Re: low-noise sot-23 BJT transistors

On Aug 2, 5:07 am, Winfield <winfieldh...@xxxxxxxxx> wrote:
Suggestions, anyone?

I've just finished sorting through my collection
of sot-23 BJT small-signal transistors, about
20 types, looking for a low-noise part. The
best I could find was mmbt5089 / pmbt6429,
which have curves showing a lowest e_n of
about 3.5nV. But this is much much worse
than the 0.4 to 0.8nV territory we're used to
with various classic TO-92 low-noise parts.

I am stocking Zetex fmmt618 and fmmt718
(npn - pnp transistors), which are large-die
high-current parts thought to have very low
r_bb' values. I purchased these thanks to
a suggestion by Fred Bartoli here last April.
But these do have rather high capacitance.
Ah, but then again, so do the famed 2sd786
and 2sb737, etc., from the old days.

Though the '5089 (and even better the '5087) have nice low noise
figures, they aren't best for low impedance sources. I did some
'speriments a few years ago with the smt parts we had kicking around
and found that the venerable '2222 (mmbt2222 in this case) did better
than I expected. As I recall, it was actually slightly better than
the mmbt4401, though that may have been a function of the particular
lots I was sampling for my test. I'm remembering measuring input-
referred voltage noise in the vicinity of 0.7-0.9nV/rtHz, at 1kHz. I
was looking for an amplifier I could use to good advantage at the
output of a standard diode DBM, source impedance of about 60 ohms when
driven from a 50 ohm source.

Cheers,
Tom

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