On Tue, Mar 02, 2010 at 09:21:25PM -0800, Donald Tillman wrote: > > On Mar 2, 2010, at 2:35 AM, Peter TB Brett wrote: > >> The usual approach is to buy SMT packages containing 2 or 4 >> transistors on >> a single piece of silicon (i.e. literally back-to-back on the wafer). >> They're invariably well-matched enough for all but the most ultra- >> precise >> applications, in my experience. > > > Which dual/quad transistors are these? Who makes them?
For example Diodes/Zetex, navigate a bit the site and you'll find at: http://www.diodes.com/products/catalog/list.php?parent-id=28 that they are: "Built with adjacent die from a single wafer: DC Current Gain, hFE, VCE(sat), VBE(sat) are matched to a 2% maximum tolerance." Besides being at the same temperature. Unfortunately Analog's SSM2210 is obsolete. But even random dual transistor pairs (I've used BC857BS dual PNP) have very good matching on average. It's not guaranteed and some may stand out as being much worse than average, but often temperature tracking (I'm repeating myself) dominates the variations between individual transistors in actual circuits. > > (And back-to-back? Are you sure? That doesn't sound right. That would > have to involve separate processes for each side, and so the transistors > wouldn't be matched.) No, they are side by side. > > Most dual transistors I've seen have the transistors on separate dies. Huh? We must live on different planets. The ones I've seen are diffused as neighbours on the same wafer. > And so there's no matching and no offset spec; it's just like picking up > 2 or 4 individual transistors. Some have specs, some have not. But they are diffused together in close vicinity, and that's the important point that guarantees similarity and temperature tracking. Gabriel _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
