Transistor as an amplifier:

Figure:  Amplifier circuit, standard common-emitter configuration.

The common-emitter amplifier is planned that is why a small change in voltage in (V_in) alters the small current by the base of the transistor and the current amplification of the transistor combined with the properties of the circuit mean that small swings in V_in generate large alters in V_out.

Several configurations of single transistor amplifier are probable, with some voltage gain, some providing current gain, and some both of them.

From cell phones to televisions, large numbers of products that involve amplifiers for sound reproduction, radio transmission, and signal processing. The very fist discrete transistor audio amplifiers barely supplied some hundred milliwatts, but power and audio fidelity steadily increased as better transistors became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to some hundred watts are common and comparatively inexpensive.