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Date: Fri, 29 Nov 2002 09:52:28 -0500
From: Raymond Rogers
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Subject: Re: photodetector circuit, high speed with good ambient light rejection
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Rene Tschaggelar wrote:
> Raymond Rogers wrote:
>> You have had several suggestions here. They must have had much better
>> luck than I had. I have always had to use a common base
>> configuration. The emmitter reverse biases the diode to a fixed low
> It is perhaps less a matter of luck than dependent on the application.
> My application is getting the frequency of a modelocked solidstate
> laser. While the average power to the photodiode is in the order
> of a few mW, the lightpulses are several ps downto a few 100fs,
> at a rep-rate of 100MHz or so, meaning the peak power is huge, and
> the photodiode output just limited by the bandwith of the diode.
> We get 6-20mVpp into 50 Ohms just from the diode alone, the diode
> being an AEPX65 with a BW between few 100MHz and 1GHz or so I believe.
Your right, and lowering the impedance lower than 50 ohms won't improve
the risetime; which is one of the two points of a cascode circuit. Let's
do a first order analysis. R=50 ohms, C=7pf
1/2piRC ~ 500MHz
whereas the BW given by the rise time is
~ .35/RT = 350MHz
Thus the risetime is being internally limited to 350MHz and lowering the
50 ohms will not effect the response substantially. Of course you also
have layout capacitance which has to be kept well under 7pf to preserve
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