What is (and going on) with Q19 in this schematic?

[Stonent]

Did the designer take a bit of liberty with the schematic? Or is that not a standard PNP BJT?

There's another part of the circuit where it looks like he put all 3 leads of a PNP on the bottom but this part the fact that there's 4 leads coming out has me confused.

[Simon]

I take it this is a schematic of the inside of an IC ? they can do that in IC's and the schematic is more of an "equivalent" than what is really inside

[Stonent]

I take it this is a schematic of the inside of an IC ? they can do that in IC's and the schematic is more of an "equivalent" than what is really inside

Yeah supposedly the insides of a 555.

[Simon]

Well if your looking at making one from discrete transistors then you best take the block diagram as your lowest level as that shows comparators and gates and a flip flop an a discharge transistor that is "normal"

[codeboy2k]

As simon says, they make them that way on the IC.  Planar fabrication technologies lends itself to making a transistor with multiple emittors, multiple bases, or multiple collectors.  They can make certain areas wider or narrower than other areas to increase/decrease current densities in fixed ratios (good for making bandgap voltage references for example.

Here's an image of the construction of a dual emitter BJT. Your drawing is dual collector, but if you look at the base, flip it over top to bottom, and put just one emitter on top, you can have two collectors on the bottom. 

[AG6QR]

If you want to know how to translate that functionally into something you understand, you might want to have a look here:

http://shop.emscdn.com/catalog/emskits/555/kit/555_datasheet_RevA3.pdf

This is the data sheet (including schematic) for Evil Mad Scientist's oversized replica of the 555, made with discrete components on a PCB.  The 555 functionality is replicated with common 3906 and 3904 PNP and NPN transistors.  You can have a giant 555, which works (almost) exactly like the common miniature version, except that it lets you probe the internals as its working.

That's a clever project idea.  Not terribly practical and a lot more expensive than a genuine 555, but I'm still tempted to get one, build it, and use it.  It's very cute.

[c4757p]

As for (and going on with), it's part of a multiple-output current mirror. The first collector (from left, tied to the base) is sensing the reference current, and the V_BE from that controls both the second collector and Q9, to give two proportional currents.

Oops, I was too slow...

[AndersAnd]

Well if your looking at making one from discrete transistors then you best take the block diagram as your lowest level as that shows comparators and gates and a flip flop an a discharge transistor that is "normal"

Or simply look at the discrete 555 schematic already made for The “Three Fives” Discrete 555 Timer Kit

Datasheet for “Three Fives” Discrete 555 Timer Kit with schematic  diagram http://shop.emscdn.com/catalog/emskits/555/kit/555_datasheet_RevA3.pdf

[edavid]

Hans Camenzind died last year, but his book is still available to download:  http://www.designinganalogchips.com/

Highly recommended!

[Zero999]

As above it's a transistor with two collectors. It behaves like two transistors with the bases and emitters connected together and the collectors left separate.

[Stonent]

If you want to know how to translate that functionally into something you understand, you might want to have a look here:

http://shop.emscdn.com/catalog/emskits/555/kit/555_datasheet_RevA3.pdf

This is the data sheet (including schematic) for Evil Mad Scientist's oversized replica of the 555, made with discrete components on a PCB.  The 555 functionality is replicated with common 3906 and 3904 PNP and NPN transistors.  You can have a giant 555, which works (almost) exactly like the common miniature version, except that it lets you probe the internals as its working.

That's a clever project idea.  Not terribly practical and a lot more expensive than a genuine 555, but I'm still tempted to get one, build it, and use it.  It's very cute.

Aha! That's really what I was looking for. Once I saw the kit mentioned I thought it might be a way to use up the 50 or so transistors I bought "just because".