Why you ground common terminal of transistor?

Grounding the common terminal of a transistor, typically the emitter in a common-emitter configuration or the source in a common-source configuration, establishes a reference point for the circuit. This allows for stable operation by ensuring consistent voltage levels, which is crucial for accurate signal amplification and biasing. Grounding also helps minimize noise and interference, improving the overall performance of the transistor in the circuit. Additionally, it simplifies the design and analysis of the circuit by providing a clear reference for other components.