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
--
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
BJT base current 1/f noise
Started by ●February 10, 2018
Reply by ●February 10, 20182018-02-10
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 HobbsWhat! Can't toyLTspice spit out the noise values for each of the components _including_ the PNP parameters ?>:-} ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | It's what you learn, after you know it all, that counts.
Reply by ●February 10, 20182018-02-10
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 HobbsThat's a pretty fast transistor; could it be oscillating? -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 10, 20182018-02-10
On 02/10/18 17:40, Jim Thompson 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 > > What! Can't toyLTspice spit out the noise values for each of the > components _including_ the PNP parameters ?>:-} > > ...Jim Thompson >The simulator can only use what the models provide. Or did you think that pspice was from Hogwarts? 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 10, 20182018-02-10
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 -- 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 10, 20182018-02-10
On Sat, 10 Feb 2018 19:24:42 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 02/10/18 17:40, Jim Thompson 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 >> >> What! Can't toyLTspice spit out the noise values for each of the >> components _including_ the PNP parameters ?>:-} >> >> ...Jim Thompson >> > >The simulator can only use what the models provide. Or did you think >that pspice was from Hogwarts? > >Cheers > >Phil HobbsI know. I was just yanking your chain ;-) It seems, anymore, that even discrete device models are seriously lacking when it comes to noise. And MOS noise models for chip devices are 95% "default" :-( ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | It's what you learn, after you know it all, that counts.
Reply by ●February 10, 20182018-02-10
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 fingersYeah, 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? -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 10, 20182018-02-10
On Sat, 10 Feb 2018 17:18:55 -0500, 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. > > Cheers > > Phil HobbsAre you sure your 9V supply is quiet, or that the op amp is quiet relative to that supply? Even if you already have one of your capacitance multipliers on the supply, lowering the filter cutoff frequency might give a quick verification that it really is 1/f. Thanks for the on-topic question! Frank
Reply by ●February 11, 20182018-02-11
>"Phil Hobbs" wrote in message >news:11b6d95b-4867-8132-efc3-0926b68497eb@electrooptical.net...>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.>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.Why? I had a deko at the data sheet from Diodes Inc. It don't have any spec for 1/f. So, again, what should one expect it to be....? -- Kevin Aylward http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html
Reply by ●February 11, 20182018-02-11
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 -- 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
