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I sometimes come across with BJT symbols from which multiple lines come out of emitter or collector. Are such BJTs different kind of devices? What does it mean in solid-state level?

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These are multi collector (or multi emitter) bjts. Their operation is quite similar to the normal ones.

Multiple emitter bjt's do not conduct only if all the \$V_{BE}\$ are below \$V_\gamma\$, it is a sort of wired AND. Multiple collector bjt's collectors total current is set as usual by \$I_B\$, and if all the collectors are the same size (area at silicon level) the current is equally split.

Multi emitter bjt's are usually used to close some sort of feedback from following stages, maybe avoiding the bjt to saturate, or in logic input stages where the logic funcion is implemented directly by the wired AND, such as in this TTL AND input stage:

ttl and

Multi collector bjt's are useful in IC's since it is easy to match the ratio between the collector areas and split a polarizing current in precise ratios, as in the uA741 output stage polarization (\$Q_{13}\$ here):

ua741

At solid state level to add an emitter you simply add an emitter diffusion:

multi emitter

For a multi collector you just need more than one collector contacts.

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