Transistor as an Amplifier – Circuit Diagram, and Its Working The input of the amplifier is a voltage otherwise current, where the output will be an amplifier input signal. An amplifier circuit which uses a transistor otherwise transistors is known as a transistor amplifier. The applications of transistor amplifier circuits mainly involve in audio, radio, optical fiber communication, etc. Transistor amplifier circuits - circuitstoday Mar 14, 2011This article is a list of various types of amplifier circuits built using Transistors alone includes a Class AB amplifier using 4 transistors, then we have a headphone amplifier circuit and finally a low power amplifier using transistor. This article is solely dedicated to publish more transistor amplifier circuits.