Tuesday, 14 August 2012

Transistor Comparison With Electron Tubes

 a. Efficiency and Voltage Requirements. 

transistor power efficiency is greater than that of the electron tube because the transistor does not require heater power. In addition, it does not require warm-up time, and it does not require a large de voltage to operate. Other advantages of the transistor are its useful life (b below), its noise level (c below), and its size and construction (d below).

b. Useful Life. 

Life expectancy is a very im-portant consideration in the application of any electronic device. A transistor that is hermeti-cally sealed in glass or metal will withstand a variety of situations that an electron tube cannot withstand. For example, a transistor, even though it is immersed in water, will operate for long periods of time with very little noticeable effect on its operating frequency. It also will withstand centrifugal force, gravity, and impact tests that would completely shatter an electron tube. Although transistors are a comparatively new development, and complete data on their life expectancy are not yet available, it has been esti-mated that they can operate continuously for ap-proximately 8 years, a time much greater than the life of the average electron tube.

c. Noise Level. 

The noise level of a transistor is approximately 20 db with a frequency input of 1,000 cycles per second. In comparison, the aver-age electron tube has a lower noise level for the same frequency input. Although the noise level of a transistor is higher than that of an electron tube at this particular frequency, the noise level of the transistor is inversely proportional to the audio-frequency input. When a transistor is used with a higher frequency input, the noise level becomes considerably lower.

d. Size and Construction.

 A power amplifier electron tube is shown in A, figure 11, and a power amplifier transistor is shown in B, figure 11. The construction of the electron tube permits efficient dissipation of heat. Although the transistor must also dissipate heat, the size is notice-ably smaller. The flange type construction of the transistor cover provides beat dissipation. In some cases a special metallic heat dissipator must be used. A medium-power electron tube and a medium-power transistor are shown respectively, in C and D, figure 11. Note that the construction of the electron tube is much larger than that of the transistor. A miniature electron tube and a miniature transistor are shown in E and F, figure 11. The construction of the electron tube is again much larger than the transistor. Notice that the power transistor (B, fig. 11) is smaller than the miniature electron tube (E, fig. 11).


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