12.2 : BJT の構成

Bipolar junction transistors have different configurations based on the common contact.

The common base configuration features a shared base terminal for the input and output.

It exhibits unusually high voltage gain, making it suitable for single-stage amplifier circuits like microphone pre-amplifiers. However, it attenuates the input current signal as the collector current output is less than the emitter current input.

In contrast, the common emitter configuration, which is the most popular for transistor-based amplifiers, has a greater input impedance, current, and power gain than the common base configuration.

This configuration acts as an inverting amplifier circuit, with the output signal 180 degrees out-of-phase with the input voltage signal.

Finally, the common collector configuration, also known as the voltage follower circuit, possesses very high input impedance and relatively low output impedance.

While its voltage gain is less than unity, it provides significant current amplification. This non-inverting configuration is beneficial for impedance-matching and voltage-buffering applications.