外文翻译--信号放大器

1、At the most basic level, a signal amplifier does exactly what you expect it makes a signal bigger! However, the way in which it is done vary with the design of the actual amplifier, the type of signal, and the reason why we want to enlarge the signal. We can illustrate this by considering the common

2、 example of a “Hi-Fi” audio system.In a typical modern Hi-Fi: system, the signals will come from a unit like a CD player, FM tuner, or a Tape/Minidisk unit. The signals they produce have typical levels of the order of 100 mV or so when the music is moderately loud. This is a reasonably large voltage

3、, easy to detect with something like an oscilloscope or a voltmeter. However, the actual power levels of these signals are quite modest. Typically, these sources can only provide currents of a few milliamps, which by P=VI means powers of just a few milliwatts. A typical loudspeaker will require betw

4、een a few Watts and perhaps over 100 Watts to produce loud sound. Hence we will require some from of Power Amplifier to “boost” the signal power level from the source and make it big enough to play the music.Many practical amplifier chain together a series of analog amplifier stages to obtain a high

5、 overall voltage gain. For example, a PA system might start with voltages of the order of 0.1 mV from microphones, and boost this to perhaps 10 to 100V to drive loudspeakers. This requires an overall voltage gain of 10, so a number of voltage gain stages will be required.In many cases we wish to amp

6、lify the current signal level as well as the voltage. The example we can consider here is the signal required to drive the loudspeakers in a “Hi-Fi” system. These will tend to have a typical input impedance of the order of 8 Ohms. So to drive, say, 100 Watts into such a loudspeaker load we have to s

7、imultaneously provide a voltage of 28 Vrms and 3.5 Arms. Taking the example of a microphone as an initial source again a typical source impedance will be around 100 Ohms. Hence the microphone will provide just 1 nA when producing 0.1 mV. This means that to take this and drive 100 W into a loudspeake

8、r the amplifier system must amplify the signal current by a factor of over 10 at the same time as boosting the voltage by a similar amount. This means that the overall power gain required is 10 i.e. 180 dB!This high overall power gain is one reason it is common to spread the amplifying function into

9、 separately boxed preand poweramplifiers. The signal levels inside power amplifiers are so much larger than these weak inputs that even the slightest “leakage” from the output back to the input may cause problems. By putting the highpower (high current) and low power sections in different boxes we c

10、an help protect the input signals from harm.In practice, many devices which require high currents and powers tend to work on the basis that it is the signal voltage which determines the level of response, and they then draw the current they need in order to work. For example, it is the convention wi

11、th loudspeakers that the volume of the sound should be set by the voltage applied to the speaker. Despite this, most loudspeakers have an efficiency (the effectiveness with which electrical power is converted into acoustical power) which is highly frequency dependent. To a large extent this arises a

12、s a natural consequence of the physical properties of loudspeakers. We wont worry about the details here, but as a result a loudspeakers input impedance usually varies in quite a complicated manner with the frequency. (Sometimes also with the input level.)This kind of behavior is quite common in ele

13、ctronic systems. It means that, in information trms, the signal pattern is determined by the way the voltage varies with time, and ideally the current required is then drawn. Although the above is based on a highpower example, a similar situation can arise when a senor is able to generate a voltage

14、in response to an input stimulus but can only supply a very limited current. In these situations we require either a current amplifier or a buffer. These devices are quite similar, and in each case we are using some form of gain device and circuit to increase the signal current level. However, a cur

15、rent amplifier always tries to multiply the current by a set amount. Hence it is similar in action to a voltage amplifier which always tries to multiply the signal current by a set amount. The buffer differs from the current amplifier as it sets out to provide whatever current level is demanded from

16、 it in order to maintain the signal voltage told to assert. Hence it will have a higher current gain when connected to a more demanding load.The nature of electrical signals in electronic circuits readily enables the technology to be divided into classes.One of the classes is analog electronics; ano

17、ther is digital electronics.Both analog and digital electronics use similar electronic elements,but the manner of use is different , and the technologies appear to be quite distinct .For this reason we shall study them separately until we bring them together as they invariably unite in instrumentati

18、on and applications. Analog electronics pertains to those systems in which the electrical voltage and electrical current are analogous to physical quantities and vary continuously. Electronic circuits that reproduce music must have voltages and currents that are proportional to the sound. A high fid

19、elity amplifying system attempts to keep the analogy as true as possible.Analog electronic circuits are carefully designed to make the electrical voltages and currents follow the input signal.If an input signal doubles in amplitude,the output voltage or current also should double;this is possible be

20、cause the circuit elements are made to operate within limits that preserve the linearity. An electrical voltage that is proportional to temperature and changes smoothly as the temperature changes is an analog of temperature.If the temperature range is divided into small increments,then the temperatu

21、re may be indicated by a digital display.As the temperature (voltage) changes smoothly,a decision must be made by an electronic system as to the numerical value to be displayed as the temperature.The circuit making the decision is called an analog-to-digital converter,ADC.The inverse process is acco

22、mplished by a digital-to-analog converter,DAC. Digital electronic circuits do not require the linearity of analog circuits.Digital circuits act as electronic switches and switch from one state to another.The output state,on or off, is the only signal condition to be examined.In digital circuits the

23、output state is determined by the input signals in as direct a manner as the output voltage of an analog circuit is related to the input signal. In digital circuits the relation between input and output states are expressed as logic equations; the elements of digital electronics are called logic gat

24、es. Logic gates switch between states,on or off, very quickly so that they may operate at many megahertz in computers and other applications. As technical developments continue to provide new and amazing integrated circuits,as they have since the 1960s,both analog and digital systems will be more ca

25、pable. The designers of electronic systems using integrated circuits will have unlimited possibilities for innovation.一般情况下，信号放大器正如人们期望的一样工作-将信号放大。然而，信号的放大方式随着实际放大器的设计、信号的类型以及放大信号目的的不同而变化。这一点可以通过一个常见的高保真音响系统实例来加以说明。在典型的现代高保真系统中，信号是来自于CD播放器、调频收音机或磁带/小型磁盘机等设备。当音乐声大小适当时，这些设备产生信号的幅度大概在100毫伏左右。这种信号幅度相当高，

26、易于用示波器或电压表等仪器检测到。但是，这些信号的实际能量水平并不高。典型情况下，这些信号源只能提供毫安级的电流。根据公式P=VI，其功率只有几十毫瓦。典型的扬声器需要几十瓦到数百瓦的功率才能产生足够大的声音。因此，我们需要某些形式的功率放大器来提高来自信号源的信号功率，使其足以播放音乐。许多实际的放大器将多个放大器级联起来，以获得较高的电压增益。例如，一个功率放大系统的输入是来自于麦克风的0.1毫伏的电压，将其放大到10伏到100伏才能推动扬声器。这就要求电压的总增益达到10的九次方，因此就需要很多放大器级联起来。在很多情况下，除了信号的电压外，我们还要放大信号的电流。这里我们考虑的例子是高

27、保真系统中用来驱动扬声器的信号，其典型的输入电阻约为8欧姆。因此，要驱动100瓦的扬声器负载，就要同时提供28伏的电压和3.5安的电流信号。仍以麦克风作为初始信号源为例，典型的源阻抗在100欧左右。因此，麦克风在产生0.1毫伏的信号时，提供的电流仅为1纳安。这就表示要接受这种输入信号并去驱动100瓦的扬声器，放大电路就必须将信号的电流和电压同时放大10的九次方倍。这也就意味着总的功率增益为10的18次方，即180分贝。一般都将放大功能分散到单独设计的前级放大器和功率放大器中，其原因就在于功率增益很大。功率放大器中的信号幅度比微弱的输入信号大得多，即使输出的极微小的泄漏传输到输入端，都会引发一些

28、问题。通过将大功率（大电流）和小功率放大电路分置在不同的单元中，就可以避免输入信号受到干扰。实际上，许多需要大电流和大功率的设备往往都在特定的条件下工作，即由信号的电压决定响应的幅度，继而由设备吸收其所需要的电流而工作。例如，扬声器的音量通常是由所加电压控制。此外，大多数扬声器的效率（电能被转换为声能的效能）基本上与频率无关。在很大程度上，这是由扬声器的物理特性所产生的自然结果。这里不必考虑具体的细节，但扬声器的输入阻抗随频率的不同而呈复杂的变化（有时也与输入信号的幅度有关）。这种特点在电子系统中很常见。用信息术语来说就是信号类型取决于电压随时间变化的情况，且在理想情况下就能吸收所需的电流。尽管上述情况是基于大功率的例子，但当传感器在输入的激励下做出响应而产生一定的电压，却只能提供有限的电流时，类似的情况也会出现。这时我们就需要