中英互译文献.doc

Operational Amplifiers
In 1943 Harry Black commuted from his home in New York City at Bell Labs in New Jersey by way of a ferry. The ferry ride relaxed Harry enabling him to do some conceptual thinking. Harry had a tough problem to solve; when phone lines were extended long distance, they needed amplifiers, and undependable amplifiers limited phone service. First, initial tolerances on the gain were poor, but that problem was quickly solved wuth an adjustment. Second, even when an amplifier was adjusted correctly at the factory, the gain drifted so much during field operation that the volume was too low or the incoming speech was distorted.
Many attempts had been made to make a stable amplifier, but temperature changes and power supply voltage extremes experienced on phone lines caused uncontrollable gain drift. Passive components had much better drift characteristics than active components had, thus if an amplifier’s gain could be made dependent on passive components, the problem would be solve. During one of his ferry trips, Harry’s fertile brain conceived a novel solution for the amplifier problem, and he documented the solution while riding on the ferry.
The solution was to first build an amplifier that had more gain than the application required. Then some of the amplifier output signal was fed back to the input in a manner that makes the circuit gain (circuit is the amplifier and feedback components) dependent on the feedback circuit rather than the amplifier gain. Now the circuit gain is dependent on the passive feedback components rather than the active amplifier. This is called negative feedback, and it is the underlying operating principle for all modern day opamps. Harry had documented the first intentional feedback circuit had been built prior to that time ,but the designers ignored the effect.
I can hear the squeals of anguish coming from the manager and amplifier designers. I imagine that they said something like this, “it is hard enough to achieve 30kHz gainbandwidth (GBW), and now this fool wants me to design an amplifier with 3MHz GBW. But ,he is still going to get a circuit gain GBW of 30kHz .” Well, time has proven Harry right ,but there is a minor problem. It seems that circuit designed with large pen loop gains sometimes oscillate when the loop is closed. A lot of people investigated the instability effect, and it was pretty well understood in the 1940s, but solving stability problems involved long, tedious, and intricate calculations. Years passed without anybody making the problem solution simpler or more understandable.
In 1945 H. W. Bode presented a system for analyzing the stability of feedback system by using graphical methods. Until this time, feedback analysis was done by multiplication and division, so calculation of transfer functions was a time consuming and laborious task. Remember, engineers did not have calculators or computers until the ‘70s, Bode presented a log technique that transformed the intensely mathematical process of calculating a feedback system’s stability into graphical analysis that was simple and perceptive. Feedback system design was still complicated, but it no longer was an art dominated by a few electrical engineers kept in a small dark room. Any electrical engineer could use Bode’s methods to find the stability of a feedback circuit, so the application of feedback to machines began to grow. There really wasn’t much call for electrical feedback design until computers and transducers become of age.
The first real-time computer was the analog computer! This computer used preprogrammed equations and input data to calculate control actions. The programming was hard wired with a series of circuit that performed math operations on the data, and the hard wiring limitation eventually caused the declining popularity of the analog computer. The heart of the analog computer was a device called an operational amplifier because it could be configured to perform many mathematical operations such as multiplication, addition, subtraction, division, integration, and differentiation on the input signals. The name was shortened to the familiar op amp, as we have come to know and love them. The op amp used an amplifier with a large open loop gain, and when the loop was closed, the amplifier performed the mathematical operations dictated by the external passive components. This amplifier was very large because it was built with vacuum tubes and it required a high-voltage power supply,but it was the heart of the analog computer, thus its large size and huge power requirements were accepted. Many early op amps were designed for analog computers, an it was soon found out the op amps had other uses and were handy to have around the physics lab .
At this time general-purpose analog computers were found in universities and large company laboratories because they were critical to the research work done there. There was a parallel requirement for transducer signal conditioning in lab experiments, and op amps found their way into signal conditioning applications. As the signal conditioning applications expanded, the demand for op amps grew beyond the analog computer requirements , and even when the analog computers lost favor to digital computers, the op amps survived because of its importance in universal analog applications. Eventually digital computes replaced the analog computers, but the demand for op amps increased as measurement applications increased.
The first signal conditioning op amps were constructed with vacuum tubes prior to the introduction of transistors, so they were large and bulky. During the’50s, miniature vacuum tubes that worked from lower voltage power supplies enabled the manufacture of op amps that shrunk to the size lf a brick used in house construction, so the op amp modules were nick named bricks. Vacuum tube size and component size decreased until an op amp was shrunk to the size of a single octal vacuum tube. Transistors were commercially developed in the ‘60s, and they further reduced op amp size to several cubic inches. Most of these early op amps were made for specific applications, so they were not necessarily general purpose. The early op amps served a specific purpose, but each manufacturer had different specifications and packages; hence, there was little second sourcing among the early op amps.
ICs were developed during the late 1950s and early 1960s, but it wasn’t till the middle 1960s that Fairchild released the μA709. This was the first commercially successful IC op am. TheμA709 had its share of problems, bur any competent analog engineer could use it, and it served in many different analog applications. The major drawback of theμA709 was stability; it required external compensation and a competent analog engineer to apply it. Also, theμA709 was quite sensitive because it had a habit of self-destruction under any adverse condition. TheμA741 followed theμA709, and it is an internally compensated op amp that does not require external compensation if operated under data sheet conditions. There has been a never-ending series of new op amps released each year since then, and their performance and reliability had improved to the point where present day op amps can be used for analog applications by anybody.
The IC op amp is here to stay; the latest generation op amps cover the frequency spectrum from 5kHz GBW to beyond 1GHz GBW. The supply voltage ranges from guaranteed operation at 0.9V to absolute maximum voltage ratings of 1000V. The input current and input offset voltage has fallen so low that customers have problems verifying the specifications during incoming inspection. The op amp has truly become the universal analog IC because it performs all analog tasks. It can function as a line driver, comparator (one bit A/D), amplifier, level shifter, oscillator, filter, signal conditioner, actuator driver, current source, voltage source, and etc. The designer’s problem is how to rapidly select the correct circuit /op amp combination and then, how to calculate the passive component values that yield the desired transfer function in the circuit.
The op amp will continue to be a vital component of analog design because it is such a fundamental component. Each generation of electronic equipment integrates more functions on silicon and takes more of the analog circuit inside the IC. As digital applications increase, analog applications also increase because the predominant supply of data and interface applications are in the real world, and the real world is an analog world. Thus , each new generation of electronic equipment creates requirements for new analog circuit; hence, new generations of op amps are required to fulfill these requirements. Analog design, and op amp design, is a fundamental skill that will be required far into the future.

放 大 器
1943年，哈利·布莱克乘火车或渡船从位于纽约市的家去新泽西州的贝尔实验室上班。在上班途中，哈利能够送下来，思考一些概念上的问题。哈利需要解决一个很棘手的问题：电话线在用于长途传输时就需要放大器，而性能不可靠的放大器限制了电话业务的扩展。首先，增益容差性能很差；但是，通过使用调节器，这个问题很快就解决了。第二，即使放大器在工厂里被调节正确，但在现场工作是增益还是漂移地很厉害，以至于音量太低或输入语音发生畸变。
为了制造出稳定的放大器，哈利已经进行了多次尝试；但是，温度变化和电话线上出现的供电电压极限使得增益漂移无法控制。无源器件比有源器件具有好得多漂移特性；假如能使放大器增益仅由无源器件决定的话，那么这个问题就会解决。在乘渡船上下班途中，哈利构想一个新颖的、解决放大器问题的办法，并在途中将它记录下来。
这个解决方案是这样的：首先设计一个增益比实际需求高的放大器，然后，将放大器输出信号的一部分反馈输入端，这使得电路增益（这里的电路由放大器和反馈器件组成）由反馈电路决定，而不是放大器增益决定，这样的话，电路增益就取决于无源反馈器件，而不是有源放大器。这个方案被称为“负反馈“，它是现代运算放大器的基本工作原理。哈利在乘坐渡船途中记录了第一个有意加入反馈的电路。此前，也一定有人无意中使用过反馈电路，但设计者忽视了这种作用！
管理者和放大器设计者可能会发生痛苦的抱怨。他们可能会说：“获得30kHz的增益带宽就够难了，现在这个傻瓜要我设计增益带宽为30MHz的放大器，而他还是想得到一个增益带宽为30kHz电路”。然而，时间已经证明哈利是正确，但有一个小问题哈利没有详细讨论——振荡问题。在环路闭合时候，开环增益设计得很大的电路有时会发生振荡。很多人都研究了这个不稳定现象，在20世纪40年代人们对它有了清晰的认识；不过，解决稳定问题需要长时间单调复杂的计算。许多年过去了，没有人能将解法简化或使之易于理解。
1945年，波特用图形化方法展示了一个用于分析反馈系统稳定性的系统。那时，反馈分析是用乘法和除法完成的。因此，计算传输函数是一件耗时费力的工作。需要知道的是：直到20世纪70年代，工程师才有了计算器和计算机。波特采用了一种对数方法——将分析反馈系统稳定的数学过程转换为容易的、好理解的图形化分析。虽然反馈系统的设计依然很复杂，但它再也不是一件只有少数电气工程师掌握的技术。任何电气工程师都可以使用波特的方法确定反馈电路的稳定性，并越来越多地将反馈应用到机器设计当中。直到计算机和传感出现后，才产生了对电子反馈设计的真正迫切需要。
第一台实时计算机是模拟计算机。这台计算机使用预先编程的数学公式和输入数据来计算控制动作。编程是对一系列能完成对输入数据进行数学操作电脑进行硬连线，硬连线的限制最终导致模拟计算机没有普及。模拟计算机的心脏是运算放大器；通过配置，它可以对输入数据进行多种数学运算，如乘法、除法、加法、减法、积分和微分。随着人们逐步了解并喜欢运算放大器，它们名字就简化成了大家熟知“op amp”（运放）。运放使用具有一个很大开环增益的放大器，当电路形成闭合环路时，放大器就会执行由外部无源元件控制的数学运算。这个放大器体积很大，因为它是用真空管 做的，而且需要一个大电压电源。但由于他是模拟计算机的心脏，所以人们还是接受了它这种大体积和大电压。许多早期运放是为模拟计算机设计的，人们很快就发现运放还有其他用途，在屋里实验室也很容易得到运放。
那个时候，在大学和大型公司的实验室里就能够看到通用模拟计算机；因为对那里进行的研究工作而言，计算机是至关重要的。同时，在实验室中也需要对传感器信号进行调理，运放在信号调理方面也找到用武之地。随着信号调理应用范围的拓展，其对运放需要的增长超过了模拟计算机。甚至在模拟计算机逊色于数字计算机之后，运放因其在通用模拟应用中的重要性而并未受到冷落。数字计算机最终代替了模拟计算机，但是运放的需求量却随着测量应用的增长而增长了。
在晶体管出现之前，第一个用于信号调理的运放是用真空管构建的，因此它体积很大。20世纪50年代，低电压工作的小型真空管使运放的体积缩小到一块砖的大小，因此运放模块有了一个“砖块“的绰号。真空管和组件的体积不断地缩小，直至运放缩至一只八角真空管的大小。在60年代，晶体管实现了商业开发，这进一步将运放的体积减至几个立方英寸。大多数早期运放是为特定应用而制造的，所以它们不一定通用；早期运放是为某种特定应用服务的，但每个生产商的技术上指标和封装都不同。
50年代末60年代初，集成电路开发出来了。但直到60年代中期，仙童公司才发布了μA709。它是首片取得商业成功的集成运放。虽然μA709有自身的问题，然而任何一位称职的模拟工程师都能使用它，在许多不同的模块应用中都可以使用它。μA709的主要缺陷在稳定性，他需要外部补偿。μA709也很敏感；一旦不利条件出现，他就容易自毁。在μA709之后出现了内部补偿μA741；当在数据手册要求的条件下工作时，μA741是不需要外部补偿的。从那以后，新系列的运放就不断出现；今天，运放的功能和可靠性已经提高到了这种程度——任何人都能在模拟应用它。
今天，集成运放仍然存在。最新一代运放可以覆盖5kHzGBW至1GHzGBW的频频范围。供电电压范围从可靠运行的 0.9V到最大绝对指标的1000V。由于输入电流和输入偏置电压很低，用户在验证指标时遇到难题。运放已经真正成为通用模拟集成电路，因为它可完成所有模拟任务。它可以用做线驱动器，比较器，放大器，电平偏移器，振荡器，滤波器，信号调理器，执行部件驱动器，电流源，电压源等等。设计者面临额问题是如何迅速选址正确的电路运放组合，然后如何计算能产生期望电路传输函数的无源元件。
运放是一种如此基础的部件，它将继续作为模拟设计的关键部件。每一代电子设备在硅片上集成啦更多功能，将更多的模拟电路置于电路内部。随着数字应用增加，模拟应用也会增加；因为大量的数据应用和接口应用都在现实世界中，而现实世界是一个模拟的世界。因此，每一代新型电子设备需要新型的模拟电路，就需要新一代运放去满足上述需求。未来，模拟设计和运放设计将是一项必备的基本技能。
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