Friday, 27 May 2011

How to Identify Terminals (Emitter,Base,Collector) of Various Transistor (BJT) Packages (visually)

Transistors are available in a wide range of package types for various applications. Those with mounting studs or heat sinks are usually power transistors. Low-power and medium-power transistors are usually found in smaller metal or plastic cases. Still another package classification is for high-frequency devices. We should be familiar with common transistor packages and be able to identify the emitter, base, and 
collector terminals. 

Manufacturers generally classify their bipolar junction transistors into three broad categories: 
  • general-purpose/small-signal devices
  • power devices
  • RF (radio frequency/microwave) devices.

General-purpose/small-signal transistors are generally used for low- or medium-power amplifiers or switching circuits. The packages are either plastic or metal cases. Certain types of packages contain multiple transistors. Figure below illustrates common plastic cases

Figure below shows packages called metal cans,

Figure below shows multiple-transistor packages. Some of the multiple-transistor packages such as the dual in-line (DIP) and the small-outline (SO) are the same as those used for many integrated circuits. Typical pin connections are shown so you can identify the emitter, base. and collector.

Power Transistors Power transistors are used to handle large currents (typically more than 1 Amp) and/or large voltages. For example, the final audio stage in a stereo system uses a power transistor amplifier to drive the speakers. Figure below shows some common package configurations. In most applications, the metal tab or the metal case is common to the collector and is thermally connected to a heat sink for heat dissipation. Notice in part (g) how the small transistor chip is mounted inside the much larger package.

 RF transistors are designed to operate at extremely high frequencies and are commonly used for various purposes in communications systems and other high-frequency applications. Their unusual shapes and lead configurations are designed to optimize certain high-frequency parameters. Figure below shows some examples. 

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