A Bipolar Junction Transistor (BJT) is fundamentally called a current-controlled device because its output current is primarily governed by its input current.
Understanding Current Control in BJTs
The core reason a BJT operates as a current-controlled device lies in the relationship between the current flowing into its base terminal (the input) and the current flowing through its collector terminal (the output).
As highlighted by the reference:
- The output current is dependent on the input currents.
- The collector current is controlled by the base current.
This means that a small change in the base current results in a much larger, proportional change in the collector current. This amplification effect is quantified by the BJT's current gain, often denoted by Beta ($\beta$), which is the ratio of the collector current ($I_C$) to the base current ($I_B$).
$I_C = \beta \times I_B$
Therefore, by controlling the small base current entering the BJT, you effectively control the larger collector current that flows between the collector and emitter terminals. This direct control of output current via input current defines it as a current-controlled device.
In simpler terms:
- Input: A small base current signal.
- Control Mechanism: The transistor structure amplifies this base current effect.
- Output: A larger, controlled collector current.
This characteristic is crucial for its use in amplification and switching circuits, where a small current signal can be used to manage a larger current flow.
