It is not entirely accurate to describe Bipolar Junction Transistors (BJTs) as purely current-controlled devices. While BJTs can be controlled by current at the base terminal to regulate the collector current, they also exhibit voltage dependencies. The base-emitter junction of a BJT operates similarly to a diode in forward bias, where a small base current controls a larger collector current. However, the collector-emitter voltage also plays a crucial role in determining the operating point and characteristics of the transistor. Therefore, while current at the base terminal influences the transistor’s behavior, voltage across the collector-emitter terminals also significantly affects its operation.
BJTs are often referred to as current-controlled current devices because the base current controls the magnitude of the collector current. By adjusting the base current, the collector current can be varied proportionally. This characteristic makes BJTs suitable for applications requiring precise current amplification or switching, where controlling current flow is critical for circuit operation.
Transistors, including BJTs, do control current in a sense that the base current determines the amount of current flowing through the collector-emitter path. In amplification applications, small variations in the base current result in corresponding changes in the collector current, allowing transistors to amplify signals or act as switches in electronic circuits. Thus, while transistors do not strictly control current independently of other factors like voltage, they effectively regulate current flow based on input signals and biasing conditions.
BJTs are commonly used as current amplifiers in various electronic circuits. By controlling the base current, the BJT can amplify a small input signal current to a larger output current at the collector terminal. This capability is exploited in applications such as audio amplifiers, signal processing circuits, and switching circuits where precise control and amplification of current are necessary for proper operation.
Field Effect Transistors (FETs) are often described as current-controlled devices due to the relationship between the gate voltage and the drain-source current. In FETs, the gate terminal controls the conductivity of the channel between the source and drain terminals. By varying the gate-source voltage (Vgs), the FET can modulate the drain-source current (Ids). This voltage-controlled characteristic distinguishes FETs from BJTs, where current at the base terminal predominantly controls the collector current. Therefore, FETs are referred to as current-controlled devices because the gate voltage determines the amount of current flowing through the device, offering advantages in certain circuit designs and applications compared to BJT-based designs.