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AE: Bipolar Junction Transistor
Transcript of AE: Bipolar Junction Transistor
BJTs made possible the design and manufacture of lightweight, inexpensive miniature electronic devices desirable in modern apparatus. The transistor is only about 5 decades old, yet it is replacing the vacuum tubes in almost all applications. The reasons are obviously its advantages over vacuum tubes such as compact size, light weight, more resistive to shocks and vibrations, low operating voltage, long life and so on. Transistor Identification Transistors are made in many physical forms.
Some are low power, medium power and high power transistor types.
The larger the unit, the greater the power rating of the device. Transistor Action To use a BJT, we connect it so that:
Its emitter is its negative terminal.
The collector is several volts positive of its emitter.
The base is 0.7V (or slightly more) positive of its emitter. Transistor Configurations Transistor Applications Some of the common applications of a transistor include the following:
An amplifier The word transistor is derived from the combination of two words, “Transfer-Resistance”.
It means that it is a device, which transfers a low resistance into a circuit having high resistance. Transistors in general are classified as bipolar or unipolar type. The bipolar type has two PN-Junctions, while unipolar types have only one PN-Junction. A Bipolar Junction Transistor (BJT) is defined as a three-terminal semiconductor device, whose operation depends upon the flow of electric charge carriers within the solid. It is a current-controlled device.
They are constructed in the form of two semiconductor junctions, rather like two PN junction diodes joined together. The transistor has three terminals called Emitter (E), Base (B), and Collector (C), and two PN-junctions called C-B junction and E-B junction. The Collector-Base diode, however, is reverse-biased and normally you would expect it to pass no current. But if the E-B diode is conducting high current, this influences the reverse biased C-B diode to pass almost as much reverse current. There are many transistor case designs and these are defined by Transistor Outline (TO) designations (e.g. TO3, TO92.) Low–power, small-signal transistors are sealed in a metal, plastic or epoxy package.
In this type the three leads (E, B, and C) emerge from the bottom of the package High power transistors are usually designed to be mounted onto a metal frame, which acts as a heat sink (to conduct heat away).
These types of packages may have two or three leads. If only two leads are present, the metal case will be the collector, and the other two will be the base and emitter. Under these conditions, we find that:
A small base current flows into the base.
A much larger current flows into the collector.
The base and collector currents flow out of the emitter. The plastic case has a flat surface and the metal case has a tag on its rim.
These help to identify the terminal wires. 1. Common Base (CB) Configuration: In this configuration, the base is made common to the emitter and collector of the transistor. 2. Common Emitter (CE) Configuration: In this configuration, the emitter is made common to the base and the collector of the transistor. 3. Common Collector (CC) Configuration: In this configuration, the collector is made common to the base and emitter of the transistor. 1. Transistor Switch When a transistor operates as a switch, it is either ON (current flowing), or OFF (no current), thus enabling it to switch other electronic devices ON and OFF. The small base current controls the larger collector current.
The degree of amplification is usually measured by the DC current gain, hFE. 2. Transistor Amplifier Amplification is the process of increasing the level of a weak signal.
A small change in the base current of a transistor produces a large variation in the collector current.