From: Winfield Hill
Subject: Re: photodetector circuit, high speed with good ambient light rejection
Date: 29 Nov 2002 08:48:30 -0800
Organization: Rowland Institute
References: <3DE69C97.firstname.lastname@example.org> <3DE76E53.email@example.com>
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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 level.
> 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.
I agree, if the photo-current level is high, Hobbs' common-base BJT
input stage preceding a transresistance stage is excellent for high
because it reduces the transresistance opamp's troublesome e_n-Cin
noise by moving some of the capacitance away from the opamp summing
junction, and eases the opamp's GBW requirement.
Hobbs points out that with moderate signal currents that create a high
r_e, an external bias current can be added to the BJT's emitter and
a like current taken from the collector. This approach relies on the
absence of shot noise in resistor-generated currents. It would seem
practical up to bias/signal ratios of up to say 25 or 50, but at some
point weak signals get one into trouble attempting to balance the two
bias currents and struggling with an unknown lost-current factor from
the BJT's beta. Hobbs suggests using high-beta transistors like the
MPSA18 (B>500), but watch out for their input higher capacitance (e.g.
8pF), further complicating issues at high frequencies. As is often
the case, a panoply of compromises must be examined.
Elsewhere in his writings, available on his web page, Hobbs presents
effective approaches for dealing with small signals in the presence
of high light levels, if one has available or can create an additional
un-modulated light path from the laser. This powerful approach can
even be used to partially cancel shot noise in the laser's light.