Прочитайте текст и заполните пропуски подходящими по смыслу словами There are 1_______________ power...: ответ на тест 781351 - Английский язык

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There are 1_______________ power amplifiers. These are similar to voltage amplifiers, except that their main 2__________________ is to supply a considerable amount of 3________________ i. e. voltage times current to the output or load circuit, although the ac input signal may not draw any grid current and, hence, the 4________________ power may be zero.

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There are 1_______________ power amplifiers. These are similar to voltage amplifiers, except that their main 2__________________ is to supply a considerable amount of 3________________ i. e. voltage times current to the output or load 4________________, although the ac input signal may not draw any grid current and, hence, the input power may be zero.

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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  2______________  the voltage to a high 3_______________, while the last stage builds up the power to a level sufficient to operate an output device. Audio amplifiers amplify electrical ac signals 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__________________.

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Sonar (Sound Navigation and Ranging) is employed to locate submerged enemy 1________________. Like radar it is an echo-ranging 2__________________. It operates in the following way - a ship 3________________ a sound wave into the water. Encounting an obstacle the wave is reflected back to the ship, which – since the speed of sound through water is known - determines its distance from the object from the time it took the echo to return. To create the original sound wave, the ship generates 4________________ signals in а transmitter; it then sends these to a transducer, positioned on its keel as deeply in the water as possible. The transducer converts the signals into sound and transmits them through the water.