Am 11.02.2018 um 16:17 schrieb Gerhard Hoffmann:> But it could do cross spectrum. Since the 1/f seems to come from the > sample clock and not from the inputs, there still is hope.Oh no, that won't help.
BJT base current 1/f noise
Started by ●February 10, 2018
Reply by ●February 11, 20182018-02-11
Reply by ●February 11, 20182018-02-11
On Sun, 11 Feb 2018 08:05:11 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 02/10/2018 08:36 PM, John Larkin wrote: >> On Sat, 10 Feb 2018 19:39:32 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 02/10/18 18:41, John Larkin wrote: >>>> On Sat, 10 Feb 2018 17:18:55 -0500, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> Is horrible. >>>>> >>>>> I'm just debugging a nice diode laser controller for one >>>>> customer and getting ready to do another one for another (much >>>>> better) customer. It uses a ZXTP25020 PNP with a biggish >>>>> degeneration resistor and two-pole bypassing of the base to get >>>>> low noise at high frequency. >>>>> >>>>> Turns out the 1/f noise is atrocious. The total noise from 2 >>>>> Hz on up is only a couple of PPM, but it should be a factor of >>>>> 10 better than that. With a total base resistance of 1k, the >>>>> 1/f corner at 40 mA I_C is over 10 kHz. >>>>> >>>>> The bias network looks like this: >>>>> >>>>> +9V 0----------*--------*-------* | | | | | >>>>> | | | R | 4.7 | 3.3 R 39 ohm TF | nF | >>>>> nF R CCC CCC | CCC CCC | | | >>>>> | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- >>>>> | Op Amp |\ 499 499 | \ | >>>>> | --- \ / --> V --> ----- | | GND >>>>> >>>>> The emitter resistor can drop up to about 5V. >>>>> >>>>> I've got some higher-beta transistors on order (FZT788B), and >>>>> will reduce the impedance of the bias network, but I was fairly >>>>> shocked that the 1/f noise was so bad. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> >>>> That's a pretty fast transistor; could it be oscillating? >>> >>> Don't think so. I have a 1 GHz scope across the laser, and didn't >>> see anything like that, and have applied thermal-digital >>> analysis(*) to the whole thing. I'll try going to 49.9 ohms and 47 >>> nF and see if it gets better, which it should. I might need to use >>> a TCA0372 booster to drive 5-ohm resistors. :( >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> (*) i.e. heat gun, cold spray, and fingers >> >> Yeah, the base current noise into the 1K resistor string could make >> a lot of noise... assuming the signal from the opamp is perfect. >> >> Is the overall feedback optical? >> >> > >Eventually, but at the moment it's coming from the emitter resistor, so >that the loop doesn't null out the base current noise even within its >bandwidth. > >The idea is to make the bias current way sub-Poissonian, which if done >right results in useful amounts of amplitude squeezing of the light, >i.e. your detected photocurrent can actually be a few dB quieter than >full shot noise if you keep the optical loss low enough. It's more of a >checklist feature for most uses, of course, but as you've pointed out >there are a lot of crappy diode laser controllers out there, selling for >cheap, so building one that just makes the problems magically go away is >attractive. > >Building a new-to-me kind of high dynamic range system always seems to >produce a bit of buried treasure at first. The ZXTP25020 has super-low >Rbb' and Ree', so it would be an excellent candidate except for the 1/f >noise. If the higher-beta device and lower drive impedance doesn't >improve it enough, I'll have to try Darlingtonizing it with one of those >nice SiGe:C devices to return the base current to the collector circuit. > >That'd need a board spin though, and I'm super busy at the moment. > >Cheers > >Phil HobbsHow about a Sziklai pair sort of thing, with a BF862 and a PNP? Or a Pfet-PNP darlington? Disappear the base current. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 11, 20182018-02-11
On 02/11/2018 11:34 AM, John Larkin wrote:> On Sun, 11 Feb 2018 08:05:11 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 02/10/2018 08:36 PM, John Larkin wrote: >>> On Sat, 10 Feb 2018 19:39:32 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 02/10/18 18:41, John Larkin wrote: >>>>> On Sat, 10 Feb 2018 17:18:55 -0500, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>>> Is horrible. >>>>>> >>>>>> I'm just debugging a nice diode laser controller for one >>>>>> customer and getting ready to do another one for another (much >>>>>> better) customer. It uses a ZXTP25020 PNP with a biggish >>>>>> degeneration resistor and two-pole bypassing of the base to get >>>>>> low noise at high frequency. >>>>>> >>>>>> Turns out the 1/f noise is atrocious. The total noise from 2 >>>>>> Hz on up is only a couple of PPM, but it should be a factor of >>>>>> 10 better than that. With a total base resistance of 1k, the >>>>>> 1/f corner at 40 mA I_C is over 10 kHz. >>>>>> >>>>>> The bias network looks like this: >>>>>> >>>>>> +9V 0----------*--------*-------* | | | | | >>>>>> | | | R | 4.7 | 3.3 R 39 ohm TF | nF | >>>>>> nF R CCC CCC | CCC CCC | | | >>>>>> | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- >>>>>> | Op Amp |\ 499 499 | \ | >>>>>> | --- \ / --> V --> ----- | | GND >>>>>> >>>>>> The emitter resistor can drop up to about 5V. >>>>>> >>>>>> I've got some higher-beta transistors on order (FZT788B), and >>>>>> will reduce the impedance of the bias network, but I was fairly >>>>>> shocked that the 1/f noise was so bad. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>> >>>>> That's a pretty fast transistor; could it be oscillating? >>>> >>>> Don't think so. I have a 1 GHz scope across the laser, and didn't >>>> see anything like that, and have applied thermal-digital >>>> analysis(*) to the whole thing. I'll try going to 49.9 ohms and 47 >>>> nF and see if it gets better, which it should. I might need to use >>>> a TCA0372 booster to drive 5-ohm resistors. :( >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>>> (*) i.e. heat gun, cold spray, and fingers >>> >>> Yeah, the base current noise into the 1K resistor string could make >>> a lot of noise... assuming the signal from the opamp is perfect. >>> >>> Is the overall feedback optical? >>> >>> >> >> Eventually, but at the moment it's coming from the emitter resistor, so >> that the loop doesn't null out the base current noise even within its >> bandwidth. >> >> The idea is to make the bias current way sub-Poissonian, which if done >> right results in useful amounts of amplitude squeezing of the light, >> i.e. your detected photocurrent can actually be a few dB quieter than >> full shot noise if you keep the optical loss low enough. It's more of a >> checklist feature for most uses, of course, but as you've pointed out >> there are a lot of crappy diode laser controllers out there, selling for >> cheap, so building one that just makes the problems magically go away is >> attractive. >> >> Building a new-to-me kind of high dynamic range system always seems to >> produce a bit of buried treasure at first. The ZXTP25020 has super-low >> Rbb' and Ree', so it would be an excellent candidate except for the 1/f >> noise. If the higher-beta device and lower drive impedance doesn't >> improve it enough, I'll have to try Darlingtonizing it with one of those >> nice SiGe:C devices to return the base current to the collector circuit. >> >> That'd need a board spin though, and I'm super busy at the moment. >> >> Cheers >> >> Phil Hobbs > > How about a Sziklai pair sort of thing, with a BF862 and a PNP? Or a > Pfet-PNP darlington? Disappear the base current.I need it to be a PNP-type when it's done, because the lasers are usually grounded-cathode. (Keeping the high frequency output impedance high is the name of the squeezing game--with an emitter follower you get twice the shot noise.) If I could get good PFETs, that would be great, but they're scarce these days and their noise tends to be horrible. The Darlington-bandaid approach could be done with an NPN and a couple of auxiliary current sources, but it would be a bit of a mess. The advantage would be that (done right) it would have the noise of the BFP640, which is very low. Tomorrow I'll try reducing the base impedance by 10x and see if that fixes it. Because of the 39-ohm emitter resistor, it doesn't make sense to go very much lower than that. One nice thing about 1/f noise is that a 10x amplitude improvement gets you 100x lower corner frequency. At that point I can put in a low-frequency bandaid if I need to, such as sensing the voltage across the base resistor and dumping the equivalent into the emitter so that the op amp tracks out the LF noise. The trick will be to avoid a noise peak at the crossover frequency of the bandaid. 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 ●February 11, 20182018-02-11
On Saturday, February 10, 2018 at 5:19:02 PM UTC-5, Phil Hobbs wrote:> Is horrible. > > I'm just debugging a nice diode laser controller for one customer and > getting ready to do another one for another (much better) customer. It > uses a ZXTP25020 PNP with a biggish degeneration resistor and two-pole > bypassing of the base to get low noise at high frequency. > > Turns out the 1/f noise is atrocious. The total noise from 2 Hz on up > is only a couple of PPM, but it should be a factor of 10 better than > that. With a total base resistance of 1k, the 1/f corner at 40 mA I_C > is over 10 kHz. > > The bias network looks like this: > > +9V 0----------*--------*-------* > | | | > | | | > | | R > | 4.7 | 3.3 R 39 ohm TF > | nF | nF R > CCC CCC | > CCC CCC | > | | | / > | | |< ZXTP25020 > From 0--RRRR--*--RRRR--*---- | > Op Amp |\ > 499 499 | \ > | > | > --- > \ / --> > V --> > ----- > | > | > GND > > The emitter resistor can drop up to about 5V. > > I've got some higher-beta transistors on order (FZT788B), and will > reduce the impedance of the bias network, but I was fairly shocked that > the 1/f noise was so bad.No idea, did you try more than one? A tangential question; I use a pfet in 'the same'* circuit, is the pnp better.. lower noise? George H. *+/- 50 % not sure of cap values.> > 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 > https://hobbs-eo.com
Reply by ●February 11, 20182018-02-11
On Sunday, February 11, 2018 at 12:36:49 PM UTC-5, George Herold wrote:> On Saturday, February 10, 2018 at 5:19:02 PM UTC-5, Phil Hobbs wrote: > > Is horrible. > > > > I'm just debugging a nice diode laser controller for one customer and > > getting ready to do another one for another (much better) customer. It > > uses a ZXTP25020 PNP with a biggish degeneration resistor and two-pole > > bypassing of the base to get low noise at high frequency. > > > > Turns out the 1/f noise is atrocious. The total noise from 2 Hz on up > > is only a couple of PPM, but it should be a factor of 10 better than > > that. With a total base resistance of 1k, the 1/f corner at 40 mA I_C > > is over 10 kHz. > > > > The bias network looks like this: > > > > +9V 0----------*--------*-------* > > | | | > > | | | > > | | R > > | 4.7 | 3.3 R 39 ohm TF > > | nF | nF R > > CCC CCC | > > CCC CCC | > > | | | / > > | | |< ZXTP25020 > > From 0--RRRR--*--RRRR--*---- | > > Op Amp |\ > > 499 499 | \ > > | > > | > > --- > > \ / --> > > V --> > > ----- > > | > > | > > GND > > > > The emitter resistor can drop up to about 5V. > > > > I've got some higher-beta transistors on order (FZT788B), and will > > reduce the impedance of the bias network, but I was fairly shocked that > > the 1/f noise was so bad. > No idea, did you try more than one? > A tangential question; I use a pfet in 'the same'* circuit, > is the pnp better.. lower noise? > > George H.Never mind, just read your response to Tim W. GH> > *+/- 50 % not sure of cap values. > > > > > 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 > > https://hobbs-eo.com
Reply by ●February 11, 20182018-02-11
On 02/11/2018 10:17 AM, Gerhard Hoffmann wrote:> Am 11.02.2018 um 15:25 schrieb Phil Hobbs: > though, thanks. I expect that it would be >> interesting to measure the 1/f noise and drift of just the batteries, >> using the cross-power-spectrum method. Down in the <1 Hz range their >> temperature drift isn't insignificant. > > > < > https://www.researchgate.net/publication/3622215_Measurement_of_voltage_noise_in_chemical_batteries > > > > > < > http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf > > > > My 89441A that I use as a FFT analyzer is severely 1/f challenged. > I'm looking for something better. > > I'm working on a chopper amplifier that seems to break the 100pV/rt HZ, > but my FFT analyzer hides just that where it starts to be interesting. > > But it could do cross spectrum. Since the 1/f seems to come from the > sample clock and not from the inputs, there still is hope. > > cheers, GerhardNice work! For the unloaded batteries plotted on P 11, below 100 Hz the dependence seems to be much more like 1/f**2 or even 1/f**3, which looks to me like temperature drift rather than actual 1/f noise. I don't know of a mechanism for making 1/f noise at zero current, but you're clearly seeing it above about 10-100 Hz. Interesting. 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 ●February 11, 20182018-02-11
On Sun, 11 Feb 2018 12:32:55 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 02/11/2018 11:34 AM, John Larkin wrote: >> On Sun, 11 Feb 2018 08:05:11 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 02/10/2018 08:36 PM, John Larkin wrote: >>>> On Sat, 10 Feb 2018 19:39:32 -0500, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 02/10/18 18:41, John Larkin wrote: >>>>>> On Sat, 10 Feb 2018 17:18:55 -0500, Phil Hobbs >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>> >>>>>>> Is horrible. >>>>>>> >>>>>>> I'm just debugging a nice diode laser controller for one >>>>>>> customer and getting ready to do another one for another (much >>>>>>> better) customer. It uses a ZXTP25020 PNP with a biggish >>>>>>> degeneration resistor and two-pole bypassing of the base to get >>>>>>> low noise at high frequency. >>>>>>> >>>>>>> Turns out the 1/f noise is atrocious. The total noise from 2 >>>>>>> Hz on up is only a couple of PPM, but it should be a factor of >>>>>>> 10 better than that. With a total base resistance of 1k, the >>>>>>> 1/f corner at 40 mA I_C is over 10 kHz. >>>>>>> >>>>>>> The bias network looks like this: >>>>>>> >>>>>>> +9V 0----------*--------*-------* | | | | | >>>>>>> | | | R | 4.7 | 3.3 R 39 ohm TF | nF | >>>>>>> nF R CCC CCC | CCC CCC | | | >>>>>>> | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- >>>>>>> | Op Amp |\ 499 499 | \ | >>>>>>> | --- \ / --> V --> ----- | | GND >>>>>>> >>>>>>> The emitter resistor can drop up to about 5V. >>>>>>> >>>>>>> I've got some higher-beta transistors on order (FZT788B), and >>>>>>> will reduce the impedance of the bias network, but I was fairly >>>>>>> shocked that the 1/f noise was so bad. >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>> >>>>>> That's a pretty fast transistor; could it be oscillating? >>>>> >>>>> Don't think so. I have a 1 GHz scope across the laser, and didn't >>>>> see anything like that, and have applied thermal-digital >>>>> analysis(*) to the whole thing. I'll try going to 49.9 ohms and 47 >>>>> nF and see if it gets better, which it should. I might need to use >>>>> a TCA0372 booster to drive 5-ohm resistors. :( >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>>> >>>>> (*) i.e. heat gun, cold spray, and fingers >>>> >>>> Yeah, the base current noise into the 1K resistor string could make >>>> a lot of noise... assuming the signal from the opamp is perfect. >>>> >>>> Is the overall feedback optical? >>>> >>>> >>> >>> Eventually, but at the moment it's coming from the emitter resistor, so >>> that the loop doesn't null out the base current noise even within its >>> bandwidth. >>> >>> The idea is to make the bias current way sub-Poissonian, which if done >>> right results in useful amounts of amplitude squeezing of the light, >>> i.e. your detected photocurrent can actually be a few dB quieter than >>> full shot noise if you keep the optical loss low enough. It's more of a >>> checklist feature for most uses, of course, but as you've pointed out >>> there are a lot of crappy diode laser controllers out there, selling for >>> cheap, so building one that just makes the problems magically go away is >>> attractive. >>> >>> Building a new-to-me kind of high dynamic range system always seems to >>> produce a bit of buried treasure at first. The ZXTP25020 has super-low >>> Rbb' and Ree', so it would be an excellent candidate except for the 1/f >>> noise. If the higher-beta device and lower drive impedance doesn't >>> improve it enough, I'll have to try Darlingtonizing it with one of those >>> nice SiGe:C devices to return the base current to the collector circuit. >>> >>> That'd need a board spin though, and I'm super busy at the moment. >>> >>> Cheers >>> >>> Phil Hobbs >> >> How about a Sziklai pair sort of thing, with a BF862 and a PNP? Or a >> Pfet-PNP darlington? Disappear the base current. > >I need it to be a PNP-type when it's done, because the lasers are >usually grounded-cathode. (Keeping the high frequency output impedance >high is the name of the squeezing game--with an emitter follower you get >twice the shot noise.) If I could get good PFETs, that would be great, >but they're scarce these days and their noise tends to be horrible. > >The Darlington-bandaid approach could be done with an NPN and a couple >of auxiliary current sources, but it would be a bit of a mess. The >advantage would be that (done right) it would have the noise of the >BFP640, which is very low. > >Tomorrow I'll try reducing the base impedance by 10x and see if that >fixes it. Because of the 39-ohm emitter resistor, it doesn't make sense >to go very much lower than that. > >One nice thing about 1/f noise is that a 10x amplitude improvement gets >you 100x lower corner frequency. At that point I can put in a >low-frequency bandaid if I need to, such as sensing the voltage across >the base resistor and dumping the equivalent into the emitter so that >the op amp tracks out the LF noise. The trick will be to avoid a noise >peak at the crossover frequency of the bandaid. > >Cheers > >Phil HobbsHere's an idea that I've never actually built: https://www.dropbox.com/s/lr3aekzziecz7pz/Isrc_cascode_Ib.JPG?raw=1 I don't remember what those two caps are for. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 11, 20182018-02-11
On 02/11/2018 12:36 PM, George Herold wrote:> On Saturday, February 10, 2018 at 5:19:02 PM UTC-5, Phil Hobbs wrote: >> Is horrible. >> >> I'm just debugging a nice diode laser controller for one customer and >> getting ready to do another one for another (much better) customer. It >> uses a ZXTP25020 PNP with a biggish degeneration resistor and two-pole >> bypassing of the base to get low noise at high frequency. >> >> Turns out the 1/f noise is atrocious. The total noise from 2 Hz on up >> is only a couple of PPM, but it should be a factor of 10 better than >> that. With a total base resistance of 1k, the 1/f corner at 40 mA I_C >> is over 10 kHz. >> >> The bias network looks like this: >> >> +9V 0----------*--------*-------* >> | | | >> | | | >> | | R >> | 4.7 | 3.3 R 39 ohm TF >> | nF | nF R >> CCC CCC | >> CCC CCC | >> | | | / >> | | |< ZXTP25020 >> From 0--RRRR--*--RRRR--*---- | >> Op Amp |\ >> 499 499 | \ >> | >> | >> --- >> \ / --> >> V --> >> ----- >> | >> | >> GND >> >> The emitter resistor can drop up to about 5V. >> >> I've got some higher-beta transistors on order (FZT788B), and will >> reduce the impedance of the bias network, but I was fairly shocked that >> the 1/f noise was so bad. > No idea, did you try more than one? > A tangential question; I use a pfet in 'the same'* circuit, > is the pnp better.. lower noise? > > George H. > > *+/- 50 % not sure of cap values. > >>MOSFETs are generally much noisier than BJTs, although obviously they don't have the base current problem. If I can get a factor of 10 amplitude improvement I'll probably just declare victory for this iteration and start hacking more complicated topologies for the next try. 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 ●February 11, 20182018-02-11
Am 11.02.2018 um 18:52 schrieb Phil Hobbs:> On 02/11/2018 10:17 AM, Gerhard Hoffmann wrote:> For the unloaded batteries plotted on P 11, below 100 Hz the dependence > seems to be much more like 1/f**2 or even 1/f**3, which looks to me like > temperature drift rather than actual 1/f noise. I don't know of a > mechanism for making 1/f noise at zero current, but you're clearly > seeing it above about 10-100 Hz. Interesting.No, the coupling capacitor to the preamp is too small. It should be smaller than needed for -3dB. f-3dB is 0.1 Hz for the preamp, but that's still not enough to to short the 10K bias resistor efficiently via the small resistance of the DUT. I have bought some wet slug tantals, but with the 1/f of the Agilent 89441A there is no point in the replacement. I've also bought a Red Pitaya, that is small enough to run on batteries and that could eliminate ground loops, too. Cheap experiment. < https://www.redpitaya.com/c96/stemsuplabsup-125-14 > Chears, Gerhard. At least, one can see from the paper that NiCd are best, and size matters. I have some Samsung 18650 LiIon batteries to test, but that has to wait.
Reply by ●February 11, 20182018-02-11
On 02/11/2018 12:52 PM, John Larkin wrote:> On Sun, 11 Feb 2018 12:32:55 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 02/11/2018 11:34 AM, John Larkin wrote: >>> On Sun, 11 Feb 2018 08:05:11 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 02/10/2018 08:36 PM, John Larkin wrote: >>>>> On Sat, 10 Feb 2018 19:39:32 -0500, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>>> On 02/10/18 18:41, John Larkin wrote: >>>>>>> On Sat, 10 Feb 2018 17:18:55 -0500, Phil Hobbs >>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>>> >>>>>>>> Is horrible. >>>>>>>> >>>>>>>> I'm just debugging a nice diode laser controller for one >>>>>>>> customer and getting ready to do another one for another (much >>>>>>>> better) customer. It uses a ZXTP25020 PNP with a biggish >>>>>>>> degeneration resistor and two-pole bypassing of the base to get >>>>>>>> low noise at high frequency. >>>>>>>> >>>>>>>> Turns out the 1/f noise is atrocious. The total noise from 2 >>>>>>>> Hz on up is only a couple of PPM, but it should be a factor of >>>>>>>> 10 better than that. With a total base resistance of 1k, the >>>>>>>> 1/f corner at 40 mA I_C is over 10 kHz. >>>>>>>> >>>>>>>> The bias network looks like this: >>>>>>>> >>>>>>>> +9V 0----------*--------*-------* | | | | | >>>>>>>> | | | R | 4.7 | 3.3 R 39 ohm TF | nF | >>>>>>>> nF R CCC CCC | CCC CCC | | | >>>>>>>> | / | | |< ZXTP25020 From 0--RRRR--*--RRRR--*---- >>>>>>>> | Op Amp |\ 499 499 | \ | >>>>>>>> | --- \ / --> V --> ----- | | GND >>>>>>>> >>>>>>>> The emitter resistor can drop up to about 5V. >>>>>>>> >>>>>>>> I've got some higher-beta transistors on order (FZT788B), and >>>>>>>> will reduce the impedance of the bias network, but I was fairly >>>>>>>> shocked that the 1/f noise was so bad. >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>> >>>>>>> That's a pretty fast transistor; could it be oscillating? >>>>>> >>>>>> Don't think so. I have a 1 GHz scope across the laser, and didn't >>>>>> see anything like that, and have applied thermal-digital >>>>>> analysis(*) to the whole thing. I'll try going to 49.9 ohms and 47 >>>>>> nF and see if it gets better, which it should. I might need to use >>>>>> a TCA0372 booster to drive 5-ohm resistors. :( >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>>> >>>>>> (*) i.e. heat gun, cold spray, and fingers >>>>> >>>>> Yeah, the base current noise into the 1K resistor string could make >>>>> a lot of noise... assuming the signal from the opamp is perfect. >>>>> >>>>> Is the overall feedback optical? >>>>> >>>>> >>>> >>>> Eventually, but at the moment it's coming from the emitter resistor, so >>>> that the loop doesn't null out the base current noise even within its >>>> bandwidth. >>>> >>>> The idea is to make the bias current way sub-Poissonian, which if done >>>> right results in useful amounts of amplitude squeezing of the light, >>>> i.e. your detected photocurrent can actually be a few dB quieter than >>>> full shot noise if you keep the optical loss low enough. It's more of a >>>> checklist feature for most uses, of course, but as you've pointed out >>>> there are a lot of crappy diode laser controllers out there, selling for >>>> cheap, so building one that just makes the problems magically go away is >>>> attractive. >>>> >>>> Building a new-to-me kind of high dynamic range system always seems to >>>> produce a bit of buried treasure at first. The ZXTP25020 has super-low >>>> Rbb' and Ree', so it would be an excellent candidate except for the 1/f >>>> noise. If the higher-beta device and lower drive impedance doesn't >>>> improve it enough, I'll have to try Darlingtonizing it with one of those >>>> nice SiGe:C devices to return the base current to the collector circuit. >>>> >>>> That'd need a board spin though, and I'm super busy at the moment. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> How about a Sziklai pair sort of thing, with a BF862 and a PNP? Or a >>> Pfet-PNP darlington? Disappear the base current. >> >> I need it to be a PNP-type when it's done, because the lasers are >> usually grounded-cathode. (Keeping the high frequency output impedance >> high is the name of the squeezing game--with an emitter follower you get >> twice the shot noise.) If I could get good PFETs, that would be great, >> but they're scarce these days and their noise tends to be horrible. >> >> The Darlington-bandaid approach could be done with an NPN and a couple >> of auxiliary current sources, but it would be a bit of a mess. The >> advantage would be that (done right) it would have the noise of the >> BFP640, which is very low. >> >> Tomorrow I'll try reducing the base impedance by 10x and see if that >> fixes it. Because of the 39-ohm emitter resistor, it doesn't make sense >> to go very much lower than that. >> >> One nice thing about 1/f noise is that a 10x amplitude improvement gets >> you 100x lower corner frequency. At that point I can put in a >> low-frequency bandaid if I need to, such as sensing the voltage across >> the base resistor and dumping the equivalent into the emitter so that >> the op amp tracks out the LF noise. The trick will be to avoid a noise >> peak at the crossover frequency of the bandaid. >> >> Cheers >> >> Phil Hobbs > > Here's an idea that I've never actually built: > > https://www.dropbox.com/s/lr3aekzziecz7pz/Isrc_cascode_Ib.JPG?raw=1 > > I don't remember what those two caps are for.Interesting. It has the current noise of the FET at low frequency, though, and looks like it has full shot noise at high frequency on account of the caps. I'd be looking at something vaguely of that kind for the bandaid approach--next time I have a Saturday afternoon free I'll have a whack at it. 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
