Bipolar Junction Transistors (BJTs)
What are BJTs?
Bi-polar transistors are amongst the most widely used devices for amplification of all types of electrical signals in discrete circuits, (circuits made from individual components rather than integrated circuits). They are also used in circuits together with integrated circuits (I/Cs) since it is often more practical in circuits where large power or high voltage is needed to use discrete output transistors, while using low voltage, low power I/Cs for most of the signal processing. It is also often more convenient to use a discrete transistor for an individual circuit within a larger system, for which integrated circuits are not readily available. 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 for power amplification to drive a loudspeaker. Transistors come in many shapes and types. A selection of typical transistors is shown in Fig 1.1
Fig. 1.1 Typical Bipolar Transistors
1. AC127Germanium audio output transistor - found in vintage radios, and in some more modern circuits where sensitivity to heat is required.
2. BF318Silicon Video amplifier transistor uses a collector/emitter voltage (VCE) of about 150V and will amplify frequencies up to 80MHz
3. BU208ASilicon output transistor used in TVs and large screen monitors. Can deliver high power and withstand pulse VCE voltages of about 1000V. The metal case (normally bolted to a heat sink) is the collector connection.
4. BD124Silicon TV output transistor with a lower power rating.
5. BC108General purpose Silicon voltage amplifier transistor, the silver case with a small tab to identify the emitter connection is a standard TO39 package.
6. BD 131Silicon audio NPN output transistor in a TO26 package for mounting on a suitable heat sink, will dissipate 15W and is often used as part of a push-pull pair with a matched BD132 PNP transistor.