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The picture shows a detail of schematic diagram of the LM555.

LM555 detail

I presume the horizontal line is the base, but I'm not sure I understand how the transistor works. What is the effect of connecting base with collector, and what's with the second collector? Is it a current mirror of the other collector?

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  • \$\begingroup\$ I've wondered this for a while I see it in data sheets regularly. \$\endgroup\$ – Dean Aug 20 '11 at 13:11
  • \$\begingroup\$ Multiple-collector transistors (or multiple-emitter transistors for that matter) are straightforward to build on integrated circuits so they are fairly common. The collectors are often made different sizes, so the current mirror can scale its outputs. That datasheet uses an ugly symbol, though; usually the base is drawn on the other side like a normal transistor. \$\endgroup\$ – Ken Shirriff Jan 13 at 5:34
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It's a multiple collector transistor, and in this application it's being used (as it often is in ICs) as a current mirror. The Vbe drops of transistors Q20 and Q21 and resistor R6 set the current through the upper collector of Q22, and this current is then mirrored back down into Q20 by the lower collector. It also sets the current through the long tail current source transistor Q11.

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It is known as a multiple emitter transistor. There is no base as such. When signals are applied to the emitters, the current through the collector only stops flowing when all emitters are high. This performs a logic AND in a single transistor, and is faster and smaller than using diodes on the IC.

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    \$\begingroup\$ elaborate! e-lab-or-ate! \$\endgroup\$ – JustJeff Aug 20 '11 at 13:07
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    \$\begingroup\$ Diodes on the IC how do you mean? \$\endgroup\$ – Dean Aug 20 '11 at 13:11
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    \$\begingroup\$ I can see only 1 emitter, and BTW multi emitter transistors are usually still controlled via their base. \$\endgroup\$ – stevenvh Aug 20 '11 at 18:29
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    \$\begingroup\$ In case anyone looks at this answer, multiple-emitter transistors are used in TTL logic gates, providing an AND function. This performs better than earlier DTL (diode-transistor logic) circuits. But these transistors do have a base, connected to a pull-up resistor. And this is unrelated to the multiple-collector transistor in the question. \$\endgroup\$ – Ken Shirriff Jan 13 at 5:30

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