
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.
--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
Click to expand...
 
torsdag den 27. februar 2020 kl. 19.59.42 UTC+1 skrev whit3rd:
On Tuesday, February 25, 2020 at 3:55:22 PM UTC-8, George Herold wrote:
On Tuesday, February 25, 2020 at 2:13:52 PM UTC-5, plastco...@gmail.com wrote:

Russian photodiode for 13 nm, is expensive.
Example: 4keV photon,...

.. easier to measure x-ray
energy from a pulse height or total current or something.)

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.
Click to expand...

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
 
For gas sensor :
4 keV / 30 = 133ē
For scintillator - SiPM !

http://ixbt.photo/?id=photo:1330019
 
On Tuesday, February 25, 2020 at 3:55:22 PM UTC-8, George Herold wrote:
On Tuesday, February 25, 2020 at 2:13:52 PM UTC-5, plastco...@gmail.com wrote:

Russian photodiode for 13 nm, is expensive.
Example: 4keV photon,...

.. easier to measure x-ray
energy from a pulse height or total current or something.)
Click to expand...

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.
 
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