Re: low-noise sot-23 BJT transistors

Hi Win,

"Winfield" <winfieldhill@xxxxxxxxx> wrote in message news:1186056478.771956.36930@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

Something strange is going on.
One of my favorite low-noise BJTs was the 2N6429.
At a few mA and > 100 Hz there was no way it was
anywhere near 3.5 nV.

I couldn't find a data *** for the pmbt6429
(is that a Phillips part?). But I found ones for
MMBT6429 from both ON Semi and Motorola.

And their curves of en vs. freq at various collector currents
were not only different from the 2N part, but strange, too.
It doesn't make sense to me that they show 3.5 to 4 nV
at 100 kHz at all currents from 300 uA to 10 mA.

The original 2N6429 had about 1.4 nV at 1 mA --
which jibes well with the ~ 100 ohm rbb' they show
in the typical model. To get 3.5 nV, you'ld need 800 ohms
of rbb'. Did they change or mess up the chip that much?
Ccb and ft look the same. So, from them it seems to be the
same chip.
But I also note that hFE isn't shown in the SOT parts to hold
up as well at low currents. The 2N part amazingly had typical hFE of
900 at 1 mA and was still 800 at 10 uA -- at least that's what the
data *** typical showed.

Noise current In vs. freq. looks strange, too, on the SOT parts.
They show it going _up_ at low currents and freq. above 10 kHz.

Perhaps you could measure en on your SOT parts and see
if it matches the data ***.


--
Regards,
Howard
hswain@xxxxxxxxxxxxx

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