One of the important uses of transistors in electronic circuits is as an amplifier, though there are other uses like in switching, oscillators and frequency changers. Amplifiers are devices which can amplify weak signals and make them strong signals. Different types of amplifiers are used in radios and audio devices. Amplifiers used to amplify very weak signals and provide a drive to power amplifiers are known as pre-amplifiers. Power amplifiers are the output stages which provide large audio or radiofrequency signal outputs. Power amplifiers are classified into class A, B, AB and C amplifiers, depending on the region of the characteristic curve of the transistors at which they operate. Each type has a different application in electronics. While these classes are for linear amplifiers, there are also class D and E for switching designs.
