12.4 : Characteristics of BJT

The Bipolar Junction Transistor (BJT), specifically in a common-emitter configuration, exhibits distinct current-voltage characteristics crucial for understanding its behavior in electronic circuits. These characteristics are established through experimental measurements of voltage and current relationships.

For input characteristics, the base-emitter voltage is varied, maintaining a constant collector-emitter voltage. This setup reveals a Shockley-type dependence of the collector current on the base-emitter voltage. As the base-emitter voltage increases, so does the collector current, but under a controlled condition of constant collector-emitter voltage.

The output characteristics are then observed by plotting the collector current against the collector-emitter voltage while keeping the base current fixed. This relationship highlights several operational regions of the BJT. In the saturation region, the collector-emitter voltage is low, causing the collector current to approach zero, indicating minimal resistance between the collector and emitter terminals.

As the collector-emitter voltage rises, the transistor transitions into the active region, exhibiting an infinitely high collector-emitter resistance. The collector current remains relatively stable regardless of changes in the collector-emitter voltage. However, a slight increase in the collector current can be observed, which can be attributed to the early effect. This phenomenon involves a widening of the depletion region and a reduction in the base width, increasing the saturation current and thus introducing a finite output resistance.

These characteristics define the operating conditions and performance of BJTs in various applications, making them fundamental components in designing and analyzing electronic circuits.