UnitThreeElectronicApparatusandInstruments..doc

Unit Three Electronic Apparatus and Instruments Passage One Electronic Components Training target: In this part, our target is to train your reading comprehension. We have made flexible sentences strong black and marked the subject, predicate and object of them. Try to grasp the main idea of these sentences. 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. Vacuum Tubes A vacuum tube consists of an air-evacuated glass envelop 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 filament—free electrons, which migrate to the anode—a metal cylinder around the cathode is also called the plate. If an alternating voltage is applied to the cathode, electrons will only flow to the cathode 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 DC. By inserting a grid, consisting of a spiral of metal wire, between the cathode and the anode and applying a 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 lar