A device that increases the voltage or current of an electrical signal.
Power amplifiers for loudspeakers can be categorized as Class A, B, AB, C, D, G, and H. The different classes utilize different circuit types and provide varying amounts of efficiency. Class D amplifiers have become increasingly popular for active speakers.
Microphone amplifiers often referred to as preamplifiers, boost a low-level microphone signal (around a few millivolts) up to line level (around 1.2 volts). These amplifiers can be class A or AB.
Buffer amplifiers provide unity gain (no change in level) and often match impedances so that a source, like a guitar, is not adversely affected by the devices it feeds, like a chain of guitar pedals.