low-noise sot-23 BJT transistors

[Winfield]

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.

 

[Eeyore]

Some of the DIY audio folks seem to be using 2N4401/3 which are
available in SMT versions (MMBT...). Probably have less capacitance
than those matrix base Zetex parts.

I can confirm that the 2N4401 and 4403 are fairly quiet for low Z audio sources
(I've used them myself in mic amps) , implying a lower en than 3.5nV. I seems
funny to go back to using a device as old as that though.

Graham

 

[Spehro Pefhany]

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.

Some of the DIY audio folks seem to be using 2N4401/3 which are
available in SMT versions (MMBT...). Probably have less capacitance
than those matrix base Zetex parts.


Best regards,
Spehro Pefhany

 

[Tom Bruhns]

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

 

[Howard Swain]

Hi Win,

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 sheet 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 sheet 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 sheet.

 

[Howard Swain]

Hi Win,

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 datasheet do you have that shows
better results for low-Z source impedances
(e.g., under 500-ohms)?

The Motorola data sheet 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.

 

[Winfield Hill]

Hi Win,

Winfield wrote ...


The Motorola data sheet 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.

 

[Al]

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.

There is also a 2N6428A. Just a thought.

Al