Photodiode bootstrap phase rolloff

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Hi all,

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 M?/2 k? 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 M? 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.

This gets better when putting an active sink on the JFET source, but I  
thought I would spend some time optimizing the bootstrap design while  
I'm waiting for parts to arrive (~100 MHz bandwidth would be nice to  
have). It's not hard to come up with a circuit that provides >0.99 gain  
into the given load out to a few hundred MHz in simulation, for example  
by adding an RF(-ish) PNP transistor to make a complementary feedback  
pair, or simply tacking on an NPN emitter follower (bootstrapping the  
JFET drain for better effective gain as necessary).

However, while those bootstrap designs look good in isolation, it seems  
like they invariably render the complete circuit unstable on the 2 k?  
range in simulation (although they work brilliantly on the 2 M? range).  
I'll have to go through the maths properly, but this appears to be due  
to the two-pole rolloff caused by having two transistors in the path. On  
the 2 M? range this happens far above the op-amp loop bandwidth anyway,  
but this is no longer true on the 2 k? range. The bare BF862, on the  
other hand, has a lower gain to start with and rolls off earlier, but  
the single pole doesn't seem to degrade the phase margin enough to cause  
issues.

Building a sub-nV emitter follower that is good out to multiple GHz  
obviously can't be the right solution. I feel like I'm just missing an  
easy fix in the form of a lead-lag-type tweak in the right place, but I  
can't quite figure out where that would be, and much less how to make  
that work without degrading the noise performance too much.

Since my intuition for analog design is not great, I thought I'd ask  
here whether I missed something obvious. I can also post a sketch or  
LTspice file of the relevant part of the circuit if that helps, but I  
figured the question would be trivial to answer for the photodetector  
veterans among you, as presumably you would have run into this before.

Thanks!

  ? David



[1] Quantum optics. The idea is to build small (well, small as far as  
lab gear goes ? ~ 35mm x 30mm x 25mm) modules with built-in beam  
sampling optics for laser intensity monitoring and stabilisation all  
across the visible and near-IR. And yes, the range switching led to some  
rather expendable gymnastics, but the alternatives would have been even  
more painful.

Re: Photodiode bootstrap phase rolloff
On Friday, July 28, 2017 at 9:41:09 AM UTC-4, David Nadlinger wrote:
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Sounds nice.  (2MHz at 2 Meg ohm.)  I'm only a tadpole when  
it comes to PD circuits, so I can't help much.  I'm guessing  
a schematic would help those with more experience.  
How are you switching the gain?  
Maybe three orders of magnitude is too much.  
Could you use a second ADA4817 to bootstrap the PD?  
(I'm mostly a brute force type.)  

George H.  
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Re: Photodiode bootstrap phase rolloff
On Friday, July 28, 2017 at 11:42:28 AM UTC-4, George Herold wrote:
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strap the  
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Oh and what photodiode? How much is the reverse bias?  
Though I not looked at how much the capacitance changes,
you can reverse bias PD's much more than the the typical spec  
sheet claims.... that might gain you a little speed.  
GH    
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Re: Photodiode bootstrap phase rolloff
On 28.07.17 5:17 PM, George Herold wrote:
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If reality doesn't differ from the data sheets too much, it is going to  
be a Hamamatsu S6775; ~5 mm x 5 mm active area with 40 pF capacitance at  
-10 V bias.

 > How much is the reverse bias?
 > Though I not looked at how much the capacitance changes,
 > you can reverse bias PD's much more than the the typical spec
 > sheet claims.... that might gain you a little speed.

The PD package itself seems to be rather low capacitance, so I could  
indeed go to voltages higher than the 12-15 V I was going to use, and  
hence increase the "range" of the simple BF862 bootstrap. There are two  
drawbacks to this, though:

First, the dark current more than doubles going from 10 V to 30 V  
according to the datasheet (still waiting for the diodes to arrive to  
verify this). The shot noise from that still wouldn't be an issue, but  
it looks like the dark current will be the largest source of temperature  
drifts in the system.

(One important use of the photodiodes will be for intensity  
stabilisation, although the fact that there is a non-negligible amount  
of free space optics afterwards makes it hard to get better than 0.1%  
long-term stability anyway.)

Secondly, I would need an appropriate bias voltage source that can  
supply ~1 mA of current (full range at 2 k? would be ~2 mA, but  
hopefully no more than 0.5 mA of photocurrent will ever be used in  
practice). Since the PCB is going to be rather small (~ 30mm x 20mm),  
appropriately shielding a boost converter seems a bit nontrivial, so I  
would probably have to supply this externally.

Also, since there are going to be quite a few of the photodiodes in the  
experiment, I'd rather not push any parts past their documented limits  
without good reason, as I don't want to end up having to constantly fix  
the hardware.

Thanks for the suggestion, though!

  ? David

Re: Photodiode bootstrap phase rolloff
On Sunday, July 30, 2017 at 10:27:04 AM UTC-4, David Nadlinger wrote:
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OK, I've never looked at the dark current closely.  
  
I was thinking 40-50 volts, making it quite is a bit of work.  
(maybe some batteries?  I know, I don't really like batteries.)
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0.1% ! I often find some fingerprint/ dust spec, is right in the beam
making all sorts of bouncy noise.  
But don't you want to mostly take out the high frequency noise?  
(Well that's why I need a faster PD anyway.)

George H.  
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Re: Photodiode bootstrap phase rolloff
On 28.07.17 4:42 PM, George Herold wrote:
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It certainly looks like it will turn out nicer than what you can buy off  
the people who bribe physicists with red snack boxes, yes. 1 MHz would  
also be acceptable for the application, so I'm quite optimistic that  
I'll be able to make it work.

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I'm using three semiconductor switches in a T configuration to switch  
the low-impedance feedback path in and out (the middle leg dragging the  
trace to ground when not used to reduce feedthrough). Getting the  
parasitics low enough to not degrade the 2 M? range was a bit of a  
challenge, but it turns out to be just about achievable without using  
(comparatively) huge relays.

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Perhaps I should have made this clearer in the original post, but it's  
not actually the range switching that is the problem. I merely mentioned  
it to provide some background for the design choices I made, as some of  
the tradeoffs might be different if I only had a single 2 k? range to  
worry about.

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I could, but this would increase the system noise by a factor of about  
five ? the ~0.8 nV/rtHz voltage noise of the BF862 across the 40 pF  
photodiode capacitance already dominates the design, so the 4 nV/rtHz of  
the ADA4817 look quite bad. The input capacitance of the op-amp is also  
higher than that of a BF862 source follower, let alone a bootstrapped one.

  ? David

Re: Photodiode bootstrap phase rolloff
On Sunday, July 30, 2017 at 10:44:23 AM UTC-4, David Nadlinger wrote:
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Oh I was asking for my own edification.  I've been recently trying to  
speed up my PD and the switch/stray/ capacitance (grayhill rotary  
switch) is enough such that I need no compensating cap. at 100k ohm
with a 3dB point of ~800kHz (? data book is at work).. big PD ~130 pF @10V.
..  


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.

Huh, (Thanks)
I was thinking it'd only double the noise of the opamp.   I hadn't  
thought about the bootstrap cutting down the v_noise*C_in term,
but only about more speed.  

That does mean (I think) that if I bootstrap my circuit with an opamp
it'll only increase the noise a bit.  

4nV/rtHz is not too bad... (my opamp is much worse,  
8nV.. four times as bad in my thinking... noise should be  
expressed as a density. V^2/Hz and not these weird EE units. :^)

George H.

George h.  
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Photodiode bootstrap phase rolloff
One approach is to switch the whole front end and not just the feedback res
istor.  

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

For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bi
polar.  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 i
n series with the base to keep the BJT from oscillating.  

The result is a pretty good follower out to a gigahertz or so.  

Cheers

Phil Hobbs

Re: Photodiode bootstrap phase rolloff
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, snipped-for-privacy@gmail.com wrote:
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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.

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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 BF92s 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?

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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

Re: Photodiode bootstrap phase rolloff
On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger

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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  


Re: Photodiode bootstrap phase rolloff
On Tuesday, August 22, 2017 at 11:17:49 AM UTC-4, John Larkin wrote:
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 news  
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k resistor.
  
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P wraparound (shunt feedback) on the  
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:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC coupl
e the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm b
ead in series with the base to keep the BJT from oscillating.
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That's nice, Thanks.  
The stuff down the bottom (labeled comp.) is to take care of the  
bias current of the cascode?

George H.      
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Re: Photodiode bootstrap phase rolloff
On Tue, 22 Aug 2017 10:59:13 -0700 (PDT), George Herold

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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  

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Re: Photodiode bootstrap phase rolloff
On Tuesday, August 22, 2017 at 8:17:49 AM UTC-7, John Larkin wrote:
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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

Re: Photodiode bootstrap phase rolloff
On Tue, 22 Aug 2017 14:38:29 -0700 (PDT), "John Miles, KE5FX"

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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  

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Re: Photodiode bootstrap phase rolloff
David Nadlinger wrote...
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 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.


--  
 Thanks,
    - Win

Re: Photodiode bootstrap phase rolloff
On 08/03/2017 04:00 PM, Winfield Hill wrote:
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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.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Photodiode bootstrap phase rolloff
On 08/03/2017 04:50 PM, Phil Hobbs wrote:
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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
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