An amplifier is a device for increasing the power of a signal by use of an external energy source.

In an electronic amplifier, the input "signal" is usually a voltage or a current. Other types exist; a fluidic amplifier increases the power of signals represented as flow of gas or liquid, for example. Amplifiers may be classified in a variety of ways depending on their application, the frequency range they cover, or the active devices used. Ideally an amplifier increases the power of a signal without otherwise altering it; practical amplifiers have finite distortion and noise which they invariably add to the signal.

A device that converts signals from one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.

The quality of an amplifier can be characterized by a number of specifications, listed below.

The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(P_out /(P_in)). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance). For example, an audio amplifier with a gain given as 20 dB will have a voltage gain of ten (but a power gain of 100 would only occur in the event the input and output impedances were identical).

Imran Khan Niazi (Urdu: عمران خان نیازی; born 25 November 1952) is a Pakistani politician and former cricketer, playing international cricket for two decades in the late twentieth century. After retiring, he entered politics. Currently, besides his political activism, Khan is also a philanthropist, cricket commentator, Chancellor of the University of Bradford and Founder and Chairman Board of Governors of Shaukat Khanum Memorial Cancer Hospital & Research Centre.

Arguably Pakistan's most successful cricket captain, Khan played for the Pakistani cricket team from 1971 to 1992 and served as its captain intermittently throughout 1982–1992. After retiring from cricket at the end of the 1987 World Cup, he was called back to join the team in 1988. At 39, Khan led his teammates to Pakistan's first and only World Cup victory in 1992. He has a record of 3807 runs and 362 wickets in Test cricket, making him one of eight world cricketers to have achieved an 'All-rounder's Triple' in Test matches. On 14 July 2010, Khan was inducted into the ICC Cricket Hall of Fame.