[time-nuts] Photodiodes for high frequency OPLL

J. Forster jfor at quikus.com
Sat Mar 30 18:33:46 EDT 2013

The circuit I've seen is:

   +Vb-------o--|<--------------------> amp
   Vb gnd----------|

The diode is reverse biased by 50 to several hundred volts.
The two caps are DC bypass caps w/ very short leads.
The output is a 50 Ohm coax  to a broadband amp w/ 50 Ohm input.

In the limit, the amp is put right at the detector and has near-zero input Z.




> On Sat, 30 Mar 2013 10:52:54 -0700 (PDT)
> "J. Forster" <jfor at quikus.com> wrote:
>> My understanding is that you want to operate photodiodes with high
>> reverse
>> bias for the best frequency response. The bias widens the space charge
>> layer, thereby reducing the capacitance of the device. The high electric
>> fields in the SCL region also sweeps the hole-electron pairs, produced
>> by
>> photon injection, out faster, hence improving the response as well. Such
>> devices are best operated with very low capacity wiring into a virtual
>> ground.
> Could you elaborate on this circuit a little bit?
> Some terms i could google for or pointers to books/papers to read?
>> Of course, there are limits as, at some point, the device will
>> avalanche.
>> When that ocurrs, the device will have gain akin to a photomultiplier.
>> Some devices are actually designed to operate in this mode, but, since
>> the
>> thing is so near to unstable requires careful control of temperature and
>> oher parameters.
> Yes, the avalanche photodiodes are meant for this operation.
> Yes, there are kind of sensitive. But i think that's managable.
> 			Attila Kinali
> --
> The people on 4chan are like brilliant psychologists
> who also happen to be insane and gross.
> 		-- unknown

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