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Transistors are usually silicon types and are available in either NPN or PNP forms. The abbreviations refer to the sandwiching or junctions of the semiconductor materials used inside the device. Catalog specification sheets or schematics will differentiate the two different types. An NPN or PNP is selected on the basis of how it will be used in the circuit and it is important to use the one specified.

Transistors can have a wide range of applications, but these transistors will tend to cover most categories:

Linear: Transistors designed for linear applications such as low-level voltage application.

Switching: Designed to switch applications.

Power: Transistors which operate at significant power levels, usually divided into audio frequency at radio frequency power types.

Radio frequency: Transistors designed for specifically high frequency applications.

High voltage: Transistors specifically designed for high voltage.

The 2 most common ways to use a transistor are to amplify a signal or to switch a signal on and off. There are literally thousands of different types of transistor and they are rated on an extremely extensive list of criteria. They all have a unique code such as 2N2222 and the number of the transistor will be listed by the manufacturer if I am designing a specific circuit.

If the correct transistor is not available then a substitute can usually be used and the individual manufacturers provide a guide that will help match a transistor with similar parts that can be used as a substitute. The ratings do not appear on the transistor itself and to determine its characteristics I will have to look up the specific characteristics in a specifications book or look at the technical documentation on the manufacturer’s website.

The semiconductor material in a transistor is minute and for the principle of soldering the semiconductor is placed in plastic or metal cases. Signal transistors come in either of these types of cases. The plastic variety will work for most uses, but for a more precise application it may be necessary to use metal ones as they are less susceptible to stray radio frequency interference.

Signal transistors nearly always have three lead connections but sometimes have four. Power transistors can come in either metal or plastic cases and physically they are larger than signal transistors. Transistors will typically have three wire leads being the base, emitter and collector. A base is wired to a voltage or current and turns the transistor on or off. The emitter and collector lead connect to a positive or negative voltage source or ground and the position of the lead will vary on the circuit. Transistors both bipolar and unipolar are ideal for use as switches and these are sometimes known as FET switches and will feature a fourth lead, this lead grounds the case to the chassis of the circuit.

The positions of the leads means that it is vital that the transistor is placed the right way round in the circuit. Any other way will damage the transistor and may damage the other components on the circuit. Bipolar transistors are probably the most common type of transistor. A small input current is applied to the base of the transistor and this will change the amount of current that flows between the collector and the emitter. FETs ( field effect transistors), these transistors also have three connections called gate, source and drain, rather than base, collector and emitter. By applying a voltage, the gate controls the current between the source and the drain. FETs come in two types N-channel which are similar to NPN and P-channel similar to PNP. These FETS come in two sub types known as MOSFET and JFET, and should be stored in anti-static foam as static can damage them.

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