Re: 2sb737 and 2sd786 (was CD or DVD with all datasheets in the world?)



"John S. Dyson" wrote:

> In article <dnmcei024qc@xxxxxxxxxxxxxxx>,
> Winfield Hill <Winfield_member@xxxxxxxxxxx> writes:
> > martin griffith wrote...
> >>
> >> Fred Bartoli wrote:
> >>
> >>> I guess you must then have the 2SB737 I couldn't find anywhere.
> >>> I'm also searching the 2SK240 as well but you don't mention it
> >>> in AoE so... :-)
> >>
> >> 2SB737, are rare as hens teeth. I know of one company that has
> >> a few thousand, for internal use only.
> >>
> >> Any close equivalents, 2SA1085 maybe?
> >
> > The 2sb737 is a Rohm PNP with an r_bb' of 2 ohms, typical, and
> > it's the complement to their 2sd786 NPN with r_bb' of 4 ohms.
> > Both transistors are claimed to deliver e_n = 0.55nV at 10Hz if
> > operated with Ic = 10mA, and their plots show 0.4nV at 1kHz.
> >
> > The voltage-noise density, e_n, is thought to originate from
> > the transistor's collector shot noise against half its r_e,
> > plus a term from the base spreading resistance, r_bb'. Since
> > r_e goes down with Ic, and is only 2.5 ohms at 10mA, it's
> > desirable to have a very low r_bb' - in the same 2 to 4 ohm
> > territory. In the above 10mA region r_bb' becomes a dominant
> > noise source. The Johnson noise from 4 ohms is 0.256nV, so we
> > wouldn't want r_bb' much above 4 ohms to enjoy a nice low e_n
> > near 0.5nV.
> >
> > I have a healthy supply of 2sd786 transistors, which I got from
> > an s.e.d. denizen, who had lots more, IIRC. Mine do measure in
> > the 0.5nV territory. I've never crossed paths with a 2sb737.
> >
> > I'm not sure how they measure r_bb' - it can't be through e_n,
> > because the measured e_n is too high for the Johnson-noise
> > formula with r_bb' and 0.5 r_e as the only sources.
> >
> > As for close equivalents, Toshiba's 2sc3329 NPN and 2sa1316 PNP
> > transistors claim to have r_bb' = 2-ohms (if you can get 'em).
> >
> > And yes, the Hitachi (Renesas) 2sa1083 to 2sa1085 and 2sc2545
> > to 2sc2547 families claim to have 0.5nV noise at 10mA and 1kHz.
> > They don't mention r_bb', but it has to be very low. I wonder
> > if these are available somewhere.
> >
> How would the Zetex ztx788/ztx789 compare (WRT noise, not other
> features) with the above type transistors? They aren't speced
> for low noise, but appear to have most of the appropriate attributes
> of a low noise part (also the ztx688/ztx689 for NPN.) Their biggest
> problems might be the high capacitance and associated dynamic
> effects. Zetex even suggests their use for low noise, but since
> I don't have any of the cool Japanese low noise parts, I don't
> have an easy way to directly compare...

Zetex don't spec Rbb although it might be presumed to be low given the
other specs. Same reason the 2N4401/3 also work quite well for low
impedance low noise circuits.

Vceo is much much lower though. I couldn't use these parts.

Graham

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