Reverse biased diodes are not very low impedance. You can't really drive
a low impedance with them.
In fact, the high impedance of the photodiode leads to all sorts of
ugliness in noise analysis. This is well documented in Graeme's book. I
think TI also has an app note on feedback networks for the best SNR,
though Graeme has a technique to get optimal bandwidth and noise
performance.
A lot of people make a living just designing front ends for high
impedance devices. They are really hard to do optimally since high
impedance is bad for bandwidth, noise, and all things nice.
There is also a book by Hobbs on photodiode interfaces. Much of the book
is unfortunately filler, though parts are very good. I found a pirated
copy online and decided not to buy the real thing. Graeme's book is much
more detailed. The derivations of the equations are shown in painful
detail. [My dislike of Gain Technology comes from dealing with the
company. I have no personal knowledge of Graeme other than buying his
books.]
You might want to track down the pirated version of Hobbs before
spending $150. I did, then deleted it since I already owned Graeme's book.
Building Electro-Optical Systems: Making It all Work
On 4/1/2013 10:30 AM, J. Forster wrote:
I think that you should be able to take the signal right out of the
backbiased diode and run it straight into a microwave mixer w/ a fixed 7
GHz LO and frequency discrimitate the IF output from the mixer to generate
a control signal for your LASER.
In theory, if your LASERS were very, very, very good, you might be able to
achieve phase lock, but I'd not hold my breath. A discriminator can give
you frequency lock.
YMMV,
-John
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