Sages of the PIClist:

  Recently I started designing a discrete BJT op-amp (my first time) for an 
audio circuit, and in doing so have been studying the manufacturers' 
schematics of different monolithic op-amps.  In several of the circuits (the 
741 [Horowitz and Hill, p.189] and the 5532 
[http://www.dself.dsl.pipex.com/ampins/webbop/5532.htm] so far) 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.

  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?

   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.

  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?
  4a.)  Is there some reference book or website that explains these devices 
so I don't have to bother you all?

  I know that's a lot of questions, but the only thing I'd seen like this 
before had been dual-gate mosfets, which don't look very similar at all.  I 
was hoping one of you might know something, because so far the internet and 
bookshelf have been pretty dry.

Yours,
and thanks as always-

Todd Bailey


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