Understanding NPN BJT Transistor: Ic & Ib Ratios

[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)

please help me for understanding this issue
thanks,
Chen

 

[davenn]

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

 

[jim hardy]

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.

Look at your three arrangements:
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 ?

 

[NascentOxygen]

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