Transistors (transistor) is a solid-state semiconductor devices, can be used for detection, rectification, amplification, switch, voltage regulator, signal modulation, and many other features.As a kind of variable switch transistor, based on the input voltage, control the flow of electrical current, so the transistor can be as the switch of electric current, and the general mechanical switch (such as Relay, switch) different signals in the transistor is used to control, and can be very fast switching speed, switching speed can reach more than 100 GHZ in the laboratory.

Transistors are divided into two categories: bipolar transistor (BJT) and the field effect transistor (FET).
        A transistor has three pole: three of the bipolar transistor, respectively is composed of N and P type Emitter Emitter, the Base (Base) and the Collector (Collector);The field effect transistor of the three, respectively is the Source (Source), grid (Gate) and Drain (Drain) tapping.
Transistor because there are three kinds of polarity, so there are three ways of using are grounded emitter (also called amplification, CE configuration), base grounding (also called common-base amplifier, CB configuration) and the collector grounding (also known as the total set of amplification, the CC configuration, emitter with the corner).
In a bipolar transistor, the emitter to the base of small current, makes between emitter and the collector, producing large current;The mosfet, applying small voltage in the grid, to control the current between the source and drain.
        In analog circuits, transistor amplifier, audio amplifier, rf amplifier, voltage regulator circuit;In computer power supply, mainly used in switching power supply.
Transistor is applied to digital circuit, the main function is as an electronic switch.Digital circuit includes logic gate, random access memory (RAM) and the microprocessor.
        Transistor is in use, there are many attention should be paid to the maximum rating, such as maximum voltage, maximum current and maximum power.Under the conditions of excess use, can damage the interior of a transistor.Each type of transistor and unique features, such as dc power hfe, denoising NF signal-to-noise ratio, etc., can borrow by transistor specification or Data Sheet.
         Transistor in the circuit of the most common purpose should belong to the signal amplifier which on the one hand, the second is the impedance matching, signal conversion, etc., the transistor is an important component in the circuit, many precision components are mainly made of transistor.