[Laser] Beginner preamp questions

[John Matz]

Hi  Steve,

I think I understand things well enough to try a couple of answers.  Someone
else I'm sure will chime in.

Yes, you need an AF preamp.  Direct to phones is not very sensitive.  The
K3PGP design works fine.  The photodiode capacitance and the diode load
resistor set a bandwidth limit in the 1kHz-100 kHz area (f = 1/(2*pi*R*C).
For regular voice comms, this is adequate.  The transimpedance amplifier has
approximately zero ohms input resistance (due to the feedback) so the RC
input bandwidth is very large.  This would be good for hi-speed data or
video.  The gain/recovery can be about the same; the R in the PGP or the
feedback R in the transimpedance amp are the loads for the PD.  The noise
can be different though.  A lot depends on the device chosen, whether it has
best noise with a hi-z source, what its shot noise is, etc.  The PGP preamp
using a JFET is pretty good, I hear.  I have not built it, and I have only
the OPT101 as a transimpedance amp to compare against anyway.

The idea of a transformer as a load is good for daylight operation ...
there's little voltage produced from high ambient light, but the load Z for
the PD is not all that high (not megohms), so recovery is low and
signal/noise may not be optimum.  The xfmr might be a "hum pick-up point
too.  AC coupling with a load R into a cap into an amp R doesn't really buy
much in S/N either.  The transimpedance amp has low-Z input, so PD should
not move much with high ambient light, but the amp itself may rail.
Possibly a compromise feedback R on a t.i. amp, AC coupled to a low-noise AF
amp might work well both day and night.

I have not compared bias vs. zero-bias.  I've always biased to handle large
signals/ambient, but zero-bias should be lower noise.

More comments?

John Matz KB9II

----- Original Message -----
From: "steve kavanagh" <skavanagh73 at yahoo.ca>
To: <laser at mailman.qth.net>
Sent: Friday, March 18, 2005 7:50 AM
Subject: [Laser] Beginner preamp questions


> Well, I couldn't resist buying a couple of C$1.95 1 mW
> laser pointers, so I guess I am going to try this
> stuff ! A photodiode array removed from a junked
> CD-player seems to work OK across the room (MCW, diode
> connected directly to hi-Z headphones).
>
> So an audio preamp is next.  Could anyone comment on
> the pros and cons of
>
> (1) high impedance (e.g. K3PGP) vs transimpedance
> (e.g. WA6EJO) preamps,
>
> (2) DC coupling of diode to preamp (e.g. K3PGP) vs AC
> coupling using capacitor (e.g. WA6EJO) or transformer
> (e.g. KE4KWR) coupling, and
>
> (3) DC biasing of photodiode vs zero-bias operation ?
>
> I am mostly interested in low data rate weak signal
> work (i.e. MCW or weak signal digital modes),
> including daytime operation.
>
> Steve VE3SMA
>
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