They can be used to amplify the small output current from a logic chip so that it can operate他们放大逻辑芯片输出的小电流，从而来驱动灯、继电器或其他大电流器件A transistor may be used as a switch~and asan三极管也可用于开关（或是完全导通，流过最大电源，或是完全关断，没有电流流过）和放大器（总是不完全导通）Transistors can be damaged by heat when soldering so if you are not an expert it is wise三极管会被焊接时产生的热量所损坏，因此如果你不是这方面的专家，那么最好在焊接点与三极管管身之间的管脚上夹一个散热Whether the device in question be voltage controlled or current controlled,the amount of不管讨论的器件是受电压控制还是受电流控制，控制信号所需的功率一般远小于被控信号所用的功率It is a fundamental rule of physics that energy能量既不会被创造也不会被消灭，这正是物理学的一个基本法则If this Law is true and an overwhelming mass of experimental data suggests that it is-then iti如果这条定律是正确的—惊人的实验数据说明它是正确的—那么就不可能制造出一种器件，该器件可以吸收小能量，然后神奇的把小能量转换为大能量Usually,machines fail even to meet this limit,losing some of their input energy in the f通常，机器不能达到这个上限，会以热的形式丢失掉部分输入能量，该部分能量辐射到周围的空间，因此这部分能量不属于输出能量部分Not only would such a perpetual motion machine prove that the Law of Energy~这样的永动机不仅能够证明能量守恒定律不成立，而且它能够引导一场前所未有的技术革命，因为它可以循环的给自身提供能量并产生免费的多余能量The key to understanding how amplifiers can exist without violating the Law of Energy co~明白放大器能够存在而不违反能量守恒定律的关键在于有源元件的性质The Law of Energy conservation is not violated because the additional power is suppli 能量守恒定律并没有被违背，因为增加的功率是由外部电源提供的，通常是直流电池或等效电源Because the efficiency of an amplifier is always 100 percent or less,one can never be m由于放大器的效率一定为100%或小于100%，任何东西都不可能被做成具有永动功能的装置One key to the usefulness of these little circuits is in the engineering principle of feedback,particularly negative feedback,which这些小电路发挥作用的关键在于工程学中的反馈原理，特别是负反馈，其构成了几乎所有自动控制过程的基础The peak-to-peak amplitude of this amplifier'sOutput between cutoff and saturation remains放大器处于截至饱和状态间的峰峰值保持不变Large forward transfer function正向传递函数很大Virtually nonexistent reverse transfer function事实上不存在反向传递函数Large input impedance,Zin输入阻抗很大，Z（任何信号都可加到运算放大器上而不存在带负载问题）Small output impedance,Zout输入阻抗很小，Z（运放提供的功率没有限制）Wide bandwidth带宽宽Infinite gain增益无穷大This polarity inversion from input to output is due to the fact that the input signal is being se输入与输出间极性相反是由于输入信号被夹在运放的反向输入端，因此其相当于反相放大器However,if we apply a constant,positive voltage to the input,the op-amp output will fallNegative at a linear rate,in an attempt to produce the changing voltage across the capacitor necessary to maintain the current est可是，如果我们将产生一个恒定的正电压加到输入端，运放的输出将为一个线性的负电压，为了在电容上产生一个变化的电压，有必要维持电阻端电压产生的电流不变Transmitter:the transmitter converts the message into a signal,the form of which is suitable for transmission over the communicat发信机。

发信机把消息转变为信号，信号形式要适合在通信信道中传送Receiver:the receiver can be thought of as the inverse of the transmitter ,it changes the received signal back into a message and passes the message onto its destination which接收机：接收机可以看作是与发信机相反的东西。

他把接收到的信号变回成信息，并把消息传给终端，这个终端可以是一台扬声器、电传打字机或一个计算机数据库This unwanted noise may cause distortions of sound in a telephone,or errors in a telegraph这种人们不希望存在的噪声会引起电话声音失真，电报报文或数据出现差错Groups or supergroups are moved as single units by the communications equipment and it is not necessary for the radios to know how many channels are involved通信设备把群或者超群做为一些单一的单元来传送。

对于无限设备来说，并不需要知道含有多少路。

A radio can handle a supergroup provided sufficient bandwidth is available只要有足够的带宽，无线电可以处理一个超群The size of the group is a compromise as treating each channels individually involves far more equipment because separate filters,modulators and oscillators are required 群的大小采取折中考虑确定，因为处理每一路都需要独立的滤波器、调制器、振荡器，这比每个群所需的设备要多得多However the failure of one module will lose all of the channels associated with a group但若有一个部件失效，则会失去与这个群相关的全部话路The signal created by sampling our analog speech input is known as pulse amplitude 对模拟话音输入取样所产生的信号称为脉幅调制信号It will be seen later that most of the advantages of digital modulation,come from the transmitted pulses having two levels only,this being known as a binary system 数字调制的大多数优点来自于发送的脉冲只有两个电平，这种系统称为二进制系统To do this it is necessary to employ syncnronized switches at each end of the communication link to enable samples to be要做到这一点，必须在通信链路的两端采用同步开关，使对来自各条电路的抽样值能依次轮流传送Although each user only has periodic short time slots,the original analog signals between samples can be reconstituted at the receiver虽然每个用户只有一些周期性的短视隙，抽样值之间的原来的模拟信号却可在接受机中重新构成The advantages of optical fiber as a transmission medium are due to the fact that the fiber is made of a dielectric and the signal光纤做为传播媒介的有点在于它是由绝缘体制成而且传送的信号是光。

In this case the information source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source发信源提供电信号给发射机，发射机组成一个电子平台来驱动光源以完成对光载频的调制The optical source which provides the electrical-optical conversion may be either a semiconductor laser or light emitting diode光源是由发光二极管或半导体激光器组成，他完成光-电变换Thus there is a requirement for electrical interfacing at either end of the optical link and at present the signal processing is usually per因此，在光系统链路的两端都要求有电接口，并且再现阶段，信号处理通常是通过电路实现的In the telephone system,the use of fiber optics for interconnecting central offices within a metropolitan area and for lower levels in the在电话系统中，光纤在主要城市地区中心交换局间互连和低级交换中的使用继续迅速增加These wideband subscriber loop systems would provide access to services such as pictu 这些宽带用户环路系统将为可视电话，视频娱乐节目等业务提供通路The means of standardizing these various broadband communication services so that they can be provided in an integrated manner is no other than the broadband integrated serv将这些不同的宽带通信业务标准化的最好方法莫过于采用宽带综合业务数字网，这样能够使这些服务以集成的方式提供Fortunately,the remarkable advances in the field of electronics and fiber optics have led toThe maturation of broadband communication幸运的是，电子及光纤领域的显著近战导致了快带通信技术的成熟ATM allocates bandwidth on by constructing virtual channels and virtual paths between source and destination points on the ATM netATM通过在其网络边界内的起点和终点之间建立虚拟信道和虚拟通路，按需分配带宽The nuances and peculiarities of digital videoMake it impractical to transport real-time video in its native uncompressed format over数字视频的细微差别和特别之处也使得在ATM上传输天然的、未经压缩的实时视频是不切实际的The most important of these components is the central processing unit (cpu),or microprocessor, which acts as the “brain” of your computer.这些组件中最重要的是作为计算机“大脑”的中央处理器，也叫微处理器B ecause hard disks can hold massive amounts of information, they usually serve as your computer’s primary means of storage, holding almost all of your programs and files.因为硬盘可以容纳大量的信息，它们通常用作计算机上主要存储器，保存几乎全部程序和文件When you move the mousse with your hand, a pointer on your screen moves in the same direction. (T he pointer’s appearance might change depending on where it’s positioned on your screen.)Where you want to select an item, you point to the item and then click( press and release)the primary button, Pointing and clicking with your mouseis the main way to interact with your computer.当你用手移动鼠标时，屏幕上的指针会向相同的方向移动（指针的外观可以改变，这取决于拟将其置于屏幕上的什么位置）如果要选择某个项目先指向该项目，然后“点击”(按住不放)主按钮，使用鼠标指出并点击是与计算机交互的主要方式Both types produce sharp images, but LCD monitors have the advantage of being much thinner and lighter .CRT monitors, however, are generally more affordable.两种类型都可产生清晰的图像，但LcD显示器具有薄，轻的优势，而crt显示器通常价格较便宜The two main types of printers are inkjet printers and laser printers. Inkjet printers are the most popular printers for the home. They can print in black and white or in full color and can produce high-quality photographs when used with special paper.打印机的类型有两种，喷墨式打印机和激光打印机。