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