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Exercise 1. First try to guess the missing words. Then listen and fill in the spaces.

The introduction of vacuum __________ at the beginning of the 20th century was the starting point of the rapid growth of modern _____________. With vacuum tubes the manipulation of signals became possible, which could not be done with the early _________ and telephone circuit or with the early transmitters using high-voltage sparks to _________ radio waves.

The simplest vacuum tube, the diode, contains two ___________: the cathode, a heated filament or a small, heated, metal tube that emits electrons through thermionic emission; and the _________, or plate, which is the electron-collecting element. In diodes, the _________ emitted by the cathode are attracted to the plate only when the latter is ________ with respect to the cathode. When the plate is negatively _________, no current flows through the tube. If an alternating potential is applied to the plate, the tube passes current only during the positive halves of the cycle and thus acts as a __________.

The introduction of a third electrode, called a _______, interposed between the cathode and the anode, forms the triode, which for many years was the basic tube used for _________ current. The function of the grid is to control the current flow. At a certain negative _________, the grid, because it repels electrons, can impede the flow of electrons __________ the cathode and the anode. At lower negative potentials, the electron flow ________ on the grid potential. The grid usually consists of a network of fine _____ surrounding the cathode. The capacity of the triode to amplify depends on the small changes in the _________ between the grid and the cathode causing large changes in the number of electrons reaching the anode. Thus, the basic components of a triode _________ tube are comparable to those of the transistor which include the emitter corresponding to the heated ________ of the triode tube as the source of electrons.

Vacuum tubes have now been almost entirely ________ by transistors, which are cheaper, smaller, and more _________. Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmitters, and in scientific and military __________ that must resist strong voltage pulses which destroy transistors. The cathode ray _____ is still a base of many modern TVs and _________ monitors.

Exercise 2. Answer the following questions.

1) What is the diode used for?

2) What is the function of the additional electrode, the greed, in the vacuum triode?

3) Which devices of mentioned above are used for rectification of a direct current into alternating one?

4) What kinds of rectifying circuits with transistors or vacuum tubs do you know?

5) Do vacuum tubes have any advantages over transistors?

6) Are transistors more reliable than vacuum tubes?

7) What adjectives can be used with the noun tube?

Exercise 3. Decide whether the following statements are true or false in relation to the information in the text in the Exercise 1. If you think the statement is false, change to make it true.

1) Any vacuum tube has at least three electrodes.

2) The electric current in the vacuum tube is a flow of positively charged ions.

3) The greed in a vacuum tube needs to be heated.

4) In certain condition the greed and the plate can emit electrons by themselves.

5) The intercity of electron stream in the vacuum tube always depend on voltage between the cathode and plate.

6) Transistors usually consume more power than vacuum tubes.

7) Nowadays most of people have never seen any vacuum tube.

Date: 2015-12-11; view: 1589

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