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From: John Larkin on 23 Mar 2010 22:40 On Tue, 23 Mar 2010 18:25:12 -0700 (PDT), George Herold <ggherold(a)gmail.com> wrote: >On Mar 23, 4:33�pm, Phil Hobbs ><pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> On 3/23/2010 4:20 PM, Robert Macy wrote: >> >> >> >> >> >> > On Mar 23, 11:26 am, George Herold<ggher...(a)gmail.com> �wrote: >> >> On Mar 23, 1:42 pm, Robert Macy<m...(a)california.com> �wrote: >> >> >>> On Mar 23, 10:39 am, George Herold<ggher...(a)gmail.com> �wrote: >> >> >>>> On Mar 23, 11:33 am, John Larkin >> >> >>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> �wrote: >> >>>>> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold >> >> >>>>> <ggher...(a)gmail.com> �wrote: >> >>>>>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl> �wrote: >> >>>>>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl> �wrote: >> >> >>>>>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal >> >> >>>>>>>> Murray) wrote: >> >>>>>>>>> In article<4BA7797C.7020...(a)electrooptical.net>, >> >>>>>>>>> � Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> �writes: >> >> >>>>>>>>>> � Night lights use CdS or CdSSe photoconductors, but they don't have to >> >>>>>>>>>> have any kind of stability or repeatability, and they don't.. �(CdSSe's >> >>>>>>>>>> resistance can vary 5X due entirely to previous illumination history.) >> >> >>>>>>>>> What is the mechanism for that history quirk? >> >> >>>>>>>>> What is the time scale? �How long does it remember the history? >> >> >>>>>>>>> -- >> >>>>>>>>> These are my opinions, not necessarily my employer's. �I hate spam. >> >> >>>>>>>> Hello, >> >> >>>>>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the >> >>>>>>>> links. When at the photocell page, on the right side you have an >> >>>>>>>> application note link. They have a very nice document on the LDRs that >> >>>>>>>> includes the memory effect also. >> >> >>>>>>>> Best regards, >> >> >>>>>>>> Wim >> >>>>>>>> PA3DJSwww.tetech.nl >> >>>>>>>> without abc, PM will reach me >> >> >>>>>>> The document that I have is no longer present on the Perkinelmer >> >>>>>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf >> >>>>>>> you can find the document about LDR. I will leave it there for about >> >>>>>>> 10 days. >> >> >>>>>>> Best regards, >> >> >>>>>>> Wim >> >>>>>>> PA3DJSwww.tetech.nl-Hidequotedtext- >> >> >>>>>>> - Show quoted text - >> >> >>>>>> Thanks for the document, page 35 talks about shot noise in LDR's. �Are >> >>>>>> they just throwing equations around or is there shot noise in LDR's? >> >> >>>>>> George H. >> >> >>>>> Since photons make current, probably so. But LDRs are so messy it's >> >>>>> likely buried in other gunk. >> >> >>>>> One of my guys is building a test setup to measure shot/excess noise >> >>>>> in resistors. We need a ~~ 100M resistor to create a small (50 nA >> >>>>> maybe) bias current with below shot-level noise. It's not clear if >> >>>>> high-value cermet resistors have shot noise or not, so we plan to >> >>>>> measure a bunch. >> >> >>>>> Metal film resistors don't go to very high values; we can probably get >> >>>>> 10M and likely 22M, and maybe even 50M, so we may have to do a series >> >>>>> string. Even then I want to measure them to make sure they behave. I'm >> >>>>> guessing that axials are better than surface mount, because of the >> >>>>> bigger available surface for depositing metal. >> >> >>>>> It's an interesting problem, trying to generate a nA-range DC current >> >>>>> with low noise. Low voltage across a low-value metal-film resistor >> >>>>> doesn't work because of Johnson noise. High value resistors are noisy >> >>>>> in their own right. >> >> >>>>> John- Hide quoted text - >> >> >>>>> - Show quoted text - >> >> >>>> "> �Since photons make current, probably so. But LDRs are so messy it's >> >> >>>>> likely buried in other gunk." >> >> >>>> Hmm, Yeah this is a bit confusing. �The photons make the charge >> >>>> carriers and reduce the resistance... and as Phil H. says there is >> >>>> generation and recombination noise. �But this should only depend on >> >>>> the light level and not on how the device is biased. �For a given >> >>>> light level if I double the bias voltage (and thus double the current) >> >>>> does the noise go up? >> >> >>>> I'm using 100M and 1 G resistors from Ohmite to make a 10nA current >> >>>> source. �I'll look at the noise next chance I get. >> >> >>>> George H. >> >> >>> sorry for the VERY naive question, but would some form of junction >> >>> leakage have lower noise? �Or are junction leakages at and above >> >>> johnson?- Hide quoted text - >> >> >>> - Show quoted text - >> >> >> Not naive at all! �Phil is the expert here, but I'll answer anyway... >> >> (put my foot in mouth once again.) �So, as I understand it, the >> >> thermal (Johnson) noise from a biased junction is 1/2 of the thermal >> >> noise from a resistor that has the same resistance as the junction. (R >> >> = kT/(eV * I). �I beleive that Phil has actully used this 'trick' to >> >> make lower noise photodiode front ends... but it's a trick that is >> >> beyond my ability. >> >> >> George H. >> >> > Thank you for your reply. >> >> > Next naive question is Can the leakage be provided by a DC/DC >> > converter circuit where the supply's noise is caused by caps and >> > inductors, oh wait, no noise there. �But I'm talking a physically >> > realizable circuit. >> >> There was a big foofaraw 15 years or so back where some guy claimed to >> have figured out how to make an active circuit that functioned as a >> noiseless resistor. �It was all over IEEE Spectrum and places like that. >> >> There are various means to take quiet active things, e.g. the beta of a >> BJT or the g_m of a good JFET, and apply feedback so as to make the >> equivalent of a very quiet resistor. �I recently designed a TIA that's >> shot noise limited down to the low tens of nanoamps in a 1-MHz >> bandwidth, using techniques like that. �It's about 20 dB better than I >> thought I could do, which was a very pleasant surprise. �You just have >> to get rid of the 300 kelvin resistors. >> >> (John L. and I collaborated on it, along with one of his guys, Jonathan >> Dufour--you'll be able to buy them soon, if all goes well. �Buy >> lots--I'll have two kids in college this fall, and John's ski place >> needs a new laboratory.) ;) >> >> Cheers >> >> Phil Hobbs >> >> -- >> Dr Philip C D Hobbs >> Principal >> ElectroOptical Innovations >> 55 Orchard Rd >> Briarcliff Manor NY 10510 >> 845-480-2058 >> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text - >> >> - Show quoted text - > >Are you talking about active feedback? (Maybe those aren't the right >words?) You send back some of the signal (from a system) to damp the >response and this looks like a resistance to the system. But it�s >noiseless. You can make a room-temperature amplifier that has very low noise temperature... I think some gaasfet-based microwave amps get down to something like 50K noise temps. And you can use such an amp to simulate a resistor well below room temp. It doesn't violate the laws of thermodynamics because the amp needs power to do its thing. If you connect two resistors with wires, and they're at different temperatures, the Johnson noise difference becomes a thermal conductivity between them... the hotter one makes more noise, and pumps power into the colder one. I did the math once... it's something like 10 orders of magnitude weaker coupling than the thermal conductivity of any conceivable wires that you'd use to connect them. John
From: John Larkin on 23 Mar 2010 22:47 On Tue, 23 Mar 2010 18:39:43 -0700 (PDT), George Herold <ggherold(a)gmail.com> wrote: >On Mar 23, 5:34�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 23 Mar 2010 16:23:32 -0400, Phil Hobbs >> >> >> >> >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> >On 3/23/2010 4:16 PM, John Larkin wrote: >> >> On Tue, 23 Mar 2010 12:59:12 -0700 (PDT), George Herold >> >> <ggher...(a)gmail.com> �wrote: >> >> >>> On Mar 23, 2:58 pm, John Larkin >> >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> �wrote: >> >>>> On Tue, 23 Mar 2010 10:39:15 -0700 (PDT), George Herold >> >> >>>> <ggher...(a)gmail.com> �wrote: >> >>>>> On Mar 23, 11:33 am, John Larkin >> >>>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> �wrote: >> >>>>>> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold >> >> >>>>>> <ggher...(a)gmail.com> �wrote: >> >>>>>>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl> �wrote: >> >>>>>>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl> �wrote: >> >> >>>>>>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal >> >> >>>>>>>>> Murray) wrote: >> >>>>>>>>>> In article<4BA7797C.7020...(a)electrooptical.net>, >> >>>>>>>>>> � Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> �writes: >> >> >>>>>>>>>>> � Night lights use CdS or CdSSe photoconductors, but they don't have to >> >>>>>>>>>>> have any kind of stability or repeatability, and they don't. �(CdSSe's >> >>>>>>>>>>> resistance can vary 5X due entirely to previous illumination history.) >> >> >>>>>>>>>> What is the mechanism for that history quirk? >> >> >>>>>>>>>> What is the time scale? �How long does it remember the history? >> >> >>>>>>>>>> -- >> >>>>>>>>>> These are my opinions, not necessarily my employer's. �I hate spam. >> >> >>>>>>>>> Hello, >> >> >>>>>>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the >> >>>>>>>>> links. When at the photocell page, on the right side you have an >> >>>>>>>>> application note link. They have a very nice document on the LDRs that >> >>>>>>>>> includes the memory effect also. >> >> >>>>>>>>> Best regards, >> >> >>>>>>>>> Wim >> >>>>>>>>> PA3DJSwww.tetech.nl >> >>>>>>>>> without abc, PM will reach me >> >> >>>>>>>> The document that I have is no longer present on the Perkinelmer >> >>>>>>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf >> >>>>>>>> you can find the document about LDR. I will leave it there for about >> >>>>>>>> 10 days. >> >> >>>>>>>> Best regards, >> >> >>>>>>>> Wim >> >>>>>>>> PA3DJSwww.tetech.nl-Hidequotedtext - >> >> >>>>>>>> - Show quoted text - >> >> >>>>>>> Thanks for the document, page 35 talks about shot noise in LDR's. �Are >> >>>>>>> they just throwing equations around or is there shot noise in LDR's? >> >> >>>>>>> George H. >> >> >>>>>> Since photons make current, probably so. But LDRs are so messy it's >> >>>>>> likely buried in other gunk. >> >> >>>>>> One of my guys is building a test setup to measure shot/excess noise >> >>>>>> in resistors. We need a ~~ 100M resistor to create a small (50 nA >> >>>>>> maybe) bias current with below shot-level noise. It's not clear if >> >>>>>> high-value cermet resistors have shot noise or not, so we plan to >> >>>>>> measure a bunch. >> >> >>>>>> Metal film resistors don't go to very high values; we can probably get >> >>>>>> 10M and likely 22M, and maybe even 50M, so we may have to do a series >> >>>>>> string. Even then I want to measure them to make sure they behave. I'm >> >>>>>> guessing that axials are better than surface mount, because of the >> >>>>>> bigger available surface for depositing metal. >> >> >>>>>> It's an interesting problem, trying to generate a nA-range DC current >> >>>>>> with low noise. Low voltage across a low-value metal-film resistor >> >>>>>> doesn't work because of Johnson noise. High value resistors are noisy >> >>>>>> in their own right. >> >> >>>>>> John- Hide quoted text - >> >> >>>>>> - Show quoted text - >> >> >>>>> "> �Since photons make current, probably so. But LDRs are so messy it's >> >>>>>> likely buried in other gunk." >> >> >>>>> Hmm, Yeah this is a bit confusing. �The photons make the charge >> >>>>> carriers and reduce the resistance... and as Phil H. says there is >> >>>>> generation and recombination noise. �But this should only depend on >> >>>>> the light level and not on how the device is biased. �For a given >> >>>>> light level if I double the bias voltage (and thus double the current) >> >>>>> does the noise go up? >> >> >>>> What's amazing to me is that there are resistors that *don't* have >> >>>> shot noise. >> >> >>>> Any time you have a current in which the electrons arrive randomly, >> >>>> there's shot noise. It's pure raindrops-on-the-roof statistics. That >> >>>> happens when the electrons are knocked loose by photons or when some >> >>>> thin barrier, like a P-N junction, doles out electrons across a >> >>>> surface. Metal wire is unique in having electron interactions that >> >>>> smooth out the flow. As far as I know, all semiconductor junction >> >>>> currents and leakages have shot noise. Tubes have shot noise. >> >> >>>> The shot noise current depends only on the average current; it goes up >> >>>> as the square root of I. Of course, some devices have more noise than >> >>>> pure shot noise. >> >> >>>>> I'm using 100M and 1 G resistors from Ohmite to make a 10nA current >> >>>>> source. �I'll look at the noise next chance I get. >> >> >>>> I'd appreciate that. We'll share whatever we learn. This is not very >> >>>> easy to measure. >> >> >>>> John- Hide quoted text - >> >> >>>> - Show quoted text - >> >> >>> "> �I'd appreciate that. We'll share whatever we learn. This is not >> >>> very >> >>>> easy to measure." >> >> >>> Hmm I hadn't thought it would be that hard... but then I'm probably >> >>> missing something. �I was going to take the the 10nA current source >> >>> and fed it into a TIA opamp circuit with 100Meg as feed back resistor, >> >>> (giving me a volt of DC across the resistor) And then see how noisy it >> >>> is. >> >> >> Fine, as long as that 100M resistor has no excess noise of its own, >> >> and you account for its Johnson noise, and the TIA has very little >> >> input current noise. And all the supplies are very quiet. And >> >> everything is really well shielded. >> >> >> We're going to make a voltage divider from two identical RUTs, and AC >> >> couple that into an opamp with a gain of +1000. The AC coupling RC >> >> will add shunt Johnson noise but no excess noise of its own. We're >> >> figuring on a polystyrene cap and a 1G resistor maybe. Opamp = >> >> ADA4817. I figure we'd measure from 1 KHz to 100K maybe. >> >> >> John >> >> >If you use two FET op amps going into two scope channels, and use the >> >math functions to multiply the two inputs, FFT the results, and average >> >over N traces, you'll get just the noise of the resistors, because >> >everything else cancels out. �How big N is depends on how far below the >> >amplifier noise you want to go. >> >> Sounds like a lot of math. That would be good for extremes, like >> cryogenics maybe. >> >> Why FFT? Wouldn't the averaged product work? >> >> >> >> >Quiet power supplies are of course vital. >> >> >Sounds like a really interesting measurement. >> >> The ADA4817 has 4 nv/rthz noise, and the Johnson noise from 50M will >> be almost a microvolt per, so we should see the noise pretty well. >> Input noise current is low enough to not make trouble. It's an amazing >> opamp. >> >> If cranking up the DC voltage into the divider doesn't much increase >> the noise, we're done. If we jam 20 volts into the 100M:100M divider, >> full shot noise would make ... calculates furiously ... 13 uV/rthz, a >> huge signal. >> >> John- Hide quoted text - >> >> - Show quoted text - > >Yeah I think things will be fine. (Though I really have no idea.) >Your resistors may have 1/f noise but there shouldn't be any shot >noise. > >Still, I must order some CdS LDR's. I still don't get the shot >noise.... and to paraphrase Brian Pippard in the intro to "The >Physics of Vibration" "I'm not as smart as some others and it's nice >to have some experimental results to help guide my thinking." > >George H. If you connect a pin photodiode or a PMT, with a little gain maybe, to a scope, and hit it with a light pulse, the top of the pulse will usually be a lot noisier then the baseline (dark) level. It's pretty obvious if the light is in the roughly 1 microwatt sort of range. It's the quantized photons hitting the detector randomly... shot noise. I guess it was named for the sound of shot (as in birdshot) falling onto a surface. Hisssssss. John
From: Martin Brown on 24 Mar 2010 04:49 John Larkin wrote: > > One of my guys is building a test setup to measure shot/excess noise > in resistors. We need a ~~ 100M resistor to create a small (50 nA > maybe) bias current with below shot-level noise. It's not clear if > high-value cermet resistors have shot noise or not, so we plan to > measure a bunch. > > Metal film resistors don't go to very high values; we can probably get > 10M and likely 22M, and maybe even 50M, so we may have to do a series > string. Even then I want to measure them to make sure they behave. I'm > guessing that axials are better than surface mount, because of the > bigger available surface for depositing metal. > > It's an interesting problem, trying to generate a nA-range DC current > with low noise. Low voltage across a low-value metal-film resistor > doesn't work because of Johnson noise. High value resistors are noisy > in their own right. Try looking for the patents on Faraday detector amplifier calibration sources. They are typically around 0.1nA with fairly low noise and very high reproducibility. Not sure how much of it is trade secret. ISTR the resistors for this job are effectively single sourced and in the range 10^9 to 10^11 and some kind of doped conductive glass. They are low noise (at least for low frequencies) but also behave a little bit like they have distributed internal RC and battery behaviour which requires external compensation. Regards, Martin Brown
From: Oppie on 24 Mar 2010 09:28 "Tim Williams" <tmoranwms(a)charter.net> wrote in message news:ho83jp$dr2$1(a)news.eternal-september.org... > "Oppie" <Oppie(a)saynotospam.com> wrote in message > news:yRLpn.28952$ao7.12608(a)newsfe21.iad... >>> http://webpages.charter.net/dawill/Images/Photodiode_PWM.gif >> >> Geez, I haven't futzed with PUTs in ages. Does anybody still make them? > > PUTs aren't that hard to find. Maybe you're thinking of UJTs? > http://www.mouser.com/ProductDetail/ON-Semiconductor/2N6027G/?qs=sGAEpiMZZMtK7gk71q29LtJYaNkyBux0URy1Y7q4hnw%3d > Although I'll admit none of them are actually *in stock* right now. > > Anyway, if they're on the expensive side ($0.25 in singles?), you can > Jeorgify your own from a 2N3904 and 2N3906 ($0.06/ea in singles)... the > equivalent circuit is as indicated (i.e., like an SCR, but take the second > junction). > > Tim > Last time I worked with them was about 25 years ago. It was used as a oscillator/ramp generator for a proportional heating control system in a 'Centrifichem' blood chemistry analyzer. Oppie
From: George Herold on 24 Mar 2010 10:44
On Mar 23, 10:40 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Tue, 23 Mar 2010 18:25:12 -0700 (PDT), George Herold > > > > > > <ggher...(a)gmail.com> wrote: > >On Mar 23, 4:33 pm, Phil Hobbs > ><pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >> On 3/23/2010 4:20 PM, Robert Macy wrote: > > >> > On Mar 23, 11:26 am, George Herold<ggher...(a)gmail.com> wrote: > >> >> On Mar 23, 1:42 pm, Robert Macy<m...(a)california.com> wrote: > > >> >>> On Mar 23, 10:39 am, George Herold<ggher...(a)gmail.com> wrote: > > >> >>>> On Mar 23, 11:33 am, John Larkin > > >> >>>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >>>>> On Tue, 23 Mar 2010 06:59:08 -0700 (PDT), George Herold > > >> >>>>> <ggher...(a)gmail.com> wrote: > >> >>>>>> On Mar 23, 8:32 am, Wimpie<wimabc...(a)tetech.nl> wrote: > >> >>>>>>> On 23 mar, 12:57, Wimpie<wimabc...(a)tetech.nl> wrote: > > >> >>>>>>>> On 22 mar, 23:52, hal-use...(a)ip-64-139-1-69.sjc.megapath.net (Hal > > >> >>>>>>>> Murray) wrote: > >> >>>>>>>>> In article<4BA7797C.7020...(a)electrooptical.net>, > >> >>>>>>>>> Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> writes: > > >> >>>>>>>>>> Night lights use CdS or CdSSe photoconductors, but they don't have to > >> >>>>>>>>>> have any kind of stability or repeatability, and they don't... (CdSSe's > >> >>>>>>>>>> resistance can vary 5X due entirely to previous illumination history.) > > >> >>>>>>>>> What is the mechanism for that history quirk? > > >> >>>>>>>>> What is the time scale? How long does it remember the history? > > >> >>>>>>>>> -- > >> >>>>>>>>> These are my opinions, not necessarily my employer's. I hate spam. > > >> >>>>>>>> Hello, > > >> >>>>>>>> Go towww.perkinelmer.comandsearchfor"photocell", or follow the > >> >>>>>>>> links. When at the photocell page, on the right side you have an > >> >>>>>>>> application note link. They have a very nice document on the LDRs that > >> >>>>>>>> includes the memory effect also. > > >> >>>>>>>> Best regards, > > >> >>>>>>>> Wim > >> >>>>>>>> PA3DJSwww.tetech.nl > >> >>>>>>>> without abc, PM will reach me > > >> >>>>>>> The document that I have is no longer present on the Perkinelmer > >> >>>>>>> website. Here:www.tetech.nl/divers/BRO_PhotoconductiveCellsAndAnalogOptoiso.pdf > >> >>>>>>> you can find the document about LDR. I will leave it there for about > >> >>>>>>> 10 days. > > >> >>>>>>> Best regards, > > >> >>>>>>> Wim > >> >>>>>>> PA3DJSwww.tetech.nl-Hidequotedtext- > > >> >>>>>>> - Show quoted text - > > >> >>>>>> Thanks for the document, page 35 talks about shot noise in LDR's. Are > >> >>>>>> they just throwing equations around or is there shot noise in LDR's? > > >> >>>>>> George H. > > >> >>>>> Since photons make current, probably so. But LDRs are so messy it's > >> >>>>> likely buried in other gunk. > > >> >>>>> One of my guys is building a test setup to measure shot/excess noise > >> >>>>> in resistors. We need a ~~ 100M resistor to create a small (50 nA > >> >>>>> maybe) bias current with below shot-level noise. It's not clear if > >> >>>>> high-value cermet resistors have shot noise or not, so we plan to > >> >>>>> measure a bunch. > > >> >>>>> Metal film resistors don't go to very high values; we can probably get > >> >>>>> 10M and likely 22M, and maybe even 50M, so we may have to do a series > >> >>>>> string. Even then I want to measure them to make sure they behave. I'm > >> >>>>> guessing that axials are better than surface mount, because of the > >> >>>>> bigger available surface for depositing metal. > > >> >>>>> It's an interesting problem, trying to generate a nA-range DC current > >> >>>>> with low noise. Low voltage across a low-value metal-film resistor > >> >>>>> doesn't work because of Johnson noise. High value resistors are noisy > >> >>>>> in their own right. > > >> >>>>> John- Hide quoted text - > > >> >>>>> - Show quoted text - > > >> >>>> "> Since photons make current, probably so. But LDRs are so messy it's > > >> >>>>> likely buried in other gunk." > > >> >>>> Hmm, Yeah this is a bit confusing. The photons make the charge > >> >>>> carriers and reduce the resistance... and as Phil H. says there is > >> >>>> generation and recombination noise. But this should only depend on > >> >>>> the light level and not on how the device is biased. For a given > >> >>>> light level if I double the bias voltage (and thus double the current) > >> >>>> does the noise go up? > > >> >>>> I'm using 100M and 1 G resistors from Ohmite to make a 10nA current > >> >>>> source. I'll look at the noise next chance I get. > > >> >>>> George H. > > >> >>> sorry for the VERY naive question, but would some form of junction > >> >>> leakage have lower noise? Or are junction leakages at and above > >> >>> johnson?- Hide quoted text - > > >> >>> - Show quoted text - > > >> >> Not naive at all! Phil is the expert here, but I'll answer anyway... > >> >> (put my foot in mouth once again.) So, as I understand it, the > >> >> thermal (Johnson) noise from a biased junction is 1/2 of the thermal > >> >> noise from a resistor that has the same resistance as the junction. (R > >> >> = kT/(eV * I). I beleive that Phil has actully used this 'trick' to > >> >> make lower noise photodiode front ends... but it's a trick that is > >> >> beyond my ability. > > >> >> George H. > > >> > Thank you for your reply. > > >> > Next naive question is Can the leakage be provided by a DC/DC > >> > converter circuit where the supply's noise is caused by caps and > >> > inductors, oh wait, no noise there. But I'm talking a physically > >> > realizable circuit. > > >> There was a big foofaraw 15 years or so back where some guy claimed to > >> have figured out how to make an active circuit that functioned as a > >> noiseless resistor. It was all over IEEE Spectrum and places like that. > > >> There are various means to take quiet active things, e.g. the beta of a > >> BJT or the g_m of a good JFET, and apply feedback so as to make the > >> equivalent of a very quiet resistor. I recently designed a TIA that's > >> shot noise limited down to the low tens of nanoamps in a 1-MHz > >> bandwidth, using techniques like that. It's about 20 dB better than I > >> thought I could do, which was a very pleasant surprise. You just have > >> to get rid of the 300 kelvin resistors. > > >> (John L. and I collaborated on it, along with one of his guys, Jonathan > >> Dufour--you'll be able to buy them soon, if all goes well. Buy > >> lots--I'll have two kids in college this fall, and John's ski place > >> needs a new laboratory.) ;) > > >> Cheers > > >> Phil Hobbs > > >> -- > >> Dr Philip C D Hobbs > >> Principal > >> ElectroOptical Innovations > >> 55 Orchard Rd > >> Briarcliff Manor NY 10510 > >> 845-480-2058 > >> hobbs at electrooptical dot nethttp://electrooptical.net-Hide quoted text - > > >> - Show quoted text - > > >Are you talking about active feedback? (Maybe those aren't the right > >words?) You send back some of the signal (from a system) to damp the > >response and this looks like a resistance to the system. But its > >noiseless. > > You can make a room-temperature amplifier that has very low noise > temperature... I think some gaasfet-based microwave amps get down to > something like 50K noise temps. And you can use such an amp to > simulate a resistor well below room temp. It doesn't violate the laws > of thermodynamics because the amp needs power to do its thing. > > If you connect two resistors with wires, and they're at different > temperatures, the Johnson noise difference becomes a thermal > conductivity between them... the hotter one makes more noise, and > pumps power into the colder one. I did the math once... it's something > like 10 orders of magnitude weaker coupling than the thermal > conductivity of any conceivable wires that you'd use to connect them. > > John- Hide quoted text - > > - Show quoted text - The noise temperature of a FET opamp looking at say a 1 Meg or 10 Meg resistor can be truly tiny. (I guess noise temperatures only really make sense if you stick to a fixed input impedance.) George H. |