[Laser] OPT-101 vs PIN diode

[steve kavanagh]

Art et al:

In my experience high impedance photodiode receiver
needs good shielding too.

There is very little technical info in the
Scatterpoint article so I don't really know the
answers to your questions with regard to that specific
design.  But if you are interested you can find the
2005 issues of Scatterpoint at 

http://www.g0czd.clara.net/ukug/SP_2005/

As I understand it the bandwidth of the K3PGP front
end (and similar designs) is controlled by the
parallel combination of a resistance (gate-to-ground
resistor, FET input resistance, photodiode resistance
and stray leakage in parallel) and a capacitance
(photodiode capacitance and FET input capacitance in
parallel, plus stray capacitance from wiring).

I agree that some of these quantities are a bit hard
to define.

In the original K3PGP preamp the first stage has some
voltage gain, so the input capacitance is Cgs plus the
Miller feedback capacitance, which because of the gain
will be larger than Cgd.  

In my design the second stage is common base and has
low input impedance, resulting in first stage voltage
gain of less than one and hence no significant Miller
effect.  Therefore the FET input capacitance will be
close to Cgs+Cgd.  I have one intentional low-pass
section in my audio amp as well as several high pass
sections.  But the low pass characteristics are
dominated by the photodiode capacitance and its
variation with reverse voltage (and possibly its
resistance), it seems, because I observe a marked
difference in the output noise spectrum between dark
and daylight conditions.  My photodiode is unbiased
and AC coupled so the DC voltage on it (and hence its
capacitance) will vary with light level.  For example,
I recorded audio noise spectra for almost dark
(indoors with heavy curtains drawn during daytime),
and for pointing at the sky on a dull grey day, and
observed 3 dB bandwidths of about 150-500 Hz (dark)
and 250-1500 Hz (light).

73,
Steve VE3SMA

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