Understanding NPN BJT Transistor: Ic & Ib Ratios

• chenrim
In summary, the conversation discusses the relationship between the input and output currents in a NPN BJT transistor. The DC current gain (beta) is the ratio between the collector and base currents, with the collector current (Ic) considered as the output and the base current (Ib) as the input. This is because Ic is proportional to Ib within the linear range of the device. The conversation also mentions the different arrangements of common emitter, common base, and common collector and how they affect current and voltage gain. Finally, it is noted that the direction of current flow can differ between NPN and PNP transistors.

chenrim

Hi i have have a small question regrading to a NPN BJT transistor.

the DC current gain (beta) is the ratio between Ic/Ib where Ic and Ib are the collector and base currents respectively.

Ib is the input current, that's alright , but, i don't get why Ic considered as the output?
in NPN BJT Ic's direction is into the collector (and the electrons direction is the opposite)

thanks,
Chen

chenrim said:
Ib is the input current, that's alright , but, i don't get why Ic considered as the output?

because Ic is proportional to Ib ( at least within the linear range of the device)

there's many good www sites explaining the operations of transistors
some simple googling with give you hours of fun reading :)

Dave

chenrim said:
Ib is the input current, that's alright , but, i don't get why Ic considered as the output?

It's purely wording, not science.
"Input" and "output" usually refer to direction of signal flow, not current flow.

Common Emitter: Input goes to base, output taken from collector
this is the arrangement most often encountered
has both current gain and voltage gain

Common Base: Input goes to emitter, output taken from collector
Has voltage gain but not current gain

Common Collector, or Emitter Follower: Input goes to base, output taken from emitter
has current gain but not voltage gain

And observe that current flows into a NPN collector but out of a PNP collector.

Any help ?

Mike_In_Plano and davenn
The controlling signal is the input, the controlled signal is the output. That's a reasonable generalization.