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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Photodiodes in photovoltaic mode, speed considerations.
References: <3DACB831.F814C3D6@webaccess.net> <3DAD0481.A4004E95@NOSPAM.univ-lille1.fr> <3DAD0A0A.1F7BF2DD@webaccess.net>
Date: Wed, 16 Oct 2002 08:40:19 GMT
NNTP-Posting-Date: Wed, 16 Oct 2002 01:40:19 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Chuck Simmons wrote:
> "s.d." wrote:
> > My two cents...
> > If you want speed you may buffer your photodiode with a transimpedance
> > amplifier.
> > That's the usual way. But still you'll need negative supply.
> > At least you can try to put a low impedance load to your diode at the
> > expense of lower gain.
> > Mind that in voltage mode the diode response is logarithmic with light
> > power.
> A negative supply is outside of allowed parameters. If it costs $0.10 it
> is outside of allowed parameters. A load on the diode is not allowed
> because because I cannot specify a buffer that operates that close to
> the negative rail. I figure I can just live with 100mV off the rail and
> since I see, in devices so far, 150 to 200 mV at desired power, I'm OK.
> The problem is speed for clock selection for the digital loop which
> depends on the diode speed in photovoltaic mode. I know some clock
> limits for some devices but I can't predict because I only get reverse
> specs and I don't know how these or area relate to speed.
> BTW, we did use loading for a while but it did not work with a
> population of parts we had because the effect of the load on voltage was
> less predictable and caused laser damage (i.e., the things failed).
> Unloaded photovoltaic is safe it would seem from our experience. Some of
> the population of lasers tend to require close to burnout current
> levels. This is within spec for the lasers but a real pain for me.
> ... The times have been,
> That, when the brains were out,
> the man would die. ... Macbeth
> Chuck Simmons firstname.lastname@example.org
One can bias the diode, either forward or reverse; millivolts or volts
(at least reverse) and have zero need for a negative supply.
At least with a reverse bias, the varactor-like capacitance is a lot
lower than forward (photo voltaic generator) mode.
Another advantage is that this gives a capacitance less dependent on
doping and other process parameters, and closer to packaging parameters.
The diode resulting voltage shift can drive a comparitor, or opamp
acting as a comparitor; or in a bridge-like circuit driving the opamp
(with or withot feedback).
And there are a lot of opamps that go rail to rail with common mode
input specs wider tan the supplies (many are SOIC).
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