A Bipolar Junction Transistor (BJT) is often referred to as a “current-controlled current source” because its output current is largely determined by the input current applied to its base terminal. To comprehend why a BJT earns this designation, one must delve into the underlying principles of BJT operation.
A BJT consists of three semiconductor layers – the emitter, base, and collector – and operates in two main modes: the active mode and the saturation mode. In the active mode, the BJT can be considered as a current amplifier.
The input current to the BJT flows into the base terminal and is termed the base current (��IB). This base current controls the transistor’s ability to conduct current between the collector and emitter terminals. The current flowing through the collector (��IC), which is the output current, is directly proportional to the base current. This relationship is described by the transistor’s current gain, denoted by the symbol �β (beta).
Mathematically, this relationship is expressed as:
��=�⋅��IC=β⋅IB
Here, ��IC is the collector current, ��IB is the base current, and �β is the current gain of the transistor.
The term “current-controlled” comes from the fact that the base current controls the flow of current from the collector to the emitter. However, the second part of the designation, “current source,” requires further explanation.
In the active mode, a BJT exhibits a relatively high input impedance at the base terminal, making it appear as if the base is driven by a current source. The input current (��IB) behaves as if it is being supplied by a current source, influencing the BJT’s operation as a current-controlled device.
It’s important to note that this designation is an idealization and simplification. In practical applications, factors such as temperature, manufacturing variations, and early voltage can affect the accuracy of this model. Nonetheless, referring to a BJT as a “current-controlled current source” captures the essence of its behavior in amplifying and controlling current, forming a fundamental concept in electronic circuits and device modeling.
