On 07/18/2016 03:28 PM, SilverLeo wrote:> Il giorno domenica 17 luglio 2016 09:56:50 UTC+2, SilverLeo ha > scritto: > >> cut > > I'm replying to myself: I apologize, this seems the only action > Google groups let me to do. > > So: interesting the cap multiplier approach; I'll do some math, maybe > it'll be handy in the future. I cannot move the TIA close to the > phototransistor: it's a retrofit for an old device for estimate the > density of filled paper tubes (filters for very specific application) > through the detection of diffused light. The more filled is the tube, > the less is the light diffused. Phototransistor is buried inside a > cast iron frame, far away from any conditioning electronic. > > The main reason that lead to the TIA approach was the fact that > pototransistor with classic resistor on collector will fall into > saturation (Vce near or below Vcesat), actually messing up density > estimation. A TIA, as far as the opamp keeps doing its magic, will > force Vce=Vbias, running the phototransistor deep in active region.Except that with the same size resistor and power supply, both will saturate at about the same light level--the TIA doesn't help that at all. Phototransistors have horrible linearity anyway, so the swing at the collector is the least of your worries.> > This in theory: in real world application the TIA output shows 50mV > of noise voltage, that does not correspond to a 10uA of current > noise, because with the simple resistor there is no evidence of it. > This noise comes out when some valves or motors are running.If you don't understand what I said about not hanging cables on summing junctions, just trust me. Your load resistor trick will work a lot better.> > TIA input impedance should be zero (as stated in books) because of > virtual ground at inverting input. But, because emitter is at -12V, > the phototransitor will see a variable resistor matched to drop > exactly 12V for any current. So, for "small" currents this resistor > becomes big", and for big current this resistor turns "small". Is > this right,somehow?No. It's the transistor that adjusts, not the load.> > I could add a resistor from inverting input to ground (10K range) to > set a limit to input impedance,That won't make any difference except to increase the noise a little.> and a capacitor in parallel to that > resistor (nF range) to bypass everything the opamp will not be able > to follow due to bandwidth limitation. Useless or not?You really don't want to hang a capacitor to ground on a summing junction. It's very likely to make the whole thing oscillate.> > If I cannot overcome this issue I'll switch back to old, classi > resistor on collector; but, it would be nice to understand what's > going on.I recommend "The Art of Electronics", third edition, sections 2.5 (negative feedback) and Chapter 4 (op amps). Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Phototransistor fed to a transimpedance amplifier
Started by ●July 17, 2016
Reply by ●July 18, 20162016-07-18
Reply by ●July 18, 20162016-07-18
On Mon, 18 Jul 2016 16:03:14 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 07/18/2016 03:28 PM, SilverLeo wrote: >> Il giorno domenica 17 luglio 2016 09:56:50 UTC+2, SilverLeo ha >> scritto: >> >>> cut >> >> I'm replying to myself: I apologize, this seems the only action >> Google groups let me to do. >> >> So: interesting the cap multiplier approach; I'll do some math, maybe >> it'll be handy in the future. I cannot move the TIA close to the >> phototransistor: it's a retrofit for an old device for estimate the >> density of filled paper tubes (filters for very specific application) >> through the detection of diffused light. The more filled is the tube, >> the less is the light diffused. Phototransistor is buried inside a >> cast iron frame, far away from any conditioning electronic. >> >> The main reason that lead to the TIA approach was the fact that >> pototransistor with classic resistor on collector will fall into >> saturation (Vce near or below Vcesat), actually messing up density >> estimation. A TIA, as far as the opamp keeps doing its magic, will >> force Vce=Vbias, running the phototransistor deep in active region. > >Except that with the same size resistor and power supply, both will >saturate at about the same light level--the TIA doesn't help that at >all. Phototransistors have horrible linearity anyway, so the swing at >the collector is the least of your worries. > >> >> This in theory: in real world application the TIA output shows 50mV >> of noise voltage, that does not correspond to a 10uA of current >> noise, because with the simple resistor there is no evidence of it. >> This noise comes out when some valves or motors are running. > >If you don't understand what I said about not hanging cables on summing >junctions, just trust me. Your load resistor trick will work a lot better. > >> >> TIA input impedance should be zero (as stated in books) because of >> virtual ground at inverting input. But, because emitter is at -12V, >> the phototransitor will see a variable resistor matched to drop >> exactly 12V for any current. So, for "small" currents this resistor >> becomes big", and for big current this resistor turns "small". Is >> this right,somehow? > >No. It's the transistor that adjusts, not the load. > >> >> I could add a resistor from inverting input to ground (10K range) to >> set a limit to input impedance, > >That won't make any difference except to increase the noise a little. > >> and a capacitor in parallel to that >> resistor (nF range) to bypass everything the opamp will not be able >> to follow due to bandwidth limitation. Useless or not? > >You really don't want to hang a capacitor to ground on a summing >junction. It's very likely to make the whole thing oscillate.An NPN cascode transistor would isolate the cable capacitance from the opamp. If the wiring is symmetric (shielded pair) a common-mode inductor might help with noise. But if the noise is getting into the base of the phototransistor, that won't help. Might make things worse. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 18, 20162016-07-18
On 2016-07-17 04:14, Phil Hobbs wrote:> Why not put the TIA at the sensor? > > Putting long leads on the summing junction of an op amp is a > guaranteed way of getting into trouble. >... or the combination of the innards of a PMT plus a way too long socket, resonating all over the place. Don't ask ...> With equal resistors and the same power rails, your TIA and load > resistor circuits will saturate at about the same light level. > > What's it for? >Silvio: In a pinch you can try 100ohms or so in series with the TIA input, right at the TIA opamp. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●July 18, 20162016-07-18
On Mon, 18 Jul 2016 14:10:45 -0700, Joerg <news@analogconsultants.com> wrote:>On 2016-07-17 04:14, Phil Hobbs wrote: >> Why not put the TIA at the sensor? >> >> Putting long leads on the summing junction of an op amp is a >> guaranteed way of getting into trouble. >> > >... or the combination of the innards of a PMT plus a way too long >socket, resonating all over the place. Don't ask ... > > >> With equal resistors and the same power rails, your TIA and load >> resistor circuits will saturate at about the same light level. >> >> What's it for? >> > >Silvio: In a pinch you can try 100ohms or so in series with the TIA >input, right at the TIA opamp.Run a fiberoptic cable from the light source to the photodiode. Fiber is 100% EMI immune. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 18, 20162016-07-18
On 2016-07-18 14:19, John Larkin wrote:> On Mon, 18 Jul 2016 14:10:45 -0700, Joerg <news@analogconsultants.com> > wrote: > >> On 2016-07-17 04:14, Phil Hobbs wrote: >>> Why not put the TIA at the sensor? >>> >>> Putting long leads on the summing junction of an op amp is a >>> guaranteed way of getting into trouble. >>> >> >> ... or the combination of the innards of a PMT plus a way too long >> socket, resonating all over the place. Don't ask ... >> >> >>> With equal resistors and the same power rails, your TIA and load >>> resistor circuits will saturate at about the same light level. >>> >>> What's it for? >>> >> >> Silvio: In a pinch you can try 100ohms or so in series with the TIA >> input, right at the TIA opamp. > > Run a fiberoptic cable from the light source to the photodiode. Fiber > is 100% EMI immune. >It's all a matter of field strength. I learned that the hard way as a teenager. In high school physics class we learned about dielectric materials and isolators but not about loss tangents and the like. So I had the marvelous idea of using a large high voltage cap from a discarded TV set for the Pi-filter in my kilowatt-class ham radio amp. It had the required voltage ratimng alright. Free stuff! Woohoo! It worked for a while and then ... tzzzt ... *BAM* ... THWOCK ... splatter. The previously white ceramic of the capacitor had turned into green molten glass pebbles. Luckily I didn't get any in my eyes. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●July 18, 20162016-07-18
On Mon, 18 Jul 2016 15:34:19 -0700, Joerg <news@analogconsultants.com> wrote:>On 2016-07-18 14:19, John Larkin wrote: >> On Mon, 18 Jul 2016 14:10:45 -0700, Joerg <news@analogconsultants.com> >> wrote: >> >>> On 2016-07-17 04:14, Phil Hobbs wrote: >>>> Why not put the TIA at the sensor? >>>> >>>> Putting long leads on the summing junction of an op amp is a >>>> guaranteed way of getting into trouble. >>>> >>> >>> ... or the combination of the innards of a PMT plus a way too long >>> socket, resonating all over the place. Don't ask ... >>> >>> >>>> With equal resistors and the same power rails, your TIA and load >>>> resistor circuits will saturate at about the same light level. >>>> >>>> What's it for? >>>> >>> >>> Silvio: In a pinch you can try 100ohms or so in series with the TIA >>> input, right at the TIA opamp. >> >> Run a fiberoptic cable from the light source to the photodiode. Fiber >> is 100% EMI immune. >> > >It's all a matter of field strength. I learned that the hard way as a >teenager. In high school physics class we learned about dielectric >materials and isolators but not about loss tangents and the like. So I >had the marvelous idea of using a large high voltage cap from a >discarded TV set for the Pi-filter in my kilowatt-class ham radio amp. >It had the required voltage ratimng alright. Free stuff! Woohoo! It >worked for a while and then ... tzzzt ... *BAM* ... THWOCK ... splatter. >The previously white ceramic of the capacitor had turned into green >molten glass pebbles. Luckily I didn't get any in my eyes.Are you trying out for the sound effects position for the next Batman and Robin movie ?>:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Reply by ●July 18, 20162016-07-18
On Monday, July 18, 2016 at 4:03:24 PM UTC-4, Phil Hobbs wrote:> On 07/18/2016 03:28 PM, SilverLeo wrote: > > Il giorno domenica 17 luglio 2016 09:56:50 UTC+2, SilverLeo ha > > scritto: > > > >> cut > > > > I'm replying to myself: I apologize, this seems the only action > > Google groups let me to do. > > > > So: interesting the cap multiplier approach; I'll do some math, maybe > > it'll be handy in the future. I cannot move the TIA close to the > > phototransistor: it's a retrofit for an old device for estimate the > > density of filled paper tubes (filters for very specific application) > > through the detection of diffused light. The more filled is the tube, > > the less is the light diffused. Phototransistor is buried inside a > > cast iron frame, far away from any conditioning electronic. > > > > The main reason that lead to the TIA approach was the fact that > > pototransistor with classic resistor on collector will fall into > > saturation (Vce near or below Vcesat), actually messing up density > > estimation. A TIA, as far as the opamp keeps doing its magic, will > > force Vce=Vbias, running the phototransistor deep in active region. > > Except that with the same size resistor and power supply, both will > saturate at about the same light level--the TIA doesn't help that at > all. Phototransistors have horrible linearity anyway, so the swing at > the collector is the least of your worries. > > > > > This in theory: in real world application the TIA output shows 50mV > > of noise voltage, that does not correspond to a 10uA of current > > noise, because with the simple resistor there is no evidence of it. > > This noise comes out when some valves or motors are running. > > If you don't understand what I said about not hanging cables on summing > junctions, just trust me. Your load resistor trick will work a lot better. > > > > > TIA input impedance should be zero (as stated in books) because of > > virtual ground at inverting input. But, because emitter is at -12V, > > the phototransitor will see a variable resistor matched to drop > > exactly 12V for any current. So, for "small" currents this resistor > > becomes big", and for big current this resistor turns "small". Is > > this right,somehow? > > No. It's the transistor that adjusts, not the load. > > > > > I could add a resistor from inverting input to ground (10K range) to > > set a limit to input impedance, > > That won't make any difference except to increase the noise a little. > > > and a capacitor in parallel to that > > resistor (nF range) to bypass everything the opamp will not be able > > to follow due to bandwidth limitation. Useless or not? > > You really don't want to hang a capacitor to ground on a summing > junction. It's very likely to make the whole thing oscillate.Hey, I think it's a good idea for people who don't understand TIA's and input C, to hang more on the inverting input and see what happens. George H.> > > > > If I cannot overcome this issue I'll switch back to old, classi > > resistor on collector; but, it would be nice to understand what's > > going on. > > I recommend "The Art of Electronics", third edition, sections 2.5 > (negative feedback) and Chapter 4 (op amps). > > Cheers > > Phil Hobbs > > > > -- > Dr Philip C D Hobbs > Principal Consultant > ElectroOptical Innovations LLC > Optics, Electro-optics, Photonics, Analog Electronics > > 160 North State Road #203 > Briarcliff Manor NY 10510 > > hobbs at electrooptical dot net > http://electrooptical.net
Reply by ●July 18, 20162016-07-18
On Monday, July 18, 2016 at 5:19:35 PM UTC-4, John Larkin wrote:> On Mon, 18 Jul 2016 14:10:45 -0700, Joerg <news@analogconsultants.com> > wrote: > > >On 2016-07-17 04:14, Phil Hobbs wrote: > >> Why not put the TIA at the sensor? > >> > >> Putting long leads on the summing junction of an op amp is a > >> guaranteed way of getting into trouble. > >> > > > >... or the combination of the innards of a PMT plus a way too long > >socket, resonating all over the place. Don't ask ... > > > > > >> With equal resistors and the same power rails, your TIA and load > >> resistor circuits will saturate at about the same light level. > >> > >> What's it for? > >> > > > >Silvio: In a pinch you can try 100ohms or so in series with the TIA > >input, right at the TIA opamp. > > Run a fiberoptic cable from the light source to the photodiode. Fiber > is 100% EMI immune. > >I like the fiber idea, you've gotta keep the (input) end clean. George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●July 18, 20162016-07-18
On Mon, 18 Jul 2016 18:28:33 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Monday, July 18, 2016 at 5:19:35 PM UTC-4, John Larkin wrote: >> On Mon, 18 Jul 2016 14:10:45 -0700, Joerg <news@analogconsultants.com> >> wrote: >> >> >On 2016-07-17 04:14, Phil Hobbs wrote: >> >> Why not put the TIA at the sensor? >> >> >> >> Putting long leads on the summing junction of an op amp is a >> >> guaranteed way of getting into trouble. >> >> >> > >> >... or the combination of the innards of a PMT plus a way too long >> >socket, resonating all over the place. Don't ask ... >> > >> > >> >> With equal resistors and the same power rails, your TIA and load >> >> resistor circuits will saturate at about the same light level. >> >> >> >> What's it for? >> >> >> > >> >Silvio: In a pinch you can try 100ohms or so in series with the TIA >> >input, right at the TIA opamp. >> >> Run a fiberoptic cable from the light source to the photodiode. Fiber >> is 100% EMI immune. >> >> >I like the fiber idea, you've gotta keep the (input) end clean. > >George H.Sure, like you gotta keep the phototransistor clean. The big fat cheap plastic fiber, like toslink, has a big aperature to let light in, but it has a lot of attenuation and wouldn't like high temperatures. There is fat glass fiber, but it tends to be expensive. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●July 19, 20162016-07-19
