An amplifier can be defined as an electronic device or system that has the capability to increase power, current, or voltage of an electric signal. It takes power from an electric supply and releases a controlled output matching with the shape of input signal but with much higher amplitude.
There can be four fundamental types, which are commonly used. These four types are Voltage amplifiers, which are used for amplifying voltage; Current amplifiers that are used for increasing or amplifying current, Transconductance, which is used to convert input voltage into a relevant current output; and Transresistance, which is used to convert input current into voltage output. Those devices, which are commonly used in audio equipment, broadcasting and wireless communication applications are known as audio power amplifiers and these are used to amplify low-power audio signals.
Many technologies, like DSL and ISDN, make use of amplifiers called repeaters to extend their range. In DSL they are called “loop repeaters” or “loop extenders”. Without amplifiers, only consumers that live close to hubs of the existing telephone exchanges would be able to use DSL or ISDN to access the Internet. The signal strength decays quickly through the standard telephone wires used to carry the digital data, which is why amplifiers are required.