I have a paper somewhere that discusses GBW-product and optimization of the two-OpAmp composite. Quite nicely done, as I recall... unfortunately I haven't re-found it :-( ...Jim Thompson
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Thinking outside the box... producing elegant solutions,
by understanding what nature is hiding.
"It is not in doing what you like, but in liking what you do that
is the secret of happiness." -James Barrie
No warning feels extremely unfair, but having a couple last conversations is extremely hard too. There's so much you want to say but gosh you don't want to be too emotional or look like you're coming unglued yourself! It feels "unfair" any way you slice it I suppose.
We mostly talked about his grandkids, some old movies he wanted to see again, girls, and boats. Seems as reasonable an assortment of topics as any other :)
Yeah, life's not fair. :^) I keep telling my kids this, but I think you only learn it through hard knocks. Me mum passed away about 1 year ago now. Alzheimer's, which is a particularly icky way to go. When I heard, after shock, my next emotion was relief.
Ouch! That is probably the hardest way to go, at least for the family. My mother was getting pretty bad at the end but it wasn't Alzheimer's. She was 95, with heart and kidney problems that were taking their toll on the brain. I understand your comment about "relief".
On Tue, 29 May 2018 09:21:19 -0700, Jim Thompson wrote:
That sounds like something I would have saved if I had seen it, so I searched my SED notes folder without luck but found some old app notes and a post from Win Hill that might be of interest to someone. (Probably old hat to Phil):
Burr Brown Application Bulletin FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE
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(composite amp discussion pp 9-13)
From: Winfield Hill Newsgroups: sci.electronics.design Subject: Re: Photodiode wich is fast enough to detect +50Mhz analog (sinus) signal?? Date: 11 Aug 2004 03:31:18 -0700
Yannick wrote...
capacitance.
your
rolloff
I posted the circuit previously in this thread (20 July) and described in detail how it works. Here's the circuit again: | Rf R2 adjustable | ,---/\/\---+---/\/\--/\/\----, | | '--||--' | C2 R3 | nA-sensitivity wideband | | Cf '--||--/\/\-- gnd | transresistance amplifier | | | | | __ ,-||--/\/\--+ correction network details | input O--+---|+ \ | __ | R2 C2 = Rf Cf | | >-+-/\/\-+-|- \ | R3 C2 sets bandwidth | ,-|-_/ | | >-----+--- | | | gnd --|+_/ | gnd --/\/\--+-/\/\--' composite amplifier The undesired Rf capacitance Cf is canceled by the R2 C2 network. R3 is used to limit the upper frequency of this cancellation. R2 C2 and R3 constitute the standard R-C-R trick. I thought of this about 18 years ago, and have used it with great success since then. Later I learned that it had been described in an old Keithley manual, and probably in many other places years before that. BTW, the circuit above will outperform (sensitivity, bandwidth, SNR, phase accuracy) any of the resonant schemes you've been contemplating here. It's not true that feedback makes things more noisy. In this circuit feedback (and a high-performance composite amplifier) insures that all of the signal current is used by the amplifier, rather than becoming uselessly drained away by the input-node capacitance. Thus feedback actually improves the SNR. Using a resonant input doesn't solve the capacitance problem because if a high enough Q is used for a solution, it simply creates insurmountable phase-error problems.
That's a lot. You could drive a cascode emitter to keep the load impedance way down on the photodiode. The transistor collector, much lower capacitance, can drive the TIA.
The problem there is that you should have a standing current in the cascode transistor, to keep the emitter impedance low. That makes an offset and maybe messes up the TIA.
Hence the notion that a photodiode has two leads. Take the fast stuff from one end and the slow signal from the other. It's the Tektronix "feed-beside" idea.
I'm expecting the first article of my GHz o/e converter any day now. It uses the feed-beside idea.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Dmitriy, What's the advantage of this? At one point in your circuit evolution I thought you were going to move the non-inverting input away from ground to bias the photodiode (PD). (You can reduce PD capacitance ~factor of 3 or so with reverse bias.) Have you read Phil Hobb's book? Most of his ideas can be found for free online... check his website.
OK, what do you find better about zero bias operation? I should admit that for many years I ran all my PD's at zero bias. I thought this gave me better 'zero' light detection. (No DC offset with no light... but the dark current from PDs is generally pretty low.) Running with some bias has two main advantages.
1.) reduced C.. faster
2.) Higher saturation current (light intensity) without bias the electrons build up in the junction and it saturates.. more light gives no more electrons.
George H. (who is addicted to reading... I need to find a few new fiction writers)
Zero bias is better in one respect: you can get zero leakage current. For jobs such as very wide range, very slow photometers, that's a win. Garry Epeldauer et al. wrote a beautiful paper about getting 14 orders of magnitude in photocurrent, if you don't mind being stuck with millihertz bandwidths:
Crappy PN photodiodes and solar cells don't respond well to large reverse bias either.
For just about anything else, zero bias is a complete crock.
With almost any PIN diode, APD, MPPC, (etc) zero bias is a disaster. Applying reverse bias to a PIN diode can reduce its capacitance by a factor of 7 or so, which reduces the high frequency noise by the same factor.
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
http://hobbs-eo.com
I know of a large organization that has wasted about a million dollars a year, since 2002, by running a lot of very expensive Hamamatsu photodiodes at zero bias.
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--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
I suspect I know the organization. ;) They have some very good folks though.
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
http://hobbs-eo.com
(Well thanks for trashing the first ~20 years of my PD career. :^)
In defense of zero bias it's got great simplicity, doesn't break if put in backwards*. And if your DMM has a 200uA scale a ~10mm^2 PD makes an easy light detector. Lots of applications don't care so much about speed. (a bow to the lm324, I never used one, but enjoyed the schematics.)
George H.
*I've never tried breaking a PD by forward biasing hard. my bpw34 looks about the same volume as a 1/4 watt resistor, so ~1/4 W, 0.6V and 0.4A
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