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.

 

[alphy]

Hi Chen,

Thank you for your question. The Ic and Ib ratios in a NPN BJT transistor are important for understanding how the transistor operates and how it can be used in circuits. I can help clarify why Ic is considered the output in this case.

First, it is important to understand that transistors are current-controlled devices. This means that the amount of current flowing through the base (Ib) controls the amount of current flowing through the collector (Ic). The base current acts as a switch, turning the transistor on and allowing current to flow through the collector.

Now, let's look at the direction of current flow in a NPN BJT transistor. As you mentioned, the direction of Ic is into the collector, but this is because the current in the transistor is carried by majority charge carriers, which in this case are holes. These holes flow from the base into the collector, creating the Ic current. This direction of current flow is opposite to the direction of electron flow, but it is still considered the output because it is the current that is being controlled and used in the circuit.

I hope this helps clarify the issue for you. If you have any further questions, please don't hesitate to ask. Understanding the operation of transistors is an important aspect of being a scientist, and I am happy to assist in any way I can.

Best,