A transistor amplifier is an electronic device that increases the strength of an electrical signal. It uses a transistor, which is a semiconductor component, to control the flow of current. By applying a small input signal to the transistor, it can produce a larger output signal, making it useful in various applications like audio equipment and radio transmission. Transistor amplifiers come in different configurations, such as common emitter, common collector, and common base, each offering unique characteristics. They are essential in modern electronics, enabling devices to function effectively by boosting weak signals for clearer sound or improved communication.