An amplifier is a device that changes a signal with smaller energy content into the same signal with larger energy. There are many parameters which are important while choosing an amplifier. The gain is one of the important parameters. This is the ratio between the powers output and input. This is evaluated in decibels.

The output dynamic range, which is also in decibels, is measured as the range between the lowest and highest utilitarian output levels. This is also quoted as the amplifier dynamic range. The bandwidth is another parameter, which is the difference in the upper-half and lower-half power points. The rise time is the time the output takes to change in the final level from 10% to 90%.

The settling time is the time taken by the output to settle within some percentage of the final value. Slew rate is nothing but the utmost rate of change in the output variable. Sometimes noise is brought in the process of amplification and this is called the Noise factor, which is evaluated in decibels, again. Another term used is the efficiency, which is the measure of input power that is enforced to the amplifier’s output. Amplifiers are given different ratings, with Class A being the most inefficient, and the efficiency increasing as the ratings increase to B, AB, D etc.

There are various types of amplifiers, like electronic amplifiers, Transistor or stereo amplifiers, operational amplifiers, Video amplifiers, Car amplifiers, distributed amplifier, microwave amplifiers etc. The Denon DN-A200 Pro Stereo Amplifier is an example of stereo amplifiers; the Kenwood KAC-7203 Car Audio Amplifier is an example for car audio amplifiers etc.