A common emitter is a basic transistor configuration used in electronic circuits, particularly with bipolar junction transistors (BJTs). In this setup, the emitter terminal is common to both the input and output circuits. This configuration is popular for amplifying signals, as it provides a significant voltage gain and can also invert the phase of the input signal. In a common emitter circuit, the input signal is applied between the base and emitter, while the output is taken from the collector and emitter. This arrangement allows for effective control of the output current based on the input voltage, making it a fundamental building block in analog electronics and signal processing.