In a multi-emitter transistor (like in TTL ICs), does the transistor switch on and conduct through the emitters independently or together? That is, if there's a base current through emitter \$E_1\$, does this result in collector current through just \$E_1\$ or through both \$E_1\$ and \$E_2\$? For example, suppose \$V_{BE_1}\$ is 0.8V but \$V_{BE_2}\$ is 0.1V, so only the first is enough to switch the transistor on.

I looked at this question, and it seems to imply that if the transistor conducts, both emitters get equal current, but I'm not sure I'm interpreting that correctly.

(My question isn't about the operation of TTL, but about multi-emitter transistors in general.)

  • \$\begingroup\$ Emitters don't have to be of the same size/area. So there is no one answer for you. But basically, it's about "current density." For example, high current gain and switching speed can be better maintained in regions of higher collector current densities. They can also be used in a kind of NAND behavior. And they are often used to arrange for different current source/sinks with different values based off of a single set current. \$\endgroup\$
    – jonk
    Commented Jul 6, 2017 at 1:45

1 Answer 1


if there's a base current through emitter E1, does this result in collector current through just E1 or through both E1 and E2?

Just think what would happen in an AND gate if both emitters received current when only one was taken low: -

enter image description here

Imagine input A is low and input B is high. Input A takes current because it is low and if that caused input B to fall to 0 volts then the signal driving input B is being corrupted by that action.


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