Learnabout Electronics

- Bipolar Junction Transistors (BJTs)

Module 3.1

Bipolar Junction Transistors

What you´ll learn in Module 3

What are BJTs?

BJT circuit symbols

Fig. 3.1.1 Bipolar Junction Transistors

Bi-polar transistors are amongst the most widely used devices for amplification of all types of electrical signals in discrete circuits, i.e. circuits made from individual components rather than integrated circuits (I/Cs). BJTs are also used in circuits together with I/Cs , since it is often more practical to use discrete output transistors where a higher power output is needed than the I/C can provide. For example an integrated circuit may carry out all of the processing of the signals in a system, but then pass the processed signal to a single discrete transistor or a pair of matched transistors for power amplification to drive a loudspeaker or other output device. It is also often more convenient to use a discrete transistor for an individual circuit within a larger system, for which I/Cs are not readily available.

Transistors come in many shapes and types. A selection of typical bipolar junction transistors (BJTs) is shown in Fig 3.1.2

Typical Bipolar Junction Transistors

Fig. 3.1.2 Typical Bipolar Junction Transistors

TO-92 package variations

Fig. 3.1.3 TO-92 package variations

Many transistor package types are also available with alternative connection layouts. The TO-92 package for example has variants TO-94, TO-96, TO-97 and TO-98 all with similar physical appearance, but each with a different pin configuration. Where different package variants are available, these are usually identified on the data sheet for each particular transistor type. Typical variations, such as those for the TO-92 to 98 series of packages used for transistors such as the 2N2222 are illustrated in Fig. 3.1.3.



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