Electronic Components. Vacuum Tubes and Transistors
1. Electronic circuits consist of interconnections of electronic components. Components are classified into two categories - active or passive. Passive elements never supply more energy than they absorb; active elements can supply more energy than they absorb. Passive components include resistors, capacitors, and inductors. Components considered active include batteries, generators, vacuum tubes, and transistors.
2. A vacuum tube consists of an air-evacuated glass envelope that contains several metal electrodes.
A simple, two-element tube (diode) consists of a cathode and an anode that is connected to the positive terminal of a power supply. The cathode - a small metal tube heated by a filament - free electrons, which migrate to the anode - a metal cylinder around the cathode (also called the plate). If an alternating voltage is applied to the anode, electrons will only flow to the anode during the positive half-cycle; during the negative cycle of the alternating voltage, the anode repels the electrons, and no current passes through the tube. Diodes connected in such a way that only the positive half-cycles of an alternating current (AC) are permitted to pass are called rectifier tubes; these are used in the conversion of alternating current to direct current (DC). By inserting a grid, consisting of a spiral of metal wire, between the cathode and the anode and applying a negative voltage to the grid, the flow of electrons can be controlled. When the grid is negative, it repels electrons, and only a fraction of the electrons emitted by the cathode can reach the anode. Such a tube, called a triode, can be used as an amplifier. Small variations in voltage at the grid, such as can be produced by a radio or audio signal, will cause large variations in the flow of electrons from the cathode to the anode and, hence, in the circuitry connected to the anode.
3. Transistors are made from semiconductors. These are materials, such as silicon or germanium, that are "doped" (have minute amounts of foreign elements added) so that either an abundance or a lack of free electrons exists. In the former case, the semiconductor is called n-type, and in the latter case, p-type. By combining n-type and p-type materials, a diode can be produced. When this diode is connected to a battery so that the p-type material is positive and the n-type negative, electrons are repelled from the negative battery terminal and pass unimpeded to the p-region, which lacks electrons. With battery reversed, the electrons arriving in the p-material can pass only with difficulty to the n-material, which is already filled with free electrons, and the current is almost zero.
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4. The bipolar transistor was invented in 1948 as a replacement for the triode vacuum tube. It consists of three layers of doped material, forming two p-n (bipolar) junctions with configurations of p-n-p or n-p-n. One junction is connected to a battery so as to allow current flow (forward bias), and the other junction has a battery connected in the opposite direction (reverse bias). If the current in the forward-biased junction is varied by the addition of a signal, the current in the reverse-biased junction of the transistor will vary accordingly. The principle can be used to construct amplifiers in which a small signal applied to the forward-biased junction causes a large change in current in the reverse-biased junction.
5. Another type of transistor is the field-effect transistor (FET). Such a transistor operates on the principle of repulsion or attraction of charges due to a superimposed electric field. Amplification of current is accomplished in a manner similar to the grid control of a vacuum tube. Field-effect transistors operate more efficiently than bipolar types, because a large signal can be controlled by a very small amount of energy.
III After-text exercises
1. Agree or disagree with the next statements:
1) Passive components include batteries, generators, vacuum tubes and transistors.
2) A diode consists of a cathode and an anode that is connected to the positive terminal of a power supply.
3) When a grid is positive, it repels electrons.
4) By combining n-type and p-type materials a diode can be produced.
5) Field-effect transistors operate less efficiently than bipolar types.
Find the information about:
l) the function of the cathode;
2) the controlling of the flow of electrons;
3) such materials as semiconductors;
4) the bipolar transistor;
5) the principle of the field-effect transistor's operation.
Put questions to each other.
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