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From: Artist on 8 Feb 2010 23:44
Phil Hobbs wrote:

>> Similar, but unlike the passive photoresistor, the electromotive force
>> detector converts photons into electrical energy. In a TIA circuit it
>> can be modeled as a current source that does not require a bias.
>
> Photodiodes and photoconductors are completely different.
>
> The
>> current is proportional to the motion of a beam light across it. It is
>> very good at detecting changes in a laser light's speckled pattern
>> produced when it reflects off any but the most even of surfaces.
>
> I'd never heard of a photoelectromotive detector, but Google had.
> Interesting. From what I gather from the free literature, it's basically
> a GaAs 1-D lateral effect photodiode run at zero bias, with the output
> taken across the ends of the top layer.
>
> I haven't read any really detailed papers, because they all cost $30,
> but it relies on the carrier density responding slowly to changes in the
> illumination pattern. It's made of GaAs rather than silicon, which I'm
> guessing is because the hole mobility in GaAs is so very low, which
> essentially glues the charge density in place. (Silicon would respond
> much faster.)
>
> For static illumination, the slow carrier diffusion makes the junction E
> field flatten out locally, so the charges all recombine, and the output
> current is zero. When the illumination changes, though, you have
> photocarriers being generated in regions where the net current is
> suddenly nonzero. These carriers produce a displacement current
> immediately (not when they arrive at the electrodes), and the carriers
> still flowing in the previously illuminated region produce a net
> displacement current of the opposite sign. The two sum to zero, but
> they're spatially offset from each other, which produces a net current
> from the outputs. Like everything else in nature, it's linear for small
> displacements.
>
> It's a cute technique, which reminds me a bit of a photorefractive phase
> conjugate mirror, in which the beam interference pattern forms a
> slowly-varying hologram that corrects for phase and pointing errors.
>
> A few things I'm not clear on:
> (1) There has to be a junction in there somewhere, because it works at
> zero bias and can produce current in both directions--which in a
> symmetric structure requires a third electrode.
>
> (2) Splitting the photocarriers between the ends seems to require a
> high-resistance epi layer, which would tend to make the response slow or
> else very noisy, like a lateral effect cell--the full Johnson noise
> current of the epi layer appears in the output.
>
> (3) The diffusion and recombination currents both have full shot noise,
> and the forward-biased regions will have a very low resistance. This
> will seemingly contribute a lot of noise to the output--the noise will
> be that of the full beam's photocurrent, not merely that of the small
> difference current.
>
> So in general this doesn't sound like a low-noise device, although it's
> certainly convenient for speckle measurements--the usual alternative
> method is TV interferometry, in which you cross-correlate the speckle
> patterns before and after some change (e.g. pressurizing a tank). TV
> interferometry is slow, since it takes a minimum of two video frame times.
>
>> Right now my plan is get two of the lowest noise JFET or CMOS input op
>> amps I can find that satisfy the bandwidth requirement. One will be
>> used for the feedback resistor and the other will do the bootstrap.
>
> It would be worth finding out what the noise level of the device itself
> is. If I'm mistaken and it's actually quiet, you'd be better off making
> the bootstrap out of a single-ended BF862 JFET rather than an op amp.
> Otherwise a couple of op amps is probably fine (one of my favourites is
> the ADA4817-1).
>
> Cheers
>
> Phil Hobbs
>
>

This project had a break for awhile but is now revived. I am given new
specifications to model the emf detector with. These are 100kOhm with a
parallel capacitance of 1pf. This is a much easier task.

There was discussion in this thread about other methods of noise
reduction. The goal of using a bootstrap was primarily to get the
bandwidth. The low impedance of the emf detector meant bandwidth
challenges because to a first approximation that neglects capacitances
the bandwidth will be the GBW of the op amp divided by the noise gain.

I like the ADA4817-1. Using it may make possible doing this without
bootstrapping.

Given the 1pF parallel capacitance in the model a bootstrap would simply
add as much capacitance as it removes and add noise.

The bootstrap would also increase the equivalent resistance of the emf
detector. But the resistance in the model is high enough now, and the
GBW of the ADA4817-1 is high enough, that I do not see a need.

The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am not
going to be able to do this in one stage. Even before considering the
capacitances from the virtual ground to actual ground the parallel
capacitance of 2Mohm to get a 10MHz bandwidth would have to be 8fF. That
is not going to happen.

Whats more, because of the capacitances on the virtual ground the
feedback resistor must have a parallel capacitor for stability.

To reduce inductances of resistors and capacitors I intend to use wide
packages such as 0508 along with wide pcb traces.

Your information about emf detector physics is of much interest to me.
Thanks.

--
To reply directly remove the sj. from my email address. This is a spam
jammer.
From: George Herold on 9 Feb 2010 13:10
On Feb 8, 11:44 pm, Artist <art...(a)sj.speakeasy.net> wrote:
> Phil Hobbs wrote:
> >> Similar, but unlike the passive photoresistor, the electromotive force
> >> detector converts photons into electrical energy. In a TIA circuit it
> >> can be modeled as a current source that does not require a bias.
>
> > Photodiodes and photoconductors are completely different.
>
> > The
> >> current is proportional to the motion of a beam light across it. It is
> >> very good at detecting changes in a laser light's speckled pattern
> >> produced when it reflects off any but the most even of surfaces.
>
> > I'd never heard of a photoelectromotive detector, but Google had.
> > Interesting. From what I gather from the free literature, it's basically
> > a GaAs 1-D lateral effect photodiode run at zero bias, with the output
> > taken across the ends of the top layer.
>
> > I haven't read any really detailed papers, because they all cost $30,
> > but it relies on the carrier density responding slowly to changes in the
> > illumination pattern. It's made of GaAs rather than silicon, which I'm
> > guessing is because the hole mobility in GaAs is so very low, which
> > essentially glues the charge density in place. (Silicon would respond
> > much faster.)
>
> > For static illumination, the slow carrier diffusion makes the junction E
> > field flatten out locally, so the charges all recombine, and the output
> > current is zero. When the illumination changes, though, you have
> > photocarriers being generated in regions where the net current is
> > suddenly nonzero. These carriers produce a displacement current
> > immediately (not when they arrive at the electrodes), and the carriers
> > still flowing in the previously illuminated region produce a net
> > displacement current of the opposite sign. The two sum to zero, but
> > they're spatially offset from each other, which produces a net current
> > from the outputs. Like everything else in nature, it's linear for small
> > displacements.
>
> > It's a cute technique, which reminds me a bit of a photorefractive phase
> > conjugate mirror, in which the beam interference pattern forms a
> > slowly-varying hologram that corrects for phase and pointing errors.
>
> > A few things I'm not clear on:
> > (1) There has to be a junction in there somewhere, because it works at
> > zero bias and can produce current in both directions--which in a
> > symmetric structure requires a third electrode.
>
> > (2) Splitting the photocarriers between the ends seems to require a
> > high-resistance epi layer, which would tend to make the response slow or
> > else very noisy, like a lateral effect cell--the full Johnson noise
> > current of the epi layer appears in the output.
>
> > (3) The diffusion and recombination currents both have full shot noise,
> > and the forward-biased regions will have a very low resistance. This
> > will seemingly contribute a lot of noise to the output--the noise will
> > be that of the full beam's photocurrent, not merely that of the small
> > difference current.
>
> > So in general this doesn't sound like a low-noise device, although it's
> > certainly convenient for speckle measurements--the usual alternative
> > method is TV interferometry, in which you cross-correlate the speckle
> > patterns before and after some change (e.g. pressurizing a tank). TV
> > interferometry is slow, since it takes a minimum of two video frame times.
>
> >> Right now my plan is get two of the lowest noise JFET or CMOS input op
> >> amps I can find that satisfy the bandwidth requirement. One will be
> >> used for the feedback resistor and the other will do the bootstrap.
>
> > It would be worth finding out what the noise level of the device itself
> > is. If I'm mistaken and it's actually quiet, you'd be better off making
> > the bootstrap out of a single-ended BF862 JFET rather than an op amp.
> > Otherwise a couple of op amps is probably fine (one of my favourites is
> > the ADA4817-1).
>
> > Cheers
>
> > Phil Hobbs
>
> This project had a break for awhile but is now revived. I am given new
> specifications to model the emf detector with. These are 100kOhm with a
> parallel capacitance of 1pf. This is a much easier task.
>
> There was discussion in this thread about other methods of noise
> reduction. The goal of using a bootstrap was primarily to get the
> bandwidth. The low impedance of the emf detector meant bandwidth
> challenges because to a first approximation that neglects capacitances
> the bandwidth will be the GBW of the op amp divided by the noise gain.
>
> I like the ADA4817-1. Using it may make possible doing this without
> bootstrapping.
>
> Given the 1pF parallel capacitance in the model a bootstrap would simply
> add as much capacitance as it removes and add noise.
>
> The bootstrap would also increase the equivalent resistance of the emf
> detector. But the resistance in the model is high enough now, and the
> GBW of the ADA4817-1 is high enough, that I do not see a need.
>
> The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am not
> going to be able to do this in one stage. Even before considering the
> capacitances from the virtual ground to actual ground the parallel
> capacitance of 2Mohm to get a 10MHz bandwidth would have to be 8fF. That
> is not going to happen.
>
> Whats more, because of the capacitances on the virtual ground the
> feedback resistor must have a parallel capacitor for stability.
>
> To reduce inductances of resistors and capacitors I intend to use wide
> packages such as 0508 along with wide pcb traces.
>
> Your information about emf detector physics is of much interest to me.
> Thanks.
>
> --
> To reply directly remove the sj. from my email address. This is a spam
> jammer.- Hide quoted text -
>
> - Show quoted text -

"The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am
not
going to be able to do this in one stage."

This sounds like a stretch no matter how many stages you use. Sounds
like you may have to educate your manager about capacitance. Even
with only 1pF in the detector you've still got a few pF on the input
to the opamp, plus the PCB trace connecting the two.


"> To reduce inductances of resistors and capacitors I intend to use
wide
> packages such as 0508 along with wide pcb traces."

Well on the photodetector side of the circuit I'd keep traces short
and thin to reduce the capacitance. And be careful of where you put
the ground plane.

(But Phil H. is the expert here. BTW have you bought his book yet?)

George H.

From: Artist on 9 Feb 2010 15:35
George Herold wrote:
> On Feb 8, 11:44 pm, Artist<art...(a)sj.speakeasy.net> wrote:
>> Phil Hobbs wrote:
>>>> Similar, but unlike the passive photoresistor, the electromotive force
>>>> detector converts photons into electrical energy. In a TIA circuit it
>>>> can be modeled as a current source that does not require a bias.
>>
>>> Photodiodes and photoconductors are completely different.
>>
>>> The
>>>> current is proportional to the motion of a beam light across it. It is
>>>> very good at detecting changes in a laser light's speckled pattern
>>>> produced when it reflects off any but the most even of surfaces.
>>
>>> I'd never heard of a photoelectromotive detector, but Google had.
>>> Interesting. From what I gather from the free literature, it's basically
>>> a GaAs 1-D lateral effect photodiode run at zero bias, with the output
>>> taken across the ends of the top layer.
>>
>>> I haven't read any really detailed papers, because they all cost $30,
>>> but it relies on the carrier density responding slowly to changes in the
>>> illumination pattern. It's made of GaAs rather than silicon, which I'm
>>> guessing is because the hole mobility in GaAs is so very low, which
>>> essentially glues the charge density in place. (Silicon would respond
>>> much faster.)
>>
>>> For static illumination, the slow carrier diffusion makes the junction E
>>> field flatten out locally, so the charges all recombine, and the output
>>> current is zero. When the illumination changes, though, you have
>>> photocarriers being generated in regions where the net current is
>>> suddenly nonzero. These carriers produce a displacement current
>>> immediately (not when they arrive at the electrodes), and the carriers
>>> still flowing in the previously illuminated region produce a net
>>> displacement current of the opposite sign. The two sum to zero, but
>>> they're spatially offset from each other, which produces a net current
>>> from the outputs. Like everything else in nature, it's linear for small
>>> displacements.
>>
>>> It's a cute technique, which reminds me a bit of a photorefractive phase
>>> conjugate mirror, in which the beam interference pattern forms a
>>> slowly-varying hologram that corrects for phase and pointing errors.
>>
>>> A few things I'm not clear on:
>>> (1) There has to be a junction in there somewhere, because it works at
>>> zero bias and can produce current in both directions--which in a
>>> symmetric structure requires a third electrode.
>>
>>> (2) Splitting the photocarriers between the ends seems to require a
>>> high-resistance epi layer, which would tend to make the response slow or
>>> else very noisy, like a lateral effect cell--the full Johnson noise
>>> current of the epi layer appears in the output.
>>
>>> (3) The diffusion and recombination currents both have full shot noise,
>>> and the forward-biased regions will have a very low resistance. This
>>> will seemingly contribute a lot of noise to the output--the noise will
>>> be that of the full beam's photocurrent, not merely that of the small
>>> difference current.
>>
>>> So in general this doesn't sound like a low-noise device, although it's
>>> certainly convenient for speckle measurements--the usual alternative
>>> method is TV interferometry, in which you cross-correlate the speckle
>>> patterns before and after some change (e.g. pressurizing a tank). TV
>>> interferometry is slow, since it takes a minimum of two video frame times.
>>
>>>> Right now my plan is get two of the lowest noise JFET or CMOS input op
>>>> amps I can find that satisfy the bandwidth requirement. One will be
>>>> used for the feedback resistor and the other will do the bootstrap.
>>
>>> It would be worth finding out what the noise level of the device itself
>>> is. If I'm mistaken and it's actually quiet, you'd be better off making
>>> the bootstrap out of a single-ended BF862 JFET rather than an op amp.
>>> Otherwise a couple of op amps is probably fine (one of my favourites is
>>> the ADA4817-1).
>>
>>> Cheers
>>
>>> Phil Hobbs
>>
>> This project had a break for awhile but is now revived. I am given new
>> specifications to model the emf detector with. These are 100kOhm with a
>> parallel capacitance of 1pf. This is a much easier task.
>>
>> There was discussion in this thread about other methods of noise
>> reduction. The goal of using a bootstrap was primarily to get the
>> bandwidth. The low impedance of the emf detector meant bandwidth
>> challenges because to a first approximation that neglects capacitances
>> the bandwidth will be the GBW of the op amp divided by the noise gain.
>>
>> I like the ADA4817-1. Using it may make possible doing this without
>> bootstrapping.
>>
>> Given the 1pF parallel capacitance in the model a bootstrap would simply
>> add as much capacitance as it removes and add noise.
>>
>> The bootstrap would also increase the equivalent resistance of the emf
>> detector. But the resistance in the model is high enough now, and the
>> GBW of the ADA4817-1 is high enough, that I do not see a need.
>>
>> The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am not
>> going to be able to do this in one stage. Even before considering the
>> capacitances from the virtual ground to actual ground the parallel
>> capacitance of 2Mohm to get a 10MHz bandwidth would have to be 8fF. That
>> is not going to happen.
>>
>> Whats more, because of the capacitances on the virtual ground the
>> feedback resistor must have a parallel capacitor for stability.
>>
>> To reduce inductances of resistors and capacitors I intend to use wide
>> packages such as 0508 along with wide pcb traces.
>>
>> Your information about emf detector physics is of much interest to me.
>> Thanks.
>>
>> --
>> To reply directly remove the sj. from my email address. This is a spam
>> jammer.- Hide quoted text -
>>
>> - Show quoted text -
>
> "The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am
> not
> going to be able to do this in one stage."
>
> This sounds like a stretch no matter how many stages you use. Sounds
> like you may have to educate your manager about capacitance. Even
> with only 1pF in the detector you've still got a few pF on the input
> to the opamp, plus the PCB trace connecting the two.
>
>
> "> To reduce inductances of resistors and capacitors I intend to use
> wide
>> packages such as 0508 along with wide pcb traces."
>
> Well on the photodetector side of the circuit I'd keep traces short
> and thin to reduce the capacitance. And be careful of where you put
> the ground plane.
>
> (But Phil H. is the expert here. BTW have you bought his book yet?)
>
> George H.
>

Wider traces reduce inductance and increase capacitance.

I have not bought his book yet but I will.

--
To reply directly remove the sj. from my email address. This is a spam
jammer.
From: John Larkin on 19 Feb 2010 01:45
On Tue, 09 Feb 2010 12:35:12 -0800, Artist <artist(a)sj.speakeasy.net>
wrote:

>George Herold wrote:
>> On Feb 8, 11:44 pm, Artist<art...(a)sj.speakeasy.net> wrote:
>>> Phil Hobbs wrote:
>>>>> Similar, but unlike the passive photoresistor, the electromotive force
>>>>> detector converts photons into electrical energy. In a TIA circuit it
>>>>> can be modeled as a current source that does not require a bias.
>>>
>>>> Photodiodes and photoconductors are completely different.
>>>
>>>> The
>>>>> current is proportional to the motion of a beam light across it. It is
>>>>> very good at detecting changes in a laser light's speckled pattern
>>>>> produced when it reflects off any but the most even of surfaces.
>>>
>>>> I'd never heard of a photoelectromotive detector, but Google had.
>>>> Interesting. From what I gather from the free literature, it's basically
>>>> a GaAs 1-D lateral effect photodiode run at zero bias, with the output
>>>> taken across the ends of the top layer.
>>>
>>>> I haven't read any really detailed papers, because they all cost $30,
>>>> but it relies on the carrier density responding slowly to changes in the
>>>> illumination pattern. It's made of GaAs rather than silicon, which I'm
>>>> guessing is because the hole mobility in GaAs is so very low, which
>>>> essentially glues the charge density in place. (Silicon would respond
>>>> much faster.)
>>>
>>>> For static illumination, the slow carrier diffusion makes the junction E
>>>> field flatten out locally, so the charges all recombine, and the output
>>>> current is zero. When the illumination changes, though, you have
>>>> photocarriers being generated in regions where the net current is
>>>> suddenly nonzero. These carriers produce a displacement current
>>>> immediately (not when they arrive at the electrodes), and the carriers
>>>> still flowing in the previously illuminated region produce a net
>>>> displacement current of the opposite sign. The two sum to zero, but
>>>> they're spatially offset from each other, which produces a net current
>>>> from the outputs. Like everything else in nature, it's linear for small
>>>> displacements.
>>>
>>>> It's a cute technique, which reminds me a bit of a photorefractive phase
>>>> conjugate mirror, in which the beam interference pattern forms a
>>>> slowly-varying hologram that corrects for phase and pointing errors.
>>>
>>>> A few things I'm not clear on:
>>>> (1) There has to be a junction in there somewhere, because it works at
>>>> zero bias and can produce current in both directions--which in a
>>>> symmetric structure requires a third electrode.
>>>
>>>> (2) Splitting the photocarriers between the ends seems to require a
>>>> high-resistance epi layer, which would tend to make the response slow or
>>>> else very noisy, like a lateral effect cell--the full Johnson noise
>>>> current of the epi layer appears in the output.
>>>
>>>> (3) The diffusion and recombination currents both have full shot noise,
>>>> and the forward-biased regions will have a very low resistance. This
>>>> will seemingly contribute a lot of noise to the output--the noise will
>>>> be that of the full beam's photocurrent, not merely that of the small
>>>> difference current.
>>>
>>>> So in general this doesn't sound like a low-noise device, although it's
>>>> certainly convenient for speckle measurements--the usual alternative
>>>> method is TV interferometry, in which you cross-correlate the speckle
>>>> patterns before and after some change (e.g. pressurizing a tank). TV
>>>> interferometry is slow, since it takes a minimum of two video frame times.
>>>
>>>>> Right now my plan is get two of the lowest noise JFET or CMOS input op
>>>>> amps I can find that satisfy the bandwidth requirement. One will be
>>>>> used for the feedback resistor and the other will do the bootstrap.
>>>
>>>> It would be worth finding out what the noise level of the device itself
>>>> is. If I'm mistaken and it's actually quiet, you'd be better off making
>>>> the bootstrap out of a single-ended BF862 JFET rather than an op amp.
>>>> Otherwise a couple of op amps is probably fine (one of my favourites is
>>>> the ADA4817-1).
>>>
>>>> Cheers
>>>
>>>> Phil Hobbs
>>>
>>> This project had a break for awhile but is now revived. I am given new
>>> specifications to model the emf detector with. These are 100kOhm with a
>>> parallel capacitance of 1pf. This is a much easier task.
>>>
>>> There was discussion in this thread about other methods of noise
>>> reduction. The goal of using a bootstrap was primarily to get the
>>> bandwidth. The low impedance of the emf detector meant bandwidth
>>> challenges because to a first approximation that neglects capacitances
>>> the bandwidth will be the GBW of the op amp divided by the noise gain.
>>>
>>> I like the ADA4817-1. Using it may make possible doing this without
>>> bootstrapping.
>>>
>>> Given the 1pF parallel capacitance in the model a bootstrap would simply
>>> add as much capacitance as it removes and add noise.
>>>
>>> The bootstrap would also increase the equivalent resistance of the emf
>>> detector. But the resistance in the model is high enough now, and the
>>> GBW of the ADA4817-1 is high enough, that I do not see a need.
>>>
>>> The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am not
>>> going to be able to do this in one stage. Even before considering the
>>> capacitances from the virtual ground to actual ground the parallel
>>> capacitance of 2Mohm to get a 10MHz bandwidth would have to be 8fF. That
>>> is not going to happen.
>>>
>>> Whats more, because of the capacitances on the virtual ground the
>>> feedback resistor must have a parallel capacitor for stability.
>>>
>>> To reduce inductances of resistors and capacitors I intend to use wide
>>> packages such as 0508 along with wide pcb traces.
>>>
>>> Your information about emf detector physics is of much interest to me.
>>> Thanks.
>>>
>>> --
>>> To reply directly remove the sj. from my email address. This is a spam
>>> jammer.- Hide quoted text -
>>>
>>> - Show quoted text -
>>
>> "The manager wants a 10MHz bandwidth and 2 Mohm transimpedance. I am
>> not
>> going to be able to do this in one stage."
>>
>> This sounds like a stretch no matter how many stages you use. Sounds
>> like you may have to educate your manager about capacitance. Even
>> with only 1pF in the detector you've still got a few pF on the input
>> to the opamp, plus the PCB trace connecting the two.
>>
>>
>> "> To reduce inductances of resistors and capacitors I intend to use
>> wide
>>> packages such as 0508 along with wide pcb traces."
>>
>> Well on the photodetector side of the circuit I'd keep traces short
>> and thin to reduce the capacitance. And be careful of where you put
>> the ground plane.
>>
>> (But Phil H. is the expert here. BTW have you bought his book yet?)
>>
>> George H.
>>
>
>Wider traces reduce inductance and increase capacitance.
>
>I have not bought his book yet but I will.


Trace inductances won't matter in a 10 MHz TIA. They barely matter at
500 MHz.

John



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