自動化專業(yè)外文翻譯--運算放大器

1、<p><b>　　UNIT 2</b></p><p>　　A: The Operational Amplifier</p><p>　　One problem with electronic devices corresponding to the generalized amplifiers is that the gains, Au or A~,

2、 depend upon internal properties of the two-port system (p, fl, R~, Ro, etc.)?~ This makes design difficult since these parameters usually vary from device to device, as well as with temperature. The operational amplifie

3、r, or Op-Amp, is designed to minimize this dependence and to maximize the ease of design. An Op-Amp is an integrated circuit that has many component part such as resistors </p><p>　　A totally general analysi

4、s of the Op-Amp is beyond the scope of some texts. We will instead study one example in detail, then present the two Op-Amp laws and show how they can be used for analysis in many practical circuit applications. These tw

5、o principles allow one to design many circuits without a detailed understanding of the device physics. Hence, Op-Amps are quite useful for researchers in a variety of technical fields who need to build simple amplifiers

6、but do not want to design at the tr</p><p>　　The symbol used for an ideal Op-Amp is shown in Fig. 1-2A-1. Only three connections are shown: the positive and negative inputs, and the output. Not shown are oth

7、er connections necessary to run the Op-Amp such as its attachments to power supplies and to ground potential. The latter connections are necessary to use the Op-Amp in a practical circuit but are not necessary when con

8、sidering the ideal 0p-Amp applications we study in this chapter. The voltages at the two inputs and the output will be</p><p>　　Uo =A(U+ -U-) (1-2A-l)</p><p>　　where A is the gai

9、n of the Op-Amp and U+ and U - the voltages at inputs. In other words, the output voltage is A times the difference in potential between the two inputs.</p><p>　　Integrated circuit technology allows construc

10、tion of many amplifier circuits on a single composite "chip" of semiconductor material. One key to the success of an operational amplifier is the "cascading" of a number of transistor amplifiers to cr

11、eate a very large total gain. That is, the number A in Eq. (1-2A-1) can be on the order of 100,000 or more. (For example, cascading of five transistor amplifiers, each with a gain of 10, would yield this value for A.) A

12、second important factor is that the</p><p>　　We now can analyze the particular amplifier circuit given in Fig. 1-2A-2 using these characteristics. First, we note that the voltage at the positive input, U +

13、, is equal to the source voltage, U + = Us. Various currents are defined in part b of the figure. Applying KVL around the outer loop in Fig. 1-2A-2b and remembering that the output voltage, Uo, is measured with respect t

14、o ground, we have</p><p>　　-I1R1-I2R2+U0=0 (1-2A-2) </p><p>　　Since the Op-Amp is constructed in such a way that no current flows into eithe

15、r the positive or negative input, I- =0. KCL at the negative input terminal then yields </p><p>　　I1 = I2 </p><p>　　Using Eq. (1-2A-2)

16、and setting I1 =I2 =I,</p><p>　　U0=(R1+R2)I (1-2A-3) </p><p>　　We may use Ohm's law to find the voltage at the negative input, U-, noting the assumed current direction

17、and the fact that ground potential is zero volts:</p><p>　　(U--0)/ R1=I</p><p>　　So, U-=IR1</p><p>　　and from Eq. (1-2A-3), U- =[R1/(R1+R2)] U0&l

18、t;/p><p>　　Since we now have expressions for U+ and U-, Eq. (1-2A-l) may be used to calculate the output voltage,</p><p>　　U0 = A（U+-U-）=A[US-R1U0/(R1+R2)]</p><p>　　Gathering terms,<

19、;/p><p>　　U0 =[1+AR1/(R1+R2)]= AUS (1-2A-4)</p><p>　　and finally,</p><p>　　AU = U0/US= A(R1+R2)/( R1+R2+AR1) (1-2A-5a)</p><p>　　This is

20、the gain factor for the circuit. If A is a very large number, large enough that AR~ >> (R1+R2),the denominator of this fraction is dominated by the AR~ term. The factor A, which is in both the numerator and denomin

21、ator, then cancels out and the gain is given by the expression</p><p>　　AU =(R1+R2)/ R1 (1-2A-5b)</p><p>　　This shows that if A is very large, then the gain of the ci

22、rcuit is independent of the exact value of A and can be controlled by the choice of R1and R2. This is one of the key features of Op-Amp design the action of the circuit on signals depends only upon the external elements

23、which can be</p><p>　　easily varied by the designer and which do not depend upon the detailed character of the Op-Amp itself. Note that if A=100 000 and (R1 +R2)/R1=10, the price we have paid for this advant

24、age is that we have used a device with a voltage gain of 100 000 to produce an amplifier with a gain of 10. In some sense, by using an Op-Amp we trade off "power" for "control."</p><p>　　

25、A similar mathematical analysis can be made on any Op-Amp circuit, but this is cumbersome and there are some very useful shortcuts that involve application of the two laws of Op-Amps which we now present.</p><

26、p>　　1) The first law states that in normal Op-Amp circuits we may assume that the voltage difference between the input terminals is zero, that is,</p><p><b>　　U+ =U-</b></p><p>　　

27、2) The second law states that in normal Op-Amp circuits both of the input currents may be assumed to be zero:</p><p>　　I+ =I- =0 </p><p>　　The first law is due to the large value of the intrinsi

28、c gain A. For example, if the output of an Op- Amp is IV and A= 100 000, then ( U+ - U- )= 10-SV. This is such a small number that it can often be ignored, and we set U+ = U-. The second law comes from the construction o

29、f the circuitry inside the Op-Amp which is such that almost no current flows into either of the two inputs.</p><p>　　B: Transistors</p><p>　　Put very simply a semiconductor material is one whi

30、ch can be 'doped' to produce a predominance of electrons or mobile negative charges (N-type); or 'holes' or positive charges (P- type). A single crystal of germanium or silicon treated with both N-type do

31、pe and P-type dope forms a semiconductor diode, with the working characteristics described. Transistors are formed in a similar way but like two diodes back-to-back with a common middle layer doped in the opposite way to

32、 the two end layers, thus</p><p>　　Two configurations are obviously possible, PNP or NPN (Fig. 1-2B-l). These descriptions are used to describe the two basic types of transistors. Because a transistor contai

33、ns elements with two different polarities (i.e., 'P' and 'N' zones), it is referred to as a bipolar device, or bipolar transistor.</p><p>　　A transistor thus has three elements with three lea

34、ds connecting to these elements. To operate in a working circuit it is connected with two external voltage or polarities. One external voltage is working effectively as a diode. A transistor will, in fact, work as a diod

35、e by using just this connection and forgetting about the top half. An example is the substitution of a transistor for a diode as the detector in a simple radio. It will work just as well as a diode as it is working as a

36、diode in </p><p>　　The diode circuit can be given forward or reverse bias. Connected with forward bias, as in Fig.l-2B-2, drawn for a PNP transistor, current will flow from P to the bottom N. If a second vol

37、tage is applied to the top and bottom sections of the transistor, with the same polarity applied to the bottom, the electrons already flowing through the bottom N section will promote a </p><p>　　flow of cur

38、rent through the transistor bottom-to-top.</p><p>　　By controlling the degree of doping in the different layers of the transistor during manufacture, this ability to conduct current through the second circui

39、t through a resistor can be very marked. Effectively, when the bottom half is forward biased, the bottom section acts as a generous source of free electrons (and because it emits electrons it is called the emitter). Thes

40、e are collected readily by the top half, which is consequently called the collector, but the actual amount of current which f</p><p>　　Effectively, therefore, there are two separate 'working' circuit

41、s when a transistor is working with correctly connected polarities (Fig. 1-2B-3). One is the loop formed by the bias voltage supply encompassing the emitter and base. This is called the base circuit or input circuit. The

42、 second is the circuit formed by the collector voltage supply and all three elements of the transistor. This is called the collector circuit or output circuit. (Note: this description applies only when the emitter co<

43、/p><p>　　The particular advantage offered by this circuit is that a relatively small base current can control and instigate a very much larger collector current (or, more correctly, a small input power is capab

44、le of producing a much larger output power). In other words, the transistor works as an amplifier.</p><p>　　With this mode of working the base-emitter circuit is the input side; and the emitter through base

45、to collector circuit the output side. Although these have a common path through base and emitter, the two circuits are effectively separated by the fact that as far as polarity of the base circuit is concerned, the base

46、and upper half of the transistor are connected as a reverse biased diode. Hence there is no current flow from the base circuit into the collector circuit.</p><p>　　For the circuit to work, of course, polarit

47、ies of both the base and collector circuits have to be correct (forward bias applied to the base circuit, and the collector supply connected so that the polarity of the common element (the emitter) is the same from both

48、voltage sources). This also means that the polarity of the voltages must be correct for the type of transistor. In the case of a PNP transistor as described, the emitter voltage must be positive. It follows that both the

49、 base and collect</p><p>　　In the case of an NPN transistor, exactly the same working principles apply but the polarities of both supplies are reversed (Fig. 1-2B-4). That is to say, the emitter is always ma

50、de negative relative to base and collector ('N' for negative in the case</p><p>　　of an NPN transistor). This is also inferred by the reverse direction of the arrow on the emitter in the symbol for a

51、n NPN transistor, i.e., current flow away from the base. </p><p>　　While transistors are made in thousands of different types, the number of shapes in which they are produced is more limited and more or less

52、 standardized in a simple code -- TO (Transistor Outline) followed by a number.</p><p>　　TO1 is the original transistor shape a cylindrical 'can' with the three leads emerging in triangular pattern f

53、rom the bottom. Looking at the base, the upper lead in the 'triangle' is the base, the one to the fight (marked by a color spot) the collector and the one to the left the emitter.[2] The collector lead may also b

54、e more widely spaced from the base lead than the emitter lead.</p><p>　　In other TO shapes the three leads may emerge in similar triangular pattern (but not necessarily with the same positions for base, coll

55、ector and emitter), or in-line. Just to confuse the issue there are also sub-types of the same TO number shape with different lead designations. The TO92, for example, has three leads emerging in line parallel to a flat

56、side on an otherwise circular</p><p>　　'can' reading 1,2,3 from top to bottom with the flat side to the right looking at the base.</p><p>　　With TO92 sub-type a (TO92a): 1=emitter</p

57、><p>　　2=collector</p><p><b>　　3=base</b></p><p>　　With TO92 sub-type b (TO92b): 1=emitter</p><p><b>　　2=base</b></p><p>　　3=collector

58、</p><p>　　To complicate things further, some transistors may have only two emerging leads (the third being connected to the case internally); and some transistor outline shapes are found with more than three

59、 leads emerging from the base. These, in fact, are integrated circuits (ICs), packaged in the same outline shape as a transistor. More complex ICs are packaged in quite different form, e.g., flat packages.</p><

60、;p>　　Power transistors are easily identified by shape~ They are metal cased with an elongated bottom with two mounting holes. There will only be two leads (the emitter and base) and these will normally be marked. The

61、collector is connected internally to the can, and so connection to the collector is via one of the mounting bolts or bottom of the can.</p><p><b>　　A 運算放大器</b></p><p>　　對應(yīng)于像廣義放大

62、器這樣的電子裝置，存在的一個問題就是它們的增益AU或AI,它們?nèi)Q于雙端口系統(tǒng)(µ、β、Ri、R0等)的內(nèi)部特性。器件之間參數(shù)的分散性和溫度漂移給設(shè)計工作增加了難度。設(shè)計運算放大器或Op-Amp的目的就是使它盡可能的減少對其內(nèi)部參數(shù)的依賴性、最大程度地簡化設(shè)計工作。運算放大器是一個集成電路，在它內(nèi)部有許多電阻、晶體管等元件。就此而言，我們不再描述這些元件的內(nèi)部工作原理。</p><p>　　運算放大器的

63、全面綜合分析超越了某些教科書的范圍。在這里我們將詳細研究一個例子，然后給出兩個運算放大器定律并說明在許多實用電路中怎樣使用這兩個定律來進行分析。這兩個定律可允許一個人在沒有詳細了解運算放大器物理特性的情況下設(shè)計各種電路。因此，運算放大器對于在不同技術(shù)領(lǐng)域中需要使用簡單放大器而不是在晶體管級做設(shè)計的研究人員來說是非常有用的。在電路和電子學教科書中，也說明了如何用運算放大器建立簡單的濾波電路。作為構(gòu)建運算放大器集成電路的積木—晶體管，將在下

64、篇課文中進行討論。</p><p>　　理想運算放大器的符號如圖1-2A-1所示。圖中只給出三個管腳：正輸入、負輸入和輸出。讓運算放大器正常運行所必需的其它一些管腳，諸如電源管腳、接零管腳等并未畫出。在實際電路中使用運算放大器時，后者是必要的，但在本文中討論理想的運算放大器的應(yīng)用時則不必考慮后者。兩個輸入電壓和輸出電壓用符號U+、U-和U0 表示。每一個電壓均指的是相對于接零管腳的電位。運算放大器是差分裝置。差分

65、的意思是：相對于接零管腳的輸出電壓可由下式表示</p><p>　　U0=A（U+-U-） (1-2A-1)</p><p>　　式中 A 是運算放大器的增益，U+ 和U-是輸入電壓。換句話說，輸出電壓是A乘以兩輸入間的電位差。 </p><p>　　集成電路技術(shù)使得在非常小的一塊半導體材料的復(fù)合 “芯片”上

66、可以安裝許多放大器電路。運算放大器成功的一個關(guān)鍵就是許多晶體管放大器“串聯(lián)”以產(chǎn)生非常大的整體增益。也就是說，等式(1-2A-1)中的數(shù)A約為100,000或更多 (例如，五個晶體管放大器串聯(lián)，每一個的增益為10，那么將會得到此數(shù)值的A)。 第二個重要因素是這些電路是按照流入每一個輸入的電流都很小這樣的原則來設(shè)計制作的。第三個重要的設(shè)計特點就是運算放大器的輸出阻抗(R0)非常小。也就是說運算放大器的輸出是一個理想的電壓源。</p&

67、gt;<p>　　我們現(xiàn)在利用這些特性就可以分析圖1-2A-2所示的特殊放大器電路了。首先，注意到在正極輸入的電壓U +等于電源電壓，即U+ =US。各個電流定義如圖1-2A-2中的b圖所示。對圖 1-2A-2b的外回路應(yīng)用基爾霍夫定律，注意輸出電壓U0 指的是它與接零管腳之間的電位，我們就可得到</p><p>　　-I1R1-I2R2+U0=0

68、 (1-2A-2)</p><p>　　因為運算放大器是按照沒有電流流入正輸入端和負輸入端的原則制作的，即I- =0。那么對負輸入端利用基爾霍夫定律可得I1 = I2，</p><p>　　利用等式(1-2A-2) ，并設(shè) I1 =I2 =I， </p><p>　　U0 = (R1 +R2) I

69、 (1-2A-3)</p><p>　　根據(jù)電流參考方向和接零管腳電位為零伏特的事實，利用歐姆定律，可得負極輸入電壓U-： (U--0)/ R1=I</p><p>　　因此 U-=IR1 ，并由式 (1-2A-3)可得： U- =[R1/(R1+R2)] U0 </p><p>　　因為現(xiàn)在

70、已有了U+ 和U-的表達式，所以式(1-2A-1)可用于計算輸出電壓，</p><p>　　U0 = A（U+-U-）=A[US-R1U0/(R1+R2)]</p><p>　　綜合上述等式，可得： U0 =[1+AR1/(R1+R2)]= AUS (1-2A-4) </p><p>　　最后可得：

71、 AU = U0/US= A(R1+R2)/( R1+R2+AR1) (1-2A-5a)</p><p>　　這是電路的增益系數(shù)。如果A 是一個非常大的數(shù)，大到足夠使AR1 >> (R1 +R2)，那么分式的分母主要由AR1 項決定，存在于分子和分母的系數(shù)A 就可對消，增益可用下式表示這表明， AU =(R1

72、+R2)/ R1 (1-2A-5b)</p><p>　　如果A 非常大，那么電路的增益與A 的精確值無關(guān)并能夠通過R1和R2的選擇來控制。這是運算放大器設(shè)計的重要特征之----在信號作用下，電路的動作僅取決于能夠容易被設(shè)計者改變的外部元件，而不取決于運算放大器本身的細節(jié)特性。注意，如果A=100,000， 而(R1 +R2) /R1 =10，那么為此優(yōu)點而付出的代價是

73、用一個具有100,000倍電壓增益的器件產(chǎn)生一個具有10倍增益的放大器。從某種意義上說，使用運算放大器是以“能量”為代價來換取“控制”。</p><p>　　對各種運算放大器電路都可作類似的數(shù)學分析，但是這比較麻煩，并且存在一些非常有用的捷徑，其涉及目前我們提出的運算放大器兩個定律應(yīng)用。</p><p>　　1) 第一個定律指出：在一般運算放大器電路中，可以假設(shè)輸入 端間的電壓為零，也就是

74、說，U+ =U-</p><p>　　2) 第二個定律指出：在一般運算放大器電路中，兩個輸入電流可被假定為零： I+ =I- =0 </p><p>　　第一個定律是因為內(nèi)在增益A的值很大。例，如果運算放大器的輸出是1V，并且A=100,000, 那么(U+ = U-)=10-5 V這是一個非常小、可以忽略的數(shù)，因此可設(shè)U+ = U-。第二個定律來自于運算放大器的內(nèi)部電路結(jié)構(gòu)，

75、此結(jié)構(gòu)使得基本上沒有電流流入任何一個輸入端。</p><p><b>　　B 晶體管   </b></p><p>　　簡單地說，半導體是這樣一種物質(zhì)，它能夠通過“摻雜”來產(chǎn)生多余的電子，又稱自由電子（N型）；或者產(chǎn)生“空穴”，又稱正電荷（P型）。由N型摻雜和P型摻雜處理的鍺或硅的單晶體可形成半導體二極管，它具有我們描述過的工作特性。晶體

76、管以類似的方式形成，就象帶有公共中間層、背靠背的兩個二極管，公共中間層是以對等的方式向兩個邊緣層滲入而得，因此中間層比兩個邊緣層或邊緣區(qū)要薄的多。</p><p>　　PNP 或 NPN (圖 1-2B-1)這兩種結(jié)構(gòu)顯然是可能的。PNP或NPN被用于描述晶體管的兩個基本類型。因為晶體管包含兩個不同極性的區(qū)域（例如“P”區(qū)和“N”區(qū)），所以晶體管被叫作雙向器件，或雙向晶體管。</p><p&g

77、t;　　一個晶體管有三個區(qū)域，并從這三個區(qū)域引出三個管腳。要使工作電路運行，晶體管需與兩個外部電壓或極性連接。其中一個外部電壓工作方式類似于二極管。事實上，保留這個外部電壓并去掉上半部分，晶體管將會象二極管一樣工作。例如在簡易收音機中用晶體管代替二極管作為檢波器。在這種情況下，其所起的作用和二極管所起的作用一模一樣。 </p><p>　　可以給二極管電路加正向偏置電壓或反向偏置電壓。在加正向偏置電壓的情況

78、下，如圖1-2B-2所示的PNP 晶體管，電流從底部的P極流到中間的N極。如果第二個電壓被加到晶體管的頂部和底部兩個極之間，并且底部電壓極性相同，那么，流過中間層N區(qū)的電子將激發(fā)出從晶體管底部到頂部流過的電流。</p><p>　　在生產(chǎn)晶體管的過程中，通過控制不同層的摻雜度，經(jīng)過負載電阻流過第二個電路電流的導電能力非常顯著。實際上，當晶體管下半部為正向偏置時，底部的P區(qū)就像一個取之不竭的自由電子源（因為底部的P

79、區(qū)發(fā)射電子，所以它被稱為發(fā)射極）。這些電子被頂部P區(qū)接收，因此它被稱為集電極，但是流過這個特定電路實際電流的大小由加到中間層的偏置電壓控制，所以中間層被稱為基極。</p><p>　　因此，當晶體管外加電壓接連正確（圖1-2B-3）后工作時，實際上存在兩個獨立的“工作”電路。一個是由偏置電壓源、發(fā)射極和基極形成的回路，它被稱為基極電路或輸入電路；第二個是由集電極電壓源和晶體管的三個區(qū)共同形成的電路，它被稱為集電極

80、電路或輸出電路。（注意：本定義僅適用于發(fā)射極是兩個電路的公共端時----被稱為共發(fā)射極連接。）這是晶體管最常見的連接方式，但是，當然也存在其它兩種連接方法----共基極連接和共集電極連接。但是，在每一種情況下晶體管的工作原理是相同的。</p><p>　　本電路的突出優(yōu)點是相對小的基極電流能控制和激發(fā)出一個比它大得多的集電極電流(或更恰當?shù)卣f，一個小的輸入功率能夠產(chǎn)生一個比它大得多的輸出功率)。換句話說，晶體管的

81、作用相當于一個放大器。</p><p>　　在這種工作方式中，基極-發(fā)射極電路是輸入側(cè)；通過基極的發(fā)射極和集電極電路是輸出側(cè)。雖然基極和發(fā)射極是公共路徑，但這兩個電路實際上是獨立的，就基極電路的極性而言，基極和晶體管的集電極之間相當于一個反向偏置二極管，因此沒有電流從基極電路流到集電極電路。</p><p>　　要讓電路正常工作，當然，加在基極電路和集電極電路的電壓極性必須正確（基極電路加

82、正向偏置電壓，集電極電源的連接要保證公共端（發(fā)射極）的極性與兩個電壓源的極性相同）。這也就是說電壓極性必須和晶體管的類型相匹配。在上述的PNP型晶體管中，發(fā)射極電壓必須為正。 因此，基極和集電極相對于發(fā)射極的極性為負。PNP 型晶體管的符號在發(fā)射極上有一個指示電流方向的箭頭，總是指向基極。（在PNP型晶體管中，“P”代表正）。</p><p>　　在NPN型晶體管中，工作原理完全相同，但是兩個電源的極性正好相反（

83、圖1-2B-4）。也就是說，發(fā)射極相對于基極和集電極來說極性總是負的（在NPN型晶體管中，“N”代表負）。這一點也可以從NPN型晶體管符號中發(fā)射極上相反方向的箭頭看出來，即，電流從基極流出。 </p><p>　　盡管現(xiàn)在生產(chǎn)的晶體管有上千種不同的型號，但晶體管各種外殼形狀的數(shù)量相對有限，并盡量用一種簡單碼----TO（晶體管外形）后跟一個數(shù)字為統(tǒng)一標準。</p><p>　　TO1是一種

84、最早的晶體管外殼----即一個在底部帶有三個引腳的圓柱體“外罩”，這三個引腳在底部形成三角狀。觀看底部時，“三角形”上面的管腳是基極，其右面的管腳（由一個彩色點標出）為集電極，其左面的管腳為發(fā)射極。集電極引腳到基集引腳的間距也許比發(fā)射極到基集引腳的間距要大 。</p><p>　　在其它TO外殼中，三個引腳可能有類似的三角形形狀（但是基極、集電極和發(fā)射極的位置不一定相同），或三個引腳排成一條直線。使人容易搞亂的問

85、題是同一TO號碼的子系列產(chǎn)品其管腳位置是不一樣的 。例如，TO92 的三個管腳排成一條直線，這條直線與半圓型“外罩”的切面平行，觀看TO92的底部時，將切面沖右，從上往下讀，管腳的排序為1，2，3。</p><p>　　對于TO92子系列 a (TO92a): 1=發(fā)射極</p><p><b>　　2=集電極 </b></p><p&g

86、t;<b>　　3=基極</b></p><p>　　對于TO92子系列 b (TO92b): 1=發(fā)射極</p><p><b>　　2=基極</b></p><p><b>　　3=集電極</b></p><p>　　更容易使人搞亂的是一些晶體管只有兩個管腳（第三個管腳已

87、在里邊和外殼連接）；一些和晶體管的外形很像的外殼底部有三個以上的管腳。實際上，這些都是集成電路(ICs)，用和晶體管相同的外殼包裝的，只是看起來像晶體管。更復(fù)雜的集成電路（ICs）用不同形狀的外殼包裝，例如平面包裝。</p><p>　　根據(jù)外殼形狀非常容易識別功率晶體管。它們是金屬外殼，帶有延長的底部平面，底部平面上還有兩個安裝孔。功率晶體管只有兩個管腳（發(fā)射極和基極），通常會標明。集電極在內(nèi)部被連接到外殼上，