Re: low-noise sot-23 BJT transistors

Howard Swain wrote:
Hi Win,

Winfield wrote ...
I'd like to take noise measurements now, but my
venerable HP 4470A has just gone on the fritz.
Perhaps in a week or two I can find the time to
repair it. Meantime, I've ordered a selection
of currently-available transistors from several
manufacturers to evaluate.

Actually, all the '6429 datasheets I have in my
computer show it to be a small-die transistor,
best suited to operation below 100uA or so.
What 2N6429 data*** do you have that shows
better results for low-Z source impedances
(e.g., under 500-ohms)?

The Motorola data *** for 2N6428 and 2N6429 dated 1977.
At 1 mA and 1kHz, this shows En as about 1.4 nV per root Hz
and In as 0.8 pA per root Hz as typical. I assume this is
for the 2N6429, which had the higher hFE.
Cob is spec'ed as 3 pF max at 10 V.
fT typically peaks at 400 MHz at about 10 mA and 5 V Vce.

Where it really shown for us was with a 50 ohm source
impedance and 10 Hz. I calculate about a 6 dB noise figure
at 1 mA bias.

Hmm, the 2N6428 and 2N6429 aren't in my 1974 databook,
but they do appear in the 1983 databook, as a two-page
spec summary, no graphs, etc. There's a little table
for noise, which says Vn = 4.1nV for Rs = 500 ohms.
500-ohms has 2.9nV of Johnson noise, implying 2.9nV
is from the transistor. It also says f = 10Hz.
No sign of 1.4 nV and 0.8 pA. Weird.

.

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