Hi Todd.

Todd Bailey wrote:
> 
> I have seen components drawn into the schematic which look like a normal
> BJT with an extra collector or emitter leg drawn on.
> 
>   My questions to you:
> 
>   1.)  What are these things called?  I've been calling them "split emitter"
> or "split collector" transistors because I've seen that in print before --
> but I'm not sure I'm using the term correctly.

Multi emitter or multi collector BJT transistors.


>   2.)  I assume that these are somehow doped differently than a normal BJT,
> but I don't know how.  Does anyone?  Would a "split collector" NPN
> transistor be similar to two discrete matched NPN BJTs connected together at
> the base and emitter, but connected separately at the collector?

It wont be similar to two discrete BJT connected together. When you run
technological process on the piece of silicon and transistors are located
close to each other it is quite possible to dope the areas with high
precision and thus obtain very close charateristics of transistors.
It is very difficult and impractical to attempt to achieve that with
discrete transistors (unless they were made on the same piece of silicon
in the same cycle and equipment was setup to produce matched transistors )
You will need to find a book about technological processes used in seconductor
industry, properties od materials used as dopes etc. I had such a huge book
once ( it was in russian ) while was studing things at university but lost it
somewhere later when had no practical implementation to that knowledge.


>    3.)  Why are these used?  I assume it has to do with maintaining current
> ratios in different sections of the op amp by using a common device to set
> those currents.

Multi-emitter circuit were quite commonly used in TTL digital logic ICs
awhile ago ( who remember that raise you hand ? :)
Multi-collector are quire commonly used in integrated circuits for various
purposes.


>   4.)  Finally, do these devices exist as stand-alone in monolithic
> packages, or are they always buried deep in some silicon wafer?  What I mean
> is, are these the kind of thing I could (or would want or need to) use on a
> breadboard?

I doubt anybody would pack just a few matched transistors in very expensive
( compared to the cost of silicon piece inside of it ) package and be able
to still sell while others will be selling complete multifunctional more
complex circuits for the same price.


>   4a.)  Is there some reference book or website that explains these devices
> so I don't have to bother you all?
Art of Electronics you mentioned already, "semiconductor circuits design"
Titse Shenk? not sure if I spelled names correctly it was in russian.
Also good to read " op amps and linear integrated circuits" Honeycutt.
Some sort of "sourcebook of modern transistor circuits" might also help.
Try to search in nearest areas of knowledge as well.

Good luck with research. :)


WBR Dmitriy.
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