Bipolar Junction Transistors (BJTs) are categorized into various types based on their configurations, each with distinct characteristics and applications. The configurations are primarily differentiated by which terminal—base, emitter, or collector—is common to both the input and output circuits.

The common base configuration is noted for its high voltage gain, positioning it as an ideal choice for single-stage amplifier circuits, such as microphone pre-amplifiers. A notable characteristic of this configuration is its tendency to reduce the input current signal, as the output collector current is typically lower than the input emitter current.

Conversely, the common emitter configuration is the most widely used type in transistor-based amplifiers, owing to its superior input impedance, current, and power gain compared to the common base setup. It functions as an inverting amplifier, producing an output signal that is 180 degrees out-of-phase with the input signal, making it essential for various amplification needs.

Lastly, the common collector configuration, also known as the emitter follower or voltage follower circuit, is recognized for its exceptionally high input impedance and relatively low output impedance. Although its voltage gain is slightly less than one, it provides substantial current amplification. This configuration, which does not invert the input signal, is particularly useful for impedance matching and serving as a voltage buffer in numerous applications. Each configuration offers unique advantages, making BJTs versatile components in electronic circuit design.