Вопрос № 347364 - Английский язык

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We are finally ready to apply the knowledge we gained in previous sections about electrons, vacuum 1_________________ and transistors to some practical matters. In the following sections we shall consider a variety of circuits employing electron tubes and transistors. Circuits are 2_________________ of tubes or transistors with other components, such as resistors, capacitors and inductors, and form the 3________________ building blocks of electronic systems: radio, automatic computer and so on. To understand the systems, you must be familiar with the circuits that 4_______________ them up.

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Intensive 1______________ of radio-location began in the early forties. All these years the vacuum 2_______________ was the heart of electronics and, hence, the field of 3______________ was usually defined as the study of the motion of electrons in vacuum tubes. However, it was found that small, controlled 4______________ of gas introduced into a "vacuum tube" could make it perform switching, rectifying and other jobs. To include the category of gas-filled tubes, "vacuum tubes" became converted to "electron tubes" and the science of electronics was expanded to include the study of electron motion in gases.

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Now we shall discuss amplifier circuits, or more specifically, audio amplifiers. 1 _________ is an electron tube or transistor circuit, which 2_________________ up an ac signal applied to its input. It is called a voltage amplifier if the magnitude of the output  3  __________ . From the amplifier is considerably greater than that of the input voltage. As a matter of fact the ratio of the output voltage to the 4__________________ voltage is called the amplification or gain of the amplifier.

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When a number of 1_______________ are hooked up in series so that the output of one serves as the input to the next amplifier stage, the function of the early stages is usually to 2______________ up the voltage to a high level, while the last stage builds up the power to a level sufficient to operate an output device. Audio amplifiers amplify electrical ac 3_______________ that have a frequency range corresponding to the range of human hearing, or from about 20 to 15,000 cycles per 4________________.

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The reflection of 1_______________ waves from aircraft had been observed in England in the early 1930s and the possibility for aircraft detection was discussed in 1934. It was soon realized that radio waves would be the 2________________ alternative to the existing inadequate acoustic warning equipment, which merely listened for the sound of aircraft 3________________ and had too short a range for proper warning of approach of last aircraft. In contrast, experiments indicated that radar could give a warning when the 4_________________ was 100 miles or more away.

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The principles of 1________________ in a simplified form might be stated as follows:
1. electromagnetic radiation at high radio frequencies is 2________________ to detect and locate remote reflecting objects;
2. the radiation is sent out in pulses of a few microseconds’ duration, separated by intervals many times the duration of each pulse;
3. the pulses are returned from the reflecting objects and the returns are detected and displayed by receiving equipment placed at the point of transmission;
4. 3________________   to the objects is determined by measurement of the time it took the pulses to reach the targets from the transmission equipment and return to it;
5. the directions of the targets are determined by use of highly directive radio 4__________________.