Mr Stonebeach wrote:> > On Sep 10, 11:17 am, miso <m...@sushi.com> wrote: > > On 9/9/2012 6:50 PM, John Larkin wrote: > > > Has anybody done this? > > >https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > > > > Analog Device SSM-2220 data sheet uses that scheme. Nelson Pass amps > > have used it in production. LED biasing was a wave or sorts in high end > > audio. > > Also in SQUID readout amplifiers, like Pasquarelli & Rossi in the > ISEC'97 conference. > > > The RF transistor I presume is for low capacitance. The low beta could > > be an issue. > > What about NESG4030, hFE=400 typ, Cre=0.12pF typ? I know, costs way > too much... > > Regards, > MikkoThe LED trick is about 20 dB quieter than a bandgap, and reasonably competitive with buried zeners for noise, but of course not for stability. I haven't done this recently enough to have tempco data on modern LEDs. (I've only used the LED + emitter follower voltage reference form.) 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
LED reference current source
Started by ●September 9, 2012
Reply by ●September 10, 20122012-09-10
Reply by ●September 10, 20122012-09-10
"langwadt@fonz.dk" wrote:> > On Sep 10, 3:50 am, John Larkin > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > > Has anybody done this? > > > > https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > > > > seen it done since forever for audio amps, I believe some claim they > are > also less noisy than regular diodes, just remember to shield the LED > from light > > http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf > > looks like somewhere between red and UV will get close to zero tempco > > -LasseGood paper, thanks for the reference. 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 09:48:23 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>"langwadt@fonz.dk" wrote: >> >> On Sep 10, 3:50 am, John Larkin >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> > Has anybody done this? >> > >> > https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> > >> >> seen it done since forever for audio amps, I believe some claim they >> are >> also less noisy than regular diodes, just remember to shield the LED >> from light >> >> http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf >> >> looks like somewhere between red and UV will get close to zero tempco >> >> -Lasse > >Good paper, thanks for the reference. > >Cheers > >Phil HobbsBut an LED forward is not "stiff", so where's the benefit? Or is this just one of those "fun things to do"? ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 09:11:10 +0100, Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:>On 10/09/2012 08:26, Tim Williams wrote: >> "John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in >> message news:j5pq48d85781hruk3vj18j91s223blk0bl@4ax.com... >>>> Toob guys have been doing it for ages. Where 'ya been? >>> >>> Like, as in a VR tube and a p-channel triode? >> >> No, as shown... They do it in immitation of the SS amp guys, who have >> been doing it for even longer. >> >>> As far as doing it with semiconductors, there seems to be a lot of >>> different chemistries in LEDs, namely lots of different forward >>> voltage curves for a diveb color, and not much data about tempcos. >> >> I once had a chemistry lab about bandgap. We took a battery, LED and >> styrofoam cup of LN2, and proceeded to observe the color change upon >> putting them together. Theory from lecture said, cold --> crystal >> shrinks --> wavelength shrinks. > >That LN2 demo was always much more fun with the early generation of low >efficiency red LEDs. The quantum efficiency rockets up when the lattice >is cooled and thermal vibrations are decreased much more rigid lattice. >Unfortunately they die after a few hundred cycles or less. > >I don't recall seeing the wavelength shift perceptably though the >brightness changed by a very significant amount. Admitedly on a red led >this could be both quantum efficiency and it producing shorter >wavelength red light which is perceived as bright. >> >> So naturally, I had grabbed a green (GaP) LED out of the bin (versus red >> GaAsP, or blue GaN, which apparently are more similar to each other than >> to GaP). The professor noted its behavior and suggested I demonstrate >> with a red or blue LED instead. ;-) > >I still have a few original 1970's red, yellow and green LEDs that are >now in series with modern high efficiency ones to show them off.We used to run 50 mA through the Cree blue SiC led's as a VME bus-activity indicator, a pair of F38s and a 33 ohm resistor or something like that. As time went on and led's got better, we started blinding our customers. We use about 1 mA now, into the Osrams. I got dark adapted and tried to find the minimum current that a green Avago led needs to produce detectable light. It was around 800 pA. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 01:22:29 -0700, miso <miso@sushi.com> wrote:>On 9/9/2012 10:21 PM, Grant wrote: >> On Sun, 09 Sep 2012 18:50:25 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: >> >>> >>> Has anybody done this? >>> >>> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >> I saw it decades ago for audio amp bias, claim was green LED >> (old-style now, not high modern intensity types) balanced Vbe >> tempco. >> >> Grant. >> >The high efficiency LEDs are double heterojuntion. When they first came >out, the existing generation LED drives had insufficient compliance and >produced a dimmer light than the standard LEDs. That is the high voltage >of the double heterojunction LED made the drive chips put out less current. > >It wasn't like the LED companies gave the chip companies a heads up on >the specs of the new generation LEDs so that the new driver chips would >be available.Here are some numbers for the Osram LB T67C series, beautiful parts. https://dl.dropbox.com/u/53724080/Parts/OsramCurrents.JPG -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 04:53:36 -0700 (PDT), Mr Stonebeach <reg@wmail.fi> wrote:>On Sep 10, 11:17�am, miso <m...@sushi.com> wrote: >> On 9/9/2012 6:50 PM, John Larkin wrote: >> > Has anybody done this? >> >https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >> Analog Device SSM-2220 data sheet uses that scheme. Nelson Pass amps >> have used it in production. LED biasing was a wave or sorts in high end >> audio. > > Also in SQUID readout amplifiers, like Pasquarelli & Rossi in the >ISEC'97 conference. > >> The RF transistor I presume is for low capacitance. The low beta could >> be an issue. > > What about NESG4030, hFE=400 typ, Cre=0.12pF typ? I know, costs way >too much... > > Regards, > MikkoWow, that's impressive, except that I need PNP in this case. I figured I'd use an MMBTH81, min beta 60, kinda high capacitance but tolerable in this application, since I'm charging a capacitor anyhow. I'm doing a one-shot sort of thing that's not too critical, pulse width programmable from 2 ns to 200 ps. PNPs suck. NE97833 has the speeed and capacitance of the NESG4030, but typical beta is 30. Maybe I could live with that, since beta changes, wot, 1% per degree or something. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 01:59:59 -0700 (PDT), "langwadt@fonz.dk" <langwadt@fonz.dk> wrote:>On Sep 10, 3:50�am, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> Has anybody done this? >> >> https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> > >seen it done since forever for audio amps, I believe some claim they >are >also less noisy than regular diodes, just remember to shield the LED >from light > >http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf > >looks like somewhere between red and UV will get close to zero tempco > > >-LasseGreat data; thanks for that one. My orange Osram may compensate the Vbe pretty well. My board is only 2" square, and it's brickwalled with parts already, and it needs a power indicator anyhow, so I figured I'd use the LED twice. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Reply by ●September 10, 20122012-09-10
Jim Thompson wrote:> > On Mon, 10 Sep 2012 09:48:23 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >"langwadt@fonz.dk" wrote: > >> > >> On Sep 10, 3:50 am, John Larkin > >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >> > Has anybody done this? > >> > > >> > https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > >> > > >> > >> seen it done since forever for audio amps, I believe some claim they > >> are > >> also less noisy than regular diodes, just remember to shield the LED > >> from light > >> > >> http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf > >> > >> looks like somewhere between red and UV will get close to zero tempco > >> > >> -Lasse > > > >Good paper, thanks for the reference. > > > >Cheers > > > >Phil Hobbs > > But an LED forward is not "stiff", so where's the benefit? Or is this > just one of those "fun things to do"? > > ...Jim Thompson > --The dynamic impedance is pretty low, so in the voltage-reference mode, you can wrap an op amp around it to provide its own bias. The benefit is mainly getting low noise at low impedance and low voltage without needing BFCs. You can probably do a series-shunt feedback approach to combine the V_BE drop and constant excitation current, but I haven't looked at it in any detail. 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 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●September 10, 20122012-09-10
On 10 Sep., 17:44, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> Jim Thompson wrote: > > > On Mon, 10 Sep 2012 09:48:23 -0400, Phil Hobbs > > <pcdhSpamMeSensel...@electrooptical.net> wrote: > > > >"langw...@fonz.dk" wrote: > > > >> On Sep 10, 3:50 am, John Larkin > > >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > > >> > Has anybody done this? > > > >> >https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG > > > >> seen it done since forever for audio amps, I believe some claim they > > >> are > > >> also less noisy than regular diodes, just remember to shield the LED > > >> from light > > > >>http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tem=...> > > >> looks like somewhere between red and UV will get close to zero tempc=o> > > >> -Lasse > > > >Good paper, thanks for the reference. > > > >Cheers > > > >Phil Hobbs > > > But an LED forward is not "stiff", so where's the benefit? =A0Or is thi=s> > just one of those "fun things to do"? > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0==A0 =A0 ...Jim Thompson> > -- > > The dynamic impedance is pretty low, so in the voltage-reference mode, > you can wrap an op amp around it to provide its own bias. =A0The benefit > is mainly getting low noise at low impedance and low voltage without > needing BFCs. =A0You can probably do a series-shunt feedback approach to > combine the V_BE drop and constant excitation current, but I haven't > looked at it in any detail. > > Cheers >yeh, from Johns data it looks like they about par with Zeners in dynamic impedance -Lasse
Reply by ●September 10, 20122012-09-10
On Mon, 10 Sep 2012 11:44:58 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Jim Thompson wrote: >> >> On Mon, 10 Sep 2012 09:48:23 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >"langwadt@fonz.dk" wrote: >> >> >> >> On Sep 10, 3:50 am, John Larkin >> >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> >> > Has anybody done this? >> >> > >> >> > https://dl.dropbox.com/u/53724080/Circuits/Isrc_LED.JPG >> >> > >> >> >> >> seen it done since forever for audio amps, I believe some claim they >> >> are >> >> also less noisy than regular diodes, just remember to shield the LED >> >> from light >> >> >> >> http://www.ka-electronics.com/Images/pdf/Junction_Temperature_LED_Tempco.pdf >> >> >> >> looks like somewhere between red and UV will get close to zero tempco >> >> >> >> -Lasse >> > >> >Good paper, thanks for the reference. >> > >> >Cheers >> > >> >Phil Hobbs >> >> But an LED forward is not "stiff", so where's the benefit? Or is this >> just one of those "fun things to do"? >> >> ...Jim Thompson >> -- > >The dynamic impedance is pretty low, so in the voltage-reference mode, >you can wrap an op amp around it to provide its own bias. The benefit >is mainly getting low noise at low impedance and low voltage without >needing BFCs. You can probably do a series-shunt feedback approach to >combine the V_BE drop and constant excitation current, but I haven't >looked at it in any detail. > >Cheers > >Phil HobbsIn my circuit, I think you can add a resistor from the PNP emitter to ground, to cancel the non-zero dynamic impedance of the LED. Too bad I'm out of room on this board. My supply is pretty stable anyhow, and I don't need 10-bit accuracy on the timing here. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
