Photodiode bootstrap phase rolloff

On 08/03/2017 04:50 PM, Phil Hobbs wrote:
> On 08/03/2017 04:00 PM, Winfield Hill wrote:
>> David Nadlinger wrote...
>>>
>>> I am currently working on a range-switched photodiode design for use in 
>>> our laboratory [1]. It took me a while, but I've arrived at a design I'm 
>>> fairly happy with (in no small part thanks to Phil Hobbs' helpful 
>>> writings on the topic):
>>>
>>> My current draft uses a simple BF862 source follower at the summing node 
>>> of a ADA4817-based 2 MΩ/2 kΩ transimpedance amplifier to bootstrap the 
>>> 40 pF of photodiode capacitance (its output being AC-coupled into the PD 
>>> bias node). On paper and in SPICE, I am getting just over 2 MHz 
>>> bandwidth on the 2 MΩ range at reasonable noise levels (limited by the 
>>> single BF862's e_n), assuming fairly realistic parasitics. On the lower 
>>> gain range, however, I'm only predicting ~75 MHz of bandwidth, as the 
>>> BF862 stage quickly loses steam driving the 40 pF load due to its output 
>>> impedance.
>>
>>  You want to add an emitter-follower to the BF862, to lower Zout
>>  and isolate it from the PD capacitance its driving.  You'll want
>>  a low e_n for that transistor.  For example, I use an FMMT718
>>  running at 2 mA.  I measure a 45MHz bandwidth for the bootstrap. 
>>
>>  Please check in AoE III, chapter 8, for details on all of the
>>  above, valuable e_n measurements, and other TIA amp tricks. 
>>
>>  BTW, I don't agree with Phil concerning these TIA bootstraps;
>>  one does fine for any gain over 0.95 or so, because you're
>>  reducing the PD's effective capacitance by 10 to 20x, and
>>  thus its en-C effects against the more noisy op-amp, which
>>  is already probably more than you need.
> 
> It's a lot easier to get a smooth, predictable transfer function and low
> phase  whoopdedoos with a super good bootstrap, though, particularly
> with high capacitance.  Otherwise you have to try cancelling the
> residual RC pole in the second stage, which generally works OK except
> that the cancellation is never perfect and hence you get late-time
> settling artifacts.
> 
> Plus I often like to use high slew (and thus noisy) amps such as the
> LM6171 as the TIA because it greatly reduces artifacts due to slew
> limiting.  In my designs the first couple of stages are much faster than
> the overall TIA, because you never know what sort of nasty sharp pulses
> the customer is going to send into the photodiode.

One other thing: I much prefer the PNP wraparound trick to adding an
emitter follower, because it doesn't degrade the noise the same way.

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

Hi Phil (and others),

First of all, thank you very much for your message, and sorry for this 
late reply – I was traveling abroad and had trouble accessing my news 
server.

On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
> One approach is to switch the whole front end and not just the feedback resistor.

I initially considered switching the entire frontend as a way of 
tolerating more parasitic capacitance (vs. switching the TIA feedback 
impedance). But after figuring out a workable configuration for that, it 
seemed like sharing the frontend would be the less complex option (which 
is handy given the small target PCB size).

I suppose splitting up the frontends would allow me to trade off voltage 
noise vs. current noise on the lower gain range, though, or go for a 
higher 1/f knee but lower wideband noise, and would also make it easier 
to add in an "ideal" output lowpass filter.

> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
FET up to about 100 MHz.

Yep, this is what I was (probably in error) referring to as a 
complementary feedback pair. In fact I was even going to try the BFT92, 
as I have some BF{R,T}92s lying around. (They seem to be a good starting 
point for low-noise, medium-speed things like this – any other 
favourites I should be aware of?)

[…]

Okay, this part was supposed to be a further explanation of how I 
couldn't get the loop stable in SPICE once I put the follower or 
wraparound in. However, I had only tried follower designs that were 
stable (and didn't peak beyond unity gain) in isolation before. Just 
going with a "bare" PNP wraparound without any capacitors to roll off 
the loop gain seems to work nicely in the full circuit in SPICE. Even 
with a simple resistor load in the source, the performance looks to be 
adequate. (For posterity, the circuit in SPICE: 
http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)

Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
using active current sinks it looks like I could push the design past 
200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
for the application.

I'll have to have a closer look at the biasing given device variations 
and the drift performance once I've built the thing, but due to a 
mess-up in our department's finance office I've been waiting for a 
Digi-Key order to arrive for more than a month now…

> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
> 
> The result is a pretty good follower out to a gigahertz or so.

Although overkill for this application, I'll definitely keep that one in 
my bag of tricks. I presume you prefer the BLM18BB beads because of 
their wide resistive region to higher frequencies? I'm also not sure I 
understand how to build a unity gain follower from it, but I'll look 
into it more closely some other day.

Cheers!

  — David

On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger
<david@klickverbot.at> wrote:

>Hi Phil (and others),
>
>First of all, thank you very much for your message, and sorry for this 
>late reply &#4294967295; I was traveling abroad and had trouble accessing my news 
>server.
>
>On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
>> One approach is to switch the whole front end and not just the feedback resistor.
>
>I initially considered switching the entire frontend as a way of 
>tolerating more parasitic capacitance (vs. switching the TIA feedback 
>impedance). But after figuring out a workable configuration for that, it 
>seemed like sharing the frontend would be the less complex option (which 
>is handy given the small target PCB size).
>
>I suppose splitting up the frontends would allow me to trade off voltage 
>noise vs. current noise on the lower gain range, though, or go for a 
>higher 1/f knee but lower wideband noise, and would also make it easier 
>to add in an "ideal" output lowpass filter.
>
>> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
>FET up to about 100 MHz.
>
>Yep, this is what I was (probably in error) referring to as a 
>complementary feedback pair. In fact I was even going to try the BFT92, 
>as I have some BF{R,T}92s lying around. (They seem to be a good starting 
>point for low-noise, medium-speed things like this &#4294967295; any other 
>favourites I should be aware of?)
>
>[&#4294967295;]
>
>Okay, this part was supposed to be a further explanation of how I 
>couldn't get the loop stable in SPICE once I put the follower or 
>wraparound in. However, I had only tried follower designs that were 
>stable (and didn't peak beyond unity gain) in isolation before. Just 
>going with a "bare" PNP wraparound without any capacitors to roll off 
>the loop gain seems to work nicely in the full circuit in SPICE. Even 
>with a simple resistor load in the source, the performance looks to be 
>adequate. (For posterity, the circuit in SPICE: 
>http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)
>
>Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
>using active current sinks it looks like I could push the design past 
>200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
>for the application.
>
>I'll have to have a closer look at the biasing given device variations 
>and the drift performance once I've built the thing, but due to a 
>mess-up in our department's finance office I've been waiting for a 
>Digi-Key order to arrive for more than a month now&#4294967295;
>
>> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
>> 
>> The result is a pretty good follower out to a gigahertz or so.
>
>Although overkill for this application, I'll definitely keep that one in 
>my bag of tricks. I presume you prefer the BLM18BB beads because of 
>their wide resistive region to higher frequencies? I'm also not sure I 
>understand how to build a unity gain follower from it, but I'll look 
>into it more closely some other day.
>
>Cheers!
>
>  &#4294967295; David


Here's a circuit that switches a photodiode between two amps. It
uses's Phil's bootstrap cascode arrangement, and switches two cascode
transistors to steer the pd current.

https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1

This worked fine; the real problem was the customer.

https://www.dropbox.com/s/4lo0wfz5xrsjn90/PT10.JPG?raw=1


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

On Tuesday, August 22, 2017 at 11:17:49 AM UTC-4, John Larkin wrote:
> On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger
> <david@klickverbot.at> wrote:
> 
> >Hi Phil (and others),
> >
> >First of all, thank you very much for your message, and sorry for this 
> >late reply &ndash; I was traveling abroad and had trouble accessing my news 
> >server.
> >
> >On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
> >> One approach is to switch the whole front end and not just the feedback resistor.
> >
> >I initially considered switching the entire frontend as a way of 
> >tolerating more parasitic capacitance (vs. switching the TIA feedback 
> >impedance). But after figuring out a workable configuration for that, it 
> >seemed like sharing the frontend would be the less complex option (which 
> >is handy given the small target PCB size).
> >
> >I suppose splitting up the frontends would allow me to trade off voltage 
> >noise vs. current noise on the lower gain range, though, or go for a 
> >higher 1/f knee but lower wideband noise, and would also make it easier 
> >to add in an "ideal" output lowpass filter.
> >
> >> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
> >FET up to about 100 MHz.
> >
> >Yep, this is what I was (probably in error) referring to as a 
> >complementary feedback pair. In fact I was even going to try the BFT92, 
> >as I have some BF{R,T}92s lying around. (They seem to be a good starting 
> >point for low-noise, medium-speed things like this &ndash; any other 
> >favourites I should be aware of?)
> >
> >[&hellip;]
> >
> >Okay, this part was supposed to be a further explanation of how I 
> >couldn't get the loop stable in SPICE once I put the follower or 
> >wraparound in. However, I had only tried follower designs that were 
> >stable (and didn't peak beyond unity gain) in isolation before. Just 
> >going with a "bare" PNP wraparound without any capacitors to roll off 
> >the loop gain seems to work nicely in the full circuit in SPICE. Even 
> >with a simple resistor load in the source, the performance looks to be 
> >adequate. (For posterity, the circuit in SPICE: 
> >http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)
> >
> >Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
> >using active current sinks it looks like I could push the design past 
> >200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
> >for the application.
> >
> >I'll have to have a closer look at the biasing given device variations 
> >and the drift performance once I've built the thing, but due to a 
> >mess-up in our department's finance office I've been waiting for a 
> >Digi-Key order to arrive for more than a month now&hellip;
> >
> >> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
> >> 
> >> The result is a pretty good follower out to a gigahertz or so.
> >
> >Although overkill for this application, I'll definitely keep that one in 
> >my bag of tricks. I presume you prefer the BLM18BB beads because of 
> >their wide resistive region to higher frequencies? I'm also not sure I 
> >understand how to build a unity gain follower from it, but I'll look 
> >into it more closely some other day.
> >
> >Cheers!
> >
> >  &mdash; David
> 
> 
> Here's a circuit that switches a photodiode between two amps. It
> uses's Phil's bootstrap cascode arrangement, and switches two cascode
> transistors to steer the pd current.
> 
> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1
That's nice, Thanks.  
The stuff down the bottom (labeled comp.) is to take care of the 
bias current of the cascode?

George H.     
> 
> This worked fine; the real problem was the customer.
> 
> https://www.dropbox.com/s/4lo0wfz5xrsjn90/PT10.JPG?raw=1
> 
> 
> -- 
> 
> John Larkin         Highland Technology, Inc
> 
> lunatic fringe electronics

On Tuesday, August 22, 2017 at 8:17:49 AM UTC-7, John Larkin wrote:
> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1

Nifty.  Are AD8034s OK driving 10 uf like that, though?  

I had one customer who used a setup like U3A/B to share a low-
noise reference with several ADCs.  It oscillated big-time, but 
they actually got away with it because the effect was common to 
all of the channels.

-- john, KE5FX

On Tue, 22 Aug 2017 10:59:13 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Tuesday, August 22, 2017 at 11:17:49 AM UTC-4, John Larkin wrote:
>> On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger
>> <david@klickverbot.at> wrote:
>> 
>> >Hi Phil (and others),
>> >
>> >First of all, thank you very much for your message, and sorry for this 
>> >late reply &#4294967295; I was traveling abroad and had trouble accessing my news 
>> >server.
>> >
>> >On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
>> >> One approach is to switch the whole front end and not just the feedback resistor.
>> >
>> >I initially considered switching the entire frontend as a way of 
>> >tolerating more parasitic capacitance (vs. switching the TIA feedback 
>> >impedance). But after figuring out a workable configuration for that, it 
>> >seemed like sharing the frontend would be the less complex option (which 
>> >is handy given the small target PCB size).
>> >
>> >I suppose splitting up the frontends would allow me to trade off voltage 
>> >noise vs. current noise on the lower gain range, though, or go for a 
>> >higher 1/f knee but lower wideband noise, and would also make it easier 
>> >to add in an "ideal" output lowpass filter.
>> >
>> >> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
>> >FET up to about 100 MHz.
>> >
>> >Yep, this is what I was (probably in error) referring to as a 
>> >complementary feedback pair. In fact I was even going to try the BFT92, 
>> >as I have some BF{R,T}92s lying around. (They seem to be a good starting 
>> >point for low-noise, medium-speed things like this &#4294967295; any other 
>> >favourites I should be aware of?)
>> >
>> >[&#4294967295;]
>> >
>> >Okay, this part was supposed to be a further explanation of how I 
>> >couldn't get the loop stable in SPICE once I put the follower or 
>> >wraparound in. However, I had only tried follower designs that were 
>> >stable (and didn't peak beyond unity gain) in isolation before. Just 
>> >going with a "bare" PNP wraparound without any capacitors to roll off 
>> >the loop gain seems to work nicely in the full circuit in SPICE. Even 
>> >with a simple resistor load in the source, the performance looks to be 
>> >adequate. (For posterity, the circuit in SPICE: 
>> >http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)
>> >
>> >Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
>> >using active current sinks it looks like I could push the design past 
>> >200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
>> >for the application.
>> >
>> >I'll have to have a closer look at the biasing given device variations 
>> >and the drift performance once I've built the thing, but due to a 
>> >mess-up in our department's finance office I've been waiting for a 
>> >Digi-Key order to arrive for more than a month now&#4294967295;
>> >
>> >> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
>> >> 
>> >> The result is a pretty good follower out to a gigahertz or so.
>> >
>> >Although overkill for this application, I'll definitely keep that one in 
>> >my bag of tricks. I presume you prefer the BLM18BB beads because of 
>> >their wide resistive region to higher frequencies? I'm also not sure I 
>> >understand how to build a unity gain follower from it, but I'll look 
>> >into it more closely some other day.
>> >
>> >Cheers!
>> >
>> >  &#4294967295; David
>> 
>> 
>> Here's a circuit that switches a photodiode between two amps. It
>> uses's Phil's bootstrap cascode arrangement, and switches two cascode
>> transistors to steer the pd current.
>> 
>> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1
>That's nice, Thanks.  
>The stuff down the bottom (labeled comp.) is to take care of the 
>bias current of the cascode?
>
>George H.     

R12 bleeds a little current in to the cascode transistor to keep it
on, low emitter impedance, at low photodiode currents. That creates a
DC offset error. R6 and Q1 create a simulated equivalent current that
is used to cancel the offset. Cute but overkill maybe.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Tue, 22 Aug 2017 14:38:29 -0700 (PDT), "John Miles, KE5FX"
<jmiles@gmail.com> wrote:

>On Tuesday, August 22, 2017 at 8:17:49 AM UTC-7, John Larkin wrote:
>> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1
>
>Nifty.  Are AD8034s OK driving 10 uf like that, though?  
>
>I had one customer who used a setup like U3A/B to share a low-
>noise reference with several ADCs.  It oscillated big-time, but 
>they actually got away with it because the effect was common to 
>all of the channels.
>
>-- john, KE5FX

One of my hobbies is characterizing opamps for c-load behavior. This
one is OK.

Many RRO opamps are c-load stable with a big C. 


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com