03 chapter3 Electronic Circuit Design.pptx

第1页：第03章电子电路设计封面

电子电路设计：章节封面，标示本章主题与内容范围。

第2页：3.1 设计流程

• Once a clear problem statement is completed, a team of design ers must develop a plan of action. They usually start byenume rating the goals of the project in more detail, perhaps including prior ities so that any trade-offs that must be madelater can be proper ly evaluated (e. g., how important is cost relative to range on one charge). The next step is to identify possible approach es to achieving the stated goals. Often, this step is facilitated by a brainstorming 2 session
• After the poss ibilities have been listed, each is critical lyre vie wed to see if there are any obvious reasons for dis miss ing it from further consideration. The result ing—usually shorter—list is then subject ed to even closer scrutiny. At this point 3, the design will usually need to be broken upinto different subsystems (e. g., motors, body), and subgroup sof the design ers will repeat the process for each particular subsystem. The subsystems may then be broken up further

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第3页：3.1 设计流程：Fourth

• At each step there is a four-step process to be follow ed
• (1) First, the goals of the design are enume rated and, perhaps, prior ities are established
• (2) Second, a list is gene rated contain ing as many waysof achieving the stated goals as possible
• (3) Third, this initial list of alter nat ives is subject ed to critical review to select choices that appear to be feasible
• (4) Fourth, the remain ing options are subject ed to longe rand more thoughtful analysis; note that the design maybe partition ed into smaller subsystems at this point

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第4页：3.1 设计流程

The process may be repeated many times, with the sub gro ups coming back together and modify ing the partition ing or dealing with trade-offs in the different subassemblies (e. g., if the body is made more aero dynamic ally efficient, the motor doesn't need to beas powerful). Eventual ly, one or two approach es are sett led on, and work begins in earnest. However, by this time, each of the groups has done a substantial amount of design, since to a large extent is an exploration of alter nat ives, follow ed by analysis, follow ed by choosingthe alternative that seems best suited to the task athand 4. Sometimes the process of design includes what most people think of when you mention design, namely, synthesis

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第5页：3.1 设计流程

Synthesis is the process of creating something new, atleast to the person who creates it. Although one cancer tai nly get better at synthesis through practice and by critic ally exam in ing the process used, there isn' t much that can be done to directly teach you how to synthesize entire ly new electronic circuits. In general, the process is similar to design in that alter nat ives are listed and then subject ed to different levels of analysis. However, the difference is in the listing of alter nat ives. Rather than justenume rating all the known ways of perform ing a function, you are interested in figuring out how to perform af unction in an entire ly new way, either because no waycurrently exists or because all exist ing techniques are forsome reason in adequate for the application

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第6页：3.1 设计流程：Therefore

• Although synthesis is less systematic than design and arguably not something that can be taught, much of what you learn about the process of design will help you with synthesis as well 5. Tobe good at synthesis, you must first be familiar with as many different circuits and functions as possible. You must also learnto think about circuits at several different levels of abstraction sothat you can see how to modify circuits to improve them or alter their performance. The study of circuit design
• As a final note, synthesis is more likely to consume a large amount of time to complete and is also riskier than using known methods in that your great new idea may never work proper ly!Therefore, a good de signer will care fully re search previous efforts at solving the same and similar problems to discover asmany approach es as possible. From a business perspective, it usu ally isn't a good idea to repeat the work of others

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第7页：生词与短语：electric

本页为生词与短语（New words and phrases）列表，收录本节出现的专业词汇。

词条按原文「中文释义—音标—英文词条」的顺序识别，如与原件排版不符以课件原图为准。

第8页：课文注释（3.1 设计流程）

本页为课文注释（Notes），讲解本节难句的句式与翻译处理。

• Notes
• 1.本节涉及电子电路设计中的设计流程部分
• 2. brainstorming头脑风暴法。头脑风暴法是指将所有可能解决问题的方案都列出来，而不考虑这些方案是否切实可行
• 3. at this point此时。此句可翻译为：此时，通常需要将设计工作分割成若干个不同的子系统(如发动机、车身等)的设计，各小组的设计师针对每个子系统重复该方案审核工作
• 4.句中的since引导原因状语从句，省略了从句的主语design。此句可翻译为：然而至此为止，各个小组已经做了大量的设计工作，因为设计在很大程度上就是对各种选择方案的探索，然后进行分析，最终选定一套适合于相应任务的最佳方案
• 5.句中although引导让步状语从句，而主句中的主语由what引导的主语从句充当。此句可翻译为：虽然综合的系统性不如设计那么强，而且似乎没有什么东西可以拿来直接使用，但是所学的有关设计步骤的许多内容同样有助于综合的学习

第9页：3.2 设计分析：Analysis

• Figure 3.1 An amplifier circuit
• 3.2 Analysis for Design 1
• How does the method of analysis you are familiar with differ from the type of analysis that is used in the design process? A examples will help to explain the difference. We will analyze circuits contain ing only resist ors andsources
• First, consider the circuit shown in Figure 3.1; it is typical of the kinds of circuits. The circuit uses a model of the in put source and a controlled, or dependent, source

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第10页：3.2 设计分析：Kirchhoff

• The circuit in Figure 3.1 can be analyzed using either loop or nodal analysis. We choose nodal analysis andnote that there are only two independent nodes in the circuit: the input and the output . Our choice of us ing nodal analysis is not as arbitrary as it might seem, although for this circuit it turns out to not make a very big difference
• Using Kirchhoff's current law (KCL)2 at in Figure 3.1 leads to(3-1)
• Writing KCL for，results in(3-2)

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第11页：3.2 设计分析

• Solving eq. (3-1) for yields(3-3)
• and plugging this result into eq. (3-2) produces(3-4)
• from which the final transfer function is obtain ed:(3-5)

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第12页：3.2 设计分析

• We have deli be rat ely included most of the steps in the derivation of eq. (3-5) to illustrate that this brute-force approach to analyzing the circuit, while correct, tends tofocus concentration on the algebra, rather than 3 the circuit. An alternate analysis technique—and the one thatall experienced design ers would use—is to break the problem into a sequence of smaller problems and use afew well-known results to write the answer by inspection
• (3-6)

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第13页：3.2 设计分析

• The overall transfer function of a complex network can always be broken down as shown in eq. (3-6), but doingso is useful only if the individual transfer functions can be written by inspection, or at least with significant ly less work 4. For example, if the individual net works are all unilateral, their transfer functions can usually be deriv edwith minimal effort. A unilateral network is a network inwhich signals flow in only one direction. In other words, forthis example
• (3-7)
• or
• (3-8)

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第14页：3.2 设计分析

• Exam in ing the input side, we get
• (3-9)
• or
• (3-10)
• Substitu ting these two transfer functions back into eq. (3-6), or substitu ting eq. (3-9) into eq. (3-7) directly, leads to eq.(3-5) again, but with less effort and more import antly, a clear realization on our part of where each term in the equation comes from
• We have chosen a very simple example to begin with, sothe benefits may not be obvious at this stage. However, inmore complicated circuits, the difference in the difficulty ofsolving simultaneous equations versus writing a sequence of simpler transfer functions can be quite large. We want to introduce a notation that will be used throughout this textand that helps to cultivate the proper style of analysis

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第15页：3.2 设计分析：Additive

• In analyzing a circuit like this, it is often convenient to define an
• (3-11)
• and an
• (3-12)
• These attenuation factors will be different for different circuits, butthe concept is always the same and leads to simplified analyses
• In designing amplifiers and many other circuits, we often desire tomake the attenuation factors as close to one as possible. If the in put attenuation factor is too small, the signal is reduced prior toany amplification or process ing. Additive noise, which is always present, can then be a significant problem. On the other hand, ifthe output attenuation is too small, it means we are wasting someof our signal in driving the load

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第16页：生词与短语：resistor

本页为生词与短语（New words and phrases）列表，收录本节出现的专业词汇。

词条按原文「中文释义—音标—英文词条」的顺序识别，如与原件排版不符以课件原图为准。

第17页：课文注释（3.2 设计分析）

本页为课文注释（Notes），讲解本节难句的句式与翻译处理。

• Notes
• 1.本节涉及电子电路设计中的电路分析部分
• 2. Kirchhoff's current law (KCL)基尔霍夫电流定律。定义：对于任意一个集中参数电路中的任意一个节点或闭合面，在任何时刻，通过该节点或闭合面的所有支路电流代数和等于零
• 3. rather than而不是。此句可翻译为：以上详细介绍了式(3-5)的大部分推导步骤，目的是为了说明这种粗略的分析方法虽然正确，但是更加注重代数推导而不是电路本身
• 4. only if只有……才。此句可翻译为：一个复杂网络的传输函数总是能分解成式(3-6)所示的形式，但是只有当每个传输函数可根据观察写出或只需做较少工作就可写出时这种方法才有用
• 5. On the other hand另一方面。此句可翻译为：从另一方面来说，如果输出衰减因子太小，那就意味着在驱动负载时损失了一部分信号，因此必须提高系统增益以弥补损失的信号

第18页：3.3 Electronic

• 3.3 Electronic Systems 1
• Having present ed a basic idea of what cons titutes design and of how analysis for the sake of design differs from analysis for the sake of deter mining performance, wenext discuss electronic systems. There are three reasons for this discussion: First, it will provide motivation for the material that follows; second

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第19页：3.3.1 Electronic

• 3.3.1 Electronic versus Electric Circuits
• Strict ly speak ing, an electric circuit is a complete loop in which electro ns can flow and that includes circuits contain ing trans istors. Nevertheless, most people (but not all) would distinguish between an electric circuit and an electronic circuit—hence the title of this text. An electric circuit is a circuit that contains resist ors, capacitors, inductors, transform ers, switch esand the like, but no trans istors, integ rated circuits, or tubes (yes

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第20页：3.3 电子系统

• Figure 3.2 Mechanical analogy of passive and active devices
• A mechanical analogy may be useful. If you want to lift aheavy box, you can use a lever, as shown in Figure 3.2(a), or you can use a forklift, as shown in part (b) ofthe figure. The lever is a passive device: You are directly supp lying the required energy. The lever simply lets you exert a small force over a large distance to produce alarger force over a shorter distance. On the other hand, the forklift is an active device: You direct its motion

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第21页：3.3 电子系统

An electric analog to the lever is a transformer, as shown in Figure 3.3(a): The transformer can boost the voltage supp lied to a given load (analogous to force in Figure 3.2), but the current supp lied to the load is proportion at ely lower than the current drawn from the source (analogous to distance in Figure 3.2). The electric analog of the forklift isthe transistor circuit shown in Figure 3.3(b): It uses an in put voltage to control an output voltage, but the input source does not need to supply the power to the output. The circuit includes the input source, the transistor, the load, and as epa rate supply of energy—a battery in this case. The battery is analogous to the motor in the forklift. The transistor is a device that has a current flowing from the output to ground that is controlled by the voltage appear ing from the input to ground 2. The voltage across the load isequal to the current times the load resistance, and this voltage can be much larger than the source voltage

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第22页：图3.3 Electrical

图3.3 Electrical

本页为图示页，图中给出3.3 电子系统的结构或流程，具体图形见课件原图。

第23页：3.3 电子系统

Unlike the transformer, however, the output current fromthe transistor can also be much larger than the in put current, which is ideally zero. At this point an important difference between active and passive circuits can be pointed out. Since mechanical power is force time sdi stance per unit time (integ rated if the force is not constant), the power delivered to the load in Figure 3.2(a) is no more than the power obtain ed from the source (i. e., the person pushing on the lever). In reality, the output power is lower because of friction losses and other effects. But in Figure 3.2 (b), the power delivered to theload can be significant ly larger than that provided by the source (i. e., the driver), which is negligible in this case, since all the "source" has to do is push the lever. The power in Figure 3.3 (b) is supp lied by the motor in the forklift

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第24页：3.3 电子系统：Energy

• In the electrical case, power is equal to voltage times current. Forthe ideal transformer in Figure 3.3 (a), the power out is exact lyequal to the power in. However, the ideal transformer can only beapproximated in practice. There will always be some loss so thatthe power out will actual ly be lower than the power in and the power gain of the system is strict ly less than one. ALL passive circuits have power gains less than one
• For the transistor circuit shown in Figure 3.3 (b), however, the signal power out can be much larger than the power delivered bythe signal source, so the power gain for the signal can be greater than one. Energy is not produced in the circuit, of course; it sim ply uses the energy provided by the power supply (the batter yin Figure 3.3) to drive the load. In other words, the transistor usesthe input signal as a template to produce an output that looks likethe input

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第25页：3.3.2 Analog and

• 3.3.2 Analog and Digital Electronic Circuits
• To gain a better understanding of electronic systems, let's briefly examine two common electronic systems: an FM receiver and acomputer. Before we go into any detail, however, we need to understand a fundamental difference between these two systems:The FM receiver is an analog system (although modern receiver scertainly contain digital compo nents as well, the transmitted signal is analog), whereas the computer is a digital system (com put ers do contain analog circuits as well
• The input to an FM receiver is a frequency-modulated (FM) signal. That is, the frequency of a high-frequency carrier is changed in response to the information to be transmitted (the music in this case). There are two main reasons for not transmitting the audio signal itself:First, there wouldn't be any way to separate one station from another, and, second, the an tenn as would need to be huge. An tenn as need tobe a reason able fraction of a wavelength to work well and, as an example

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第26页：3.3 电子系统

• To have practical systems, therefore, we use high-frequency carrie rs, common ly called radio-frequency (RF) waves. To transmit the signal, we modulate (i. e., change) some parameter of the carrier wave (e. g., amplitude or frequency) with the signal. The receiver must the nde modulate the carrier to recover the original signal
• In the case of a stereo receiver, the situation is complicated further by the desire to send both left-and right-channel signals and to have old-fashion ed monaural receive rs still work (not important now, but itwas when FM was first intro duced)

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第27页：3.3 电子系统：Therefore

Therefore, if you simply de modulate an FM signal, youget the left-and the right-channel signals combined, denoted (L+R); this is what a monaural receiver uses. In addition, there is a separate sub carrier modulating the RF wave that contains the difference between the left-and right-channel signals (L-R). This sub carrier is ignore dby monaural receive rs but is demo dulated by stereo receive rs. By adding and subtract ing the twodemodulated signals, the original stereo signal can berecovered; that is, (L+R)+(L-R)=2 Land (L+R)-(L-R)=2 R. We will ignore this complication and show only amon aural receiver in the discussion that follows

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第28页：3.3 电子系统

• A block diagram of a monaural FM receiver is shown in Figure 3.4. This block diagram shows a super heterodyne receiver, which simply means that the RF wave is first brought down to an intermediate frequency (IF), goes through some signal process ing there, and is then demo dulated or converted to the audio signal 3. Hetero dyn ing is the process of com bin ing two signals to produce a beat tone and is accomplished by multiply ing the signals
• Figure 3.4 Block diagram of a monaural receiver

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第29页：3.3 电子系统

• Re turning now to the block diagram, we note that the receiver first filters the RF signal coming from the antenna to choose theone station you want to listen to. The diagonal arrow through the RF filter block is a common way of showing that the block isin some way variable; in this case, you can tune the filter to select different stations. This signal is amplified and sent to amixer, which multi plies the signal by a sine wave that is gene rated locally by the local oscillator (LO)
• To understand the operation of the mixer, consider a single sinusoidal input. Multiply ing two sine waves produces sinewaves at the sum of the two frequencies and at the difference of the two frequencies. If the two frequencies are close, the difference will be at a much lower frequency where it is less expensive to perform further signal process ing. It is also possible to use an LO frequency equal to the nominal RF frequency

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第30页：10.7 MHz

In the case of broadcast-band FM, the IF is at 10.7 MHz, so to receive a station at 100.1 MHz, the LO is set to 110.8 MHz. The output of the mixer, which is the inter mediate frequency, is then sent to the IF filter and amplifier. Most of the filter ing is done in the IF section, since it is less expensive to implement good filters at that frequency. These filters often use cryst als. The RF filter is only a preselect filter. The LO frequency is changed tomix the desired signal frequency down to the IF frequency. The IF amplifier is often an automatic gain control (AGC) amplifier that changes its gain so that the output amplitude is held constant. The output of the IF section then goes into a de modulator that converts the frequency changes into voltage changes to produce the audio output 5

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第31页：3.3 电子系统

A block diagram for a basic computer is shown in Figure 3.5. The main element in the computer is the central process ing unit (CPU). When the computer is turned on, the CPU initial ly reads some basic instr uctions stored inthe nonvolatile read-only memory (ROM). Using these basic instr uctions, and perhaps automatically loading additional information from nonvolatile mass storage(e. g., disk drive), the computer is ready to run. Information that must be access ed quickly is stored in random-access memory (RAM), and information that isnot needed im mediately is stored on some slower mass storage device, most common ly a disk drive 6. Input fromthe user is accepted from a keyboard, a mouse, and perhaps other devices; output can be directed to the screen, a printer, or other peripheral devices

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第32页：图3.5 Block diagram

图3.5 Block diagram

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第33页：3.3 电子系统：CPU

• It is beyond the scope of the present discussion to gointo more detail. For example, there are many ways oforgani zing the CPU, and the computer can include many more devices than have been shown here and can beconstructed using many different circuit techno log ies. For our purposes, it will suffice at this point to say thatthe circuits in the computer are mostly digital, and the information is mani pula ted in binary words com posed ofsequences of binary digits
• The process of design re visited. We are now ready tore visit the topic of design with reference to the two specific example systems described above. The process of design proceeds at many different levels of abstraction as noted in Section 3.1

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第34页：3.3 电子系统

In the case of a communication system (i. e., transmitter and receiver), one must first decide what form of modulation to use—for example, AM or FM. The character istics of the modulation must also be determined. For example, how far will we allow the frequency of our carrier to be changed in the FM case? These decisions were made long ago for standard broadcast systems, but even if they are already determined, many choices remain forthe de signer of the receiver itself. To start with, we have shownonly one type of receiver; it is possible to use different approach es (e. g., the direct con version receiver mentioned briefly before). As sum ing our block diagram, however, there ares till plenty of choices to make. How much filter ing should be donein the RF section? How large must the AGC control range be?Do we even want to use an AGC amplifier? (The performance will be de graded with simpler schemes like amplitude limiting, butthey are cheaper.) Do we use a local oscillator that is analog andtuned by varying some component value

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第35页：3.3 电子系统：Similar

• Similar choices exist for the computer. How large a digital word should be used (e. g., 16 bit, 32 bit) for data? How large a digital word should be used for memory address ing? What kind of CPU architecture should be used? Should we use parallel process ors?Should there be a local cache memory? There are again many more choices that could be listed, even at the block diagram level
• All of these choices involve trade-offs between cost and performance, or one performance specification over another. Someof these trade-offs are quite complex, and many depend on thedetails of how each block is implemented. There are similar lists ofchoices and trade-offs to be evaluated in the design of each individual block as well; there are, for example, several good waysto make an AGC amplifier. That is why the design process bounce sback and forth between different levels of abstraction

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第36页：3.3 电子系统：Design

• It is important to realize that optimum choices are veryrarely identifiable. Design ers should do the best they canto consider the alter nat ives and make an informed choice, but sometimes the choice boils down to an arbitrary one. The important point is to make sure that where clear-cut differ ences exist, we understand themand make informed choices. Any arbitrary choices thatare made should not have an impact on the overall system performance or cost. (If they do
• Note that neither of the systems discussed is entire ly analog or entire ly digital. Therefore, it is important for acircuit de signer to know something about both kinds of design, even though most design ers will specialize inone or the other. It is also important for the circuit design er to understand some of the system iss ues involved in the design

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第37页：3.3 电子系统

Both of these systems are too complicated for any one person to know everything from the system level down through the transistor-level circuit design and on to the solid-state device design and fabrication. Nevertheless, for the overall design team to be able to go through thekind of process described, each member must know something about the disc ipl ines that are adjacent to their own. For the circuit de signer, this means understanding the system design to some degree, as well asunder standing solid-state device physics and fabrication to some degree. For the communication system engineer, it means understanding both the mathematics underlying the system analysis techniques and the transistor-level circuits to some degree. For the engineer involved with solid-state devices, it means understanding circuits aswell as the physics and fabrication

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第38页：3.3.3 Modeling

• 3.3.3 Modeling Electronic Systems
• In the preceding sections, the idea of modeling a system at many different levels of abstraction was discussed a great deal. As an example, a communication theorist might draw a single block labeled "receiver" while consider ing different models for the communication channel and how to choose the best possible modulation scheme for a given application. The receiver block could then be further broken down as shown in Figure 3.4. The AGC amplifier could then be shown as a block diagram

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第39页：3.3 电子系统：AGC

• One of the most difficult aspects of design is choosing the appropriate level of abstraction with which to examine agiven problem. If the communication theorist thinks about the motions of the electro ns in one transistor in the middle ofthe AGC amplifier, he or she won't get very far in figuring outwhat kind of modulation scheme to use! Unfortunate ly
• A similar problem exists in deciding which represent at ion touse at any given level of abstraction. For example, signal scan be represented in the time domain or the frequency domain, and circuits can be represented by schematics orby equations. No one represent at ion is always the best

本页为英文原文段落，中文译文见本章「参考译文」部分。

第40页：3.3 电子系统：Therefore

A common problem en countered in circuit design is what level of model to use for a given device. To analyze circuits contain ing trans istors, we must have a model forhow the transistor works. The problem is that there aremany models for the transistor. Using the most complete model available at all times is not appropriate, because itmakes the analysis so difficult that it hinders the process of design. Therefore, we choose the simple st model that will retain the necessary accuracy for the task at hand. However，you should be aware at the outset that there isn't just one model for any component. You must always ask yourself, When is it appropriate to use this model?

本页为英文原文段落，中文译文见本章「参考译文」部分。

第41页：3.3 电子系统

As an example of models for electronic compo nents, consider the amplifier circuit shown in Figure 3.3(b), which is repeated in Figure 3.6 (a). In the discussion regard ing that circuit, we simply said that the transistor"has a current flowing from output to ground that is controlled by the voltage appear ing from the input tog round". We can come up with a simple model for the transistor based on this statement; it is a voltage-controlled current source. If we assume that the output isa linear function of the input and call the constant of proportionality G, we arrive at the model shown in the schematic in Figure 3.6 (b)

本页为英文原文段落，中文译文见本章「参考译文」部分。

第42页：3.3 电子系统

• Using this model, we can analyze the circuit quant itatively, whereas before we examined it onlyqualitativ ely. From Figure 3.6 (b), we see that
• (3-13)
• so that the gain from to is
• (3-14)
• We have used the simple st possible model for the transistor in this analysis, and it shows the basic operation of the device as a voltage-controlled current source. Of course, the model is overly simplistic. For example, the output current of a real transistor is not alinear function of the input voltage, and the in put impedance is not infinite. Nevertheless, this simple model allows us to see that the circuit is an amplifier

本页为英文原文段落，中文译文见本章「参考译文」部分。

第43页：图3.6 simple

图3.6 simple

本页为图示页，图中给出3.3 电子系统的结构或流程，具体图形见课件原图。

第44页：3.3 电子系统：Virtu

Virtu ally everything we do in circuit design is based onmodels. There are no pure resist ors, capacitors, inductors, batter ies, or other compo nents available to us. Every real resistor has some inductance and capacitance. Every real capacitor has some inductance and resistance, and so on for every other component weuse. In many cases, we can satisfactorily model elements as being just a resistor or capacitor, but wemust always remember that there will be times when those models fail to accurate ly predict the system'sperformance (e. g., at very high frequencies). Sometime smore than one equivalent model is available. In suchcases, we choose either the model that makes the most sense in terms of how we think about the operation ofthe element or the one that yields the simple st analysis

本页为英文原文段落，中文译文见本章「参考译文」部分。

第45页：3.3 电子系统

There are many more models that could be shown forthe operational amplifier. Each one is useful for some analysis and not for others. The job of the circuit design er is to choose the appropriate model for the jobat hand. An important question to ask, whether you are astudent or an experienced de signer, is whether the model you are using is appropriate for the analysis youare doing. If the model is too complicated, your analysis may take a long time, yield results that are difficult or impossible to interpret, or be impossible. If the model istoo simple, your results are very likely to be wrong

本页为英文原文段落，中文译文见本章「参考译文」部分。

第46页：3.3 电子系统：Much

Figure 3.7 demon strates three different levels of abstraction foran amplifier design. In Figure 3.7(a), we show a block diagram ofsome source driving an amplifier, which then drives a load. In Figure 3.7(b), we use a Thévenin model for the source, a high-level circuit model for the amplifier, and a resistive model for theload. In Figure 3.7(c), we expand the amplifier one step further toshow the trans istors and individual resist ors used to construct thedevice. Much of our effort will focus on taking a schematic likethe one in Figure 3.7(c) and finding the model elements for the model shown in Figure 3.7(b). The advantage of modeling the circuit at a higher level of abstraction is that the analysis aresimpler and our understanding of the system is greater. Even when we don't draw the circuit shown in Figure 3.7(b), we have itin mind. For example, we know what the output resistance of our amplifier needs to be if we want to be able to drive a given load without having the output attenuation factor be too small. We canalso use more complex models for the source and load if that is necessary

本页为英文原文段落，中文译文见本章「参考译文」部分。

第47页：3.3 电子系统：Block

Figure 3.7 (a) Block diagram view of an amplifier, (b) a high-level circuit model for the amplifier, and (c) a transistor-level schematic for the amplifier

本页为英文原文段落，中文译文见本章「参考译文」部分。

第48页：生词与短语：capacitor

本页为生词与短语（New words and phrases）列表，收录本节出现的专业词汇。

词条按原文「中文释义—音标—英文词条」的顺序识别，如与原件排版不符以课件原图为准。

第49页：生词与短语：template、sub

本页为生词与短语（New words and phrases）列表，收录本节出现的专业词汇。

词条按原文「中文释义—音标—英文词条」的顺序识别，如与原件排版不符以课件原图为准。

第50页：课文注释（3.3 电子系统）

本页为课文注释（Notes），讲解本节难句的句式与翻译处理。

• Notes
• 1.本节对电子系统进行了讨论，包括电子与电动电路、模拟与数字电子电路和电子系统的模型化3个部分
• 2.句中第一个that引导定语从句修饰device，该定语从句中的current由第二个that引导的定语从句所修饰。此句可翻译为：晶体管有从输出端到地的输出电流，该电流由输入端到地的电压控制。负载上流过的电压等于电流乘以负载电阻，该电压远高于电源电压
• 3.句中which引导的非限定性定语从句修饰先行词superheterodyne receiver，翻译时采用分译法，将主句和从句译成两句。此句可翻译为：该方框图展示的是一个超外差式接收机，简单地说就是射频波先降至中频，其间经过一些信号处理，然后被解调或被转换成音频信号

第51页：课文注释（3.3 电子系统）

本页为课文注释（Notes），讲解本节难句的句式与翻译处理。

• 4.句中it为形式主语，真正的主语是动词不定式to use an LO frequency。此句可翻译为：也有可能选用频率与名义上的射频频率相等的本地振荡器，此时得到的接收机称为直接变换接收机或零差接收机
• 5. convert…into将……变换成，将……转换为；句中that引导定语从句修饰de modulator，翻译时可将主句和从句分译成两句。此句可翻译为：中频部分的输出结果进入解调器，由解调器将频率变化转换成电压变化来产生音频输出
• 6.此句是由并列连词and连接而成的并列句，翻译时可按顺序将此句译为两个句中。此句可翻译为：必须快速获取的信息存放在随机读取存储器(RAM)中，而不需要立即存储的信息存放在相对较慢的大容量存储器中，其中使用最普遍的是光驱

第52页：3.1 设计流程

• 参考译文
• 第3章电子电路设计
• 3.1设计流程
• 设计流程通常始于问题的描述。例如，假设欲设计一款新的电动车，要求同现有的产品比较这种电动车充一次电后能走得更远并且生产成本更低。但撰写一份清晰的设计需求说明书并不总是那么简单。有些时候必须在这一步骤中投入大量的工作。在没有被充分理解设计目标之前开始设计工作绝非明智之举
• 一份清晰的问题说明书完成之后，设计团队必须准备一份行动计划。通常第一步是详细列出项目的各个目标，可能包括各个目标的优先级，这样即可在以后必须要做的权衡中做出更合理的评估(例如，一次充电量的大小对成本来说有多重要)。下一步就是确认达到说明书中所列目标的可能方案，这一步通常借助于“头脑风暴法”完成

第53页：译文 3.1 设计流程：通常需要将设计

• 将可能方案列出之后，再对每种方案进行严格复审，检查是否有任何明显理由将其排除出进一步考虑的范围之列。对复审结果(通常会较短)的清单还要进行更详细的审核。此时，通常需要将设计工作分割成若干个不同的子系统(如发动机、车身等)的设计，各小组的设计师针对每个子系统重复该方案审核工作。而每个子系统还有可能再细分。每一步工作都可以分为以下4步
• (1)列出设计目标，而且可能还要建立优先级
• (2)列出一份包含尽可能多的能够完成所列目标的方案清单
• (3)对这份初始清单中的每个方案进行严格复审，筛选出可行的方案
• (4)对选择的方案进行更长、更周密的分析，注意此时设计工作可能被分成若干个小的子系统

第54页：译文 3.1 设计流程

• 这个过程可能会随着各个小组的整合以及分割方式的改变或者各种不同组合方式的权衡处理(例如，假如车身能更有效地借助空气动力学，那么发动机就不需要太强大)而重复多遍
• 最终会有一个或两个方案被选定，工作便可在一定的保证中展开
• 然而至此为止，各个小组已经做了大量的设计工作，因为设计在很大程度上就是对各种选择方案的探索，然后进行分析，最终选定一套适合于相应任务的最佳方案
• 很多时候当人们提到设计时大多数人都会想到设计过程中的一个步骤，那就是综合

第55页：译文 3.1 设计流程：一般来说这与设计

• 综合就是创造新事物的过程，至少对创造它的人来说是如此
• 虽然通过实践和严格测验，所用的设计步骤总能在综合中得到更好的结果，但是要直接教会一个人去综合一个全新的电子电路并没有什么办法
• 一般来说这与设计中将各种可能方案列举出来然后进行各种级别的分析是类似的
• 两者的区别在于各种可能方案的列举
• 人们感兴趣的不只是列出所有已知的能够实现某项功能的方法，而是找到一种实现某项功能的全新方法，这是因为目前根本不存在这样的方法或者由于某种原因利用现有的技术不能很好地完成这项任务

第56页：译文 3.1 设计流程：虽然综合的系统

• 虽然综合的系统性不如设计那么强，而且似乎没有什么东西可以拿来直接使用，但是所学的有关设计步骤的许多内容同样有助于综合的学习，要想学好综合，必须先尽可能多地熟悉电路及其功能，同时还必须学会以多种不同级别的抽象概念来考虑电路，这样才能提高或改变它们的性能。通过对本书中所讲述的有关电路设计知识的学习，应该能获得对于综合与设计同样有用的知识和技巧
• 最后要指出的是，综合很可能需要花费大量的时间来完成，而且比采用已知方法的风险性更大，因为新的设想可能根本不能实现！因此，一个好的设计师会仔细研究以前解决同样或类似问题所用的方法从而来发现尽可能多的解决方法。从商业的角度来说，重复别人的工作通常并不是一个好办法

第57页：3.2 设计分析

• 3.2设计分析
• 人们所熟悉的分析方法与在设计步骤中所用的分析方法究竟有什么区别呢？这里将通过1个例子来解释这个区别。下面将对只包含电源和电阻的电路进行分析
• 首先分析图3.1所示的电路。这是一个典型的例子。这个电路采用了一个戴维南(THévenin)输入源和一个受控或关联源
• 图3.1放大器电路

第58页：译文 3.2 设计分析：中的电路

• 图3.1中的电路可用回路分析法或节点分析法进行分析。在此选用节点分析法，注意在此电路中只有两个独立的节点：输入节点和输出节点。之所以选择节点分析法并不是因为它看起来更有优势，事实证明采用什么方法对这个电路来说没有太大区别
• 在图3.1中的处运用基尔霍夫(Kirchhoff)电流定律(KCL)2可得
• (3-1)
• 在处运用KCL可得
• (3-2)

第59页：译文 3.2 设计分析：将此式代入式

• 解方程(3-1)可得
• (3-3)
• 将此式代入式(3-2)得(3-4)
• 由此得到最后的传输函数为(3-5)

第60页：译文 3.2 设计分析：的大部分推导步骤

• 以上详细介绍了式(3-5)的大部分推导步骤，目的是为了说明这种粗略的分析方法虽然正确，但是更加注重代数推导而不是电路本身。另一个可供选择的分析技巧(大多数经验丰富的设计者都会采用的技巧，就是将这个问题细分成一系列更小的问题，然后通过查阅一些众所周知的结果来得出答案。采用这种方法时首先要注意到总的传输函数可写成两个简单的传输函数之积：(3-6)
• 一个复杂网络的传输函数总是能分解成式(3-6)所示的形式，但是只有当每个传输函数可根据观察写出或只需做较少工作就可写出时这种方法才有用。例如，如果每个网络都是单向的，它们的传输函数通常经过最小的努力就可以获得。单向网络是指其中的信号只沿着一个方向传输的网络。换句话说，在这个例子中，不依赖于。式(3-6)中等号右边的每个传输函数可通过观察写出，因为它们都是电阻分压器。从电路的输出端看去，可得

第61页：译文 3.2 设计分析：代入式

• (3-7)
• 或(3-8)
• 从输入端看去，可得(3-9)
• 或(3-10)
• 将这两个传输函数代回式(3-6)，或直接将式(3-9)代入式(3-7)中，又可得到式(3-5)，但是更简便、更重要的是使方程中每一项的由来变得更为直观

第62页：译文 3.2 设计分析：此处选择

• 此处选择了一个非常简单的例子，所以这种分析方法的好处在这一阶段可能不太明显。然而在比较复杂的电路中，解联立方程和解一系列简单传输函数的难度相差还是很大的。首先介绍一个将在本章中使用的符号，同时还以借助它建立合适的分析方式。在分析这样的电路时，通常会定义一个输入衰减因子(3-11)
• 和输出衰减因子(3-12)
• 这样会比较方便

第63页：译文 3.2 设计分析

• 这些衰减因子对不同的电路来说是不相同的，但概念是相同的，这样可以简化分析
• 在设计放大器和许多其他电路时，通常希望衰减因子尽可能接近于1。如果输入衰减因子太小，那么信号在被放大或处理之前就已经被削弱了。此时总会产生常见的附加噪声，因此就导致了一个不容忽视的问题。从另一方面来说，如果输出衰减因子太小，那就意味着在驱动负载时损失了一部分信号，因此必须提高系统增益以弥补损失的信号。值得一提的是，应被最大化的通常是衰减因子而不是能量转移

第64页：3.3 电子系统

• 3.3电子系统
• 之前我们讲述了设计构成的基本思想以及以设计为目标的分析与以确定性能为目标的分析有何不同，接下来将讨论电子系统。讨论电子系统有3个原因：首先，激发读者学习后面内容的兴趣；其次，给出后续内容的总体框架，这样就更容易发现怎样使其适用于更广阔的电子工程领域；最后，借此来清晰地阐明即将介绍的内容与读者以往学习过的线性电路和信号课程有何不同

第65页：3.3.1 电子与电动电路

• 3.3.1电子与电动电路
• 严格来讲，一个电子电路是指一个完整的回路，回路中有电子流动并且其中包含有晶体管。不过，大部分人(并非全部)还是能够区分出电动电路和电子电路。电动电路是指包含有电阻、电容、电感、变压器、开关等类似器件的电路，但是不包含晶体管、集成电路或者电子管(它们确实曾被使用过)。如果不考虑电子管，集成电路包括有晶体管、电阻、电容，偶尔还有电感，电动电路与电子电路的真正区别最后归结为是否含有晶体管。那么是什么令晶体管如此特殊呢？简言之，就是它们是有源器件，而电阻、电容及类似元件为无源器件。无源器件是指既不能提供能量也不能控制能量流向的器件。有源器件是指能控制能量流向或能提供能量的器件

第66页：译文 3.3 电子系统

• 一个机械方面的类比可能有助于理解这一点。如果想提起一个很重的箱子，可以用一个杠杆，如图3.2(a)所示，或者用铲车，如图3.2(b)所示。杠杆就是无源器件：直接提供所需的能量。杠杆使人们能在距离较大的一端施加较小的作用力从而在距离较短的一端产生较大的作用力。从另外一方面来说，铲车就是一有源器件：由人控制它运动，而它利用另外一个能源来完成这项任务
• 与杠杆类似的电学器件是变压器，如图3.3(a)所示。变压器可以提高为给定负载提供的电压(类似于图3.2中的作用力)，但是提供给负载的电流会成反比地低于从电源所获得的电流(类似于图3.2中的距离)。与铲车类似的电子器件是图3.3(b)所示的晶体管电路：它用输入电压来控制输出电压，但输入源无须为输出提供能源。该电路由输入源、晶体管、负载以及一个独立的能源(此处为电池)构成。电池类似于铲车的发动机，晶体管有从输出端到地的输出电流，该电流由输入端到地的电压控制。负载上流过的电压等于电流乘以负载电阻，该电压远高于电源电压

第67页：译文 3.3 电子系统

• 图3.2机械类比：(a)无源器件；(b)有源器件
• 然而与变压器不同的是，晶体管的输出电流可以远大于理想状态下应为零的输入电流。至此可以得出一个有源电路与无源电路的重要区别。机械功率为作用力乘以距离每单位时间(如作用力不是常数应求积分)，传输到图3.2(a)中负载的功率不会多于源端(即推动杠杆的人)所提供的功率。实际上，由于摩擦损耗及其他效应的影响，输出功率会减少。但是在图3.2(b)中，传送至负载的功率可以明显大于源端(即驾驶员)所提供的功率，在本例中源端提供的功率可以忽略，因为“源”所需要做的一切就是推动操作杆。图3.2 (b)中的功率是由铲车的发动机提供的

第68页：译文 3.3 电子系统

• 在电学中，功率等于电压乘以电流。对于图3.3(a)所示的理想变压器，输出功率恰好等于输入功率。然而，理想变压器在实践中只能被近似。总有一部分能量损失，从而使得实际输出功率低于输入功率，系统的功率增益绝对小于1。所有无源电路的功率增益都小于1
• 图3.3电气示例：(a)无源电压增益；(b)有源电压增益

第69页：译文 3.3 电子系统：输出信号

• 而对于图3.3(b)所示的晶体管电路，输出信号功率可以远大于信号源所传送的功率，因而信号的功率增益可以大于1。当然，电路中并没有产生能量，它只是利用能源(图3.3中所示电池)提供的能量来驱动负载。换句话说，晶体管以输入信号为模板，产生一个看似输入的输出，但是可能有更大的功率。因为它们控制或引导着某一能源能量(如电池)，所以有源电路功率增益可以大于1
• 3.3.2模拟与数字电子电路
• 为了使读者更好地理解电子系统，下面简单介绍两个常见的电子系统：FM接收机和计算机。在做进一步分析之前，有必要了解一下这两个系统之间的根本区别。FM接收机是模拟系统(虽然现代接收机的确也包括数字元件，但传输的是模拟信号)，而计算机是数字系统(计算机也确实有模拟电路，主要集中在显示、电源供应、声音复制以及所有存储器件当中)

第70页：译文 3.3 电子系统：没有任何办法

• FM接收机的输入为调频(FM)信号。也就是说，高频载波的频率会随着所传送的信息(本例中为音乐)而变化。人们之所以不直接传送音频信号本身有两个重要原因：首先，没有任何办法能将一个电台与另一个电台区分开来；其次，需要很多天线。天线需要与波长成合理比例才能较好地工作，例如，在空气中传播的1 k Hz电波波长为186英里
• 然而在实际的电子系统中采用的是高频载波，通常称之为射频(RF)波。为了传输信号，我们根据信号来调制(即改变)载波的某个参数(如幅度或频率)。接收机则需将载波解调以恢复原始信号
• 对立体声接收机来说，情况更为复杂，因为要同时发送左右声道信号，还要使老式的单声接收机仍然能够工作(现在已不重要了，但FM刚出现时这点很重要)

第71页：译文 3.3 电子系统：单声道接收机

• 因此，如果只是解调一个FM信号，得到的是左声道和右声道的组合信号，用(L+R)表示，单声道接收机接收的正是这种信号。另外，还有一个单独的子载波可以调制含有左右声道信号差值(L-R)的射频波。单声道接收机会忽略这一子载波，而立体声接收机会对此子载波进行解调，通过将两种解调信号进行相加或相减处理即可以恢复初始立体声信号，即(L+R)+(L-R)=2 L和(L+R)-(L-R)=2 R，在后面的讨论中将忽略这个复杂的立体声接收机而只介绍单声道接收机
• 图3.4是一个单声道调频FM接收机的方框图。该方框图显示了一个超外差式接收机，这只表明射频波先被降至中频，其间经过一些信号处理，然后被解调或被转换成音频信号。超外差是一个将两种信号组合起来产生振动声音的过程，这是通过将信号相乘来完成的

第72页：译文 3.3 电子系统：单声道接收机

• 图3.4单声道接收机方框图
• 现在来看图3.4所示的方框图，从中可以看到接收机首先过滤来自天线的射频信号以选择用户想听的电台。射频滤波器上的斜线箭头是通常用来表明该模块在某些方面是可调的。在本例中，可以通过调节滤波器来选择不同的电台。该信号经放大后被送至混频器，混频器再将其与由本地振荡器产生的正弦波相乘

第73页：译文 3.3 电子系统：做进一步信号

• 为了理解混频器的工作原理，下面来考虑一个单正弦输入。将两个不同频率的正弦波相乘可得到具有两频率之和的正弦波和两频率之差的正弦波。如果两频率非常接近，则差频会低得多，做进一步信号处理时成本会更低。也有可能选用频率与名义上的射频频率相等的本地振荡器，此时得到的接收机称为直接变换接收机或零差接收机
• 在广播调频中，中频为10.7 MHz，所以为接收100.1 MHz频率的电台信号，可将本地振荡器设至110.8 MHz。混频器的输出为一中间频率，被送至中频滤波器和放大器。大部分检波工作是在中频环节完成的，因为在此频段设置高效滤波器的成本较低。此类滤波器通常使用石英晶体振荡器。射频滤波器只是一个预选滤波器。调整本地振荡器的频率，将其与信号频率混合后降至中频频率。中频放大器通常为自动增益控制(AGC)放大器，通过改变其增益可使输出振幅保持不变。中频部分的输出结果进入解调器，由解调器将频率变化转换成电压变化来产生音频输出

第74页：译文 3.3 电子系统

• 图3.5显示的是一个基本计算机的方框图，计算机的主要组成部分为中央处理器(CPU)
• 当计算机打开时，CPU开始读取存储在不可擦除只读存储器(ROM)中的一些基本指令
• 利用这些基本指令，可能还会自动从其他不可擦除大容量存储器(如光驱)中获取一些附加信息，然后计算机就可以运行了
• 必须快速获取的信息存放在随机读取存储器(RAM)中，而不需立即存储的信息存放在相对较慢的大容量存储器中，其中使用最普遍的是光驱
• 用户的输入是通过键盘、鼠标或其他器件接收的，输出可直接传送到屏幕、打印机或其他外围器件上

第75页：译文 3.3 电子系统：的组成方式

• 图3.5基本计算机方框图
• 如果继续深入讨论下去就超出本书的范围了。例如，CPU的组成方式有很多种，计算机可以包括比此处所列的多得多的器件，也可以利用不同的电路技术来构建。对我们而言，需要知道的就是计算机中的电路大部分是数字的，其所包含的信息用二进制字表示，而二进制字由一系列二进制数字组成

第76页：3.1 节中所描述的那样

• 下面以刚才介绍的两个特殊示例系统为参考再来谈谈设计的话题。设计过程在许多不同的抽象概念级别中进行，就像3.1节中所描述的那样
• 在一个通信系统中(即发射机和接收机)，设计者必须首先决定采用什么样的解调方式，例如，调幅或调频。同时还要决定调制特性。例如，在调频情况下允许载波频率改变的范围是多少。对标准广播系统而言这些早就确定了，但是即使确定了，对设计者而言，接收机本身还是有多种选择的。在此只展示了一种接收机，也可采用其他接收方式(如前面提及的直接变换接收机)。尽管采用此处介绍的方框图，但仍然可以有多种选择。在射频部分需要过滤多少种频率波？AGC的控制范围必须为多大？是否需要AGC放大器？(简单系统的性能可能会有所降低，如振幅会受到限制，然而会更便宜。)是需要使用模拟的并且通过改变某些元件调频的本地振荡器，还是使用一个单频晶体振荡器和可编程数字分频器来组成一个“数字”调频接收机？需要多少级中频滤波以及什么类型的中频滤波？要采用何种解调器？由此可以看出，仅在方框图级别就有这么多的选择

第77页：译文 3.3 电子系统

• 对计算机而言也有类似的选择。表示数据的数字应为多长(如16位、32位)？用多长的字长表示存储器寻址？采用何种CPU架构？需要采用并行处理器吗？是否需要本地高速缓冲存储器？同样，即使在方框图级别，也能列出相当多的选择
• 所有这些选择都需要在成本与性能之间或一个性能规范与另一个性能规范之间进行权衡。有些权衡是很复杂的，很多时候依赖于每个模块的具体实现细节。单个的模块同样存在着类似的多种选择和权衡，例如，有若干种实现AGC放大器的好方法，这就是设计过程会在不同抽象概念层次间反复进行的原因，从系统级方框图到单个模块完成，然后再反过来。单个模块本身通常由许多可确认的模块构成，可针对其进行同样的讨论

第78页：译文 3.3 电子系统：最佳选择

• 最佳选择极少能被确认出来，这一点很重要。设计者应尽最大努力考虑所有可能的选择并做出一个基于可靠信息的决定，但是有时候这个决定是随意得出的。关键在于要知道最明显的差别在哪里，了解这些差别然后做出可靠的选择。任何随意的选择都不应该对整个系统的性能或成本产生影响(否则就不能称之为随意了)
• 需要注意的是，此处讨论的两个系统都不是完全模拟的或完全数字的，所以对电路设计者来说对两种设计都有所了解是很重要的，虽然大部分设计者只精通其中一种。设计者还需要了解一些设计中会出现的系统问题，这同样也很重要。这两种系统十分复杂，对任何个人来说，清楚地了解从系统级设计到晶体管级设计然后到固态器件设计与制造的所有过程是不可能的。尽管如此，对于能完成刚才所介绍的设计步骤的整个团队来说，每个成员都必须知道与他们的设计工作相关的一些知识。对电路设计者来说，他们应对系统设计有所了解，同时需要对固态器件物理特性和构造有一定程度的了解。对于通信系统工程师而言，他们需要同时对系统分析技术中所蕴涵的数学知识和晶体管级电路知识有一定程度的了解。对于涉足固态器件的工程师而言，他们在了解物理特性与构造的同时还需要了解电路知识

第79页：3.3.3 电子系统的模型化

• 3.3.3电子系统的模型化
• 前面几节在多种不同抽象概念层面上对系统模型化的概念的内容进行了大量讨论。例如，一个通信理论家在考虑通信频道的不同模型以及如何为一个给定应用选择一个最佳的可能调制方案时可能只画一个简单方框图来表示“接收机”。接收机方框图可能再进一步被细分，如图3.4所示。AGC放大器可能用一个方框图表示，其中包括一个可调增益放大器以及一些其他用以表明增益可调的模块。而可调增益放大器可以用由一些可调元件及一些放大级组成的方框图表示，然后可以用晶体管级图形来表示这些放大级。当然，可以用更为详细的电路模型来表示这些晶体管，也可以用晶体管的物理结构来表示，等等。这个过程就好比剥洋葱：一次剥一层。这种不同层次的图示同样也出现在用于描述系统的数学表达中，即可以用一个简单的方程描述接收机，或者一直细分直至写出晶体管中描述电子移动的方程

第80页：译文 3.3 电子系统

• 设计中最为困难的一个方面就是选择一个合适的抽象概念层次来分析一个给定问题。如果一个通信理论家考虑AGC放大器中间级的晶体管中的电子移动，则不会为确定采用什么样的模型图而花费太多精力！遗憾的是，所有设计者经常陷入不如这个陷阱明显的陷阱当中。人们在设计任何系统时都必须能够在不同抽象层次中来回变换，并且必须小心那种固守在错误层次上的陷阱
• 在给定的抽象层次中确定采用什么样的表示方法时也存在类似的问题。例如，信号可以表示成时域信号或频域信号，电路可以用电路图表示，也可以用方程表示。没有哪一种表示方法总是最好的

第81页：译文 3.3 电子系统：电路设计

• 电路设计中的一个普遍问题是应该用哪个层次(级别)的模型来表示一个给定器件。为了分析含有晶体管的电路，必须建立一个模型来描述晶体管的工作原理。问题在于晶体管的模型是多种多样的。在任何时候都采用能够找到的最完整的模型并不是合理的方法，因为这样会使分析变得很难，从而阻碍设计进程。因而，我们选择能满足手头任务所需精度要求的最简单的模型。但是，读者必须一开始就清楚对任何元件来说并不只是存在一种模型，而且要确认什么时候用这个模型比较合适
• 电子元件模型的示例如图3.3(b)的放大器电路所示，在图3.6(a)中会再次引用。在对该电路的讨论中只是简单地提到：该晶体管“有从输出端到地的输出电流，该电流由输入端到地的电压控制”。基于这个判断可以给出该晶体管的一个简单模型，那就是一个压控电流源。如果假设输出为输入的线性函数，并设比例常数为G，则可得到图3.6(b)所示的模型图

第82页：图3.6

图3.6

本页为图示页，图中给出译文 3.3 电子系统的结构或流程，具体图形见课件原图。

第83页：译文 3.3 电子系统

• 在这个分析中采用了最简单的可用晶体管模型，它表明了该器件作为压控电流源的基本应用。当然，这个模型过于简化了。例如，实际晶体管的输出电流并不是输入电压的线性函数，输入阻抗也不是无穷大。尽管如此，这个简单模型还是可以让人明白该电路为一放大器，而且可粗略估计增益。稍后将采用比较复杂的模型使读者对该器件获得更为完整的理解
• 实际上在电路设计中所做的一切都是以模型为基础的。没有理想的电阻、电容、电感、电池和其他元件可用。每个实际电阻都有一定的电感和电容，每个实际的电容都有一定的电感和电阻，人们所用的任何其他元件也是如此。在很多时候，只用电阻和电容就能恰当地表示元件模型，但必须时刻牢记的是，有时候这类模型是无法准确描述系统性能的(如在高频条件下)。有时候有多个等效模型可用。在这种情况下，要么选择最能表示所考虑的元件工作原理的模型，要么选择分析起来最为简单的模型

第84页：译文 3.3 电子系统：每一个模型

• 还有更多的模型可以用来表示运放
• 每一个模型都适用于一些分析而不适用于其他分析
• 电路设计者的工作就是为手头的工作选择一个合适的模型
• 无论是学生还是经验丰富的设计者，都要问自己所用的模型是否适用于自己所做的分析
• 如果模型过于复杂，进行分析可能要花费很多时间，得到的结果很难或者无法解释，或者根本不可能得到结果
• 如果模型过于简单，结果很可能是错误的

第85页：图3.7

图3.7

本页为图示页，图中给出译文 3.3 电子系统的结构或流程，具体图形见课件原图。

第86页：译文 3.3 电子系统：展示了放大器设计

• 图3.7展示了放大器设计的3种不同抽象层次表示
• 在图3.7 (a)中，方框图中描述的是电源驱动放大器，放大器驱动负载
• 在图3.7 (b)中用戴维南模型表示电源，高层电路模型表示放大器，阻抗模型表示负载
• 在图3.7 (c)中将放大器做了进一步扩展，将构成放大器的晶体管和分立电阻一一列出
• 绘制图3.7 (c)所示的电路图和找出图3.7 (b)所示模型的各模型元件
• 将电路用更高一级的抽象模型表示的优点是使分析变得更加简单，同时对系统获得更好的理解
• 即使不画出如图3.7(b)所示的电路，也要在脑子里产生该电路图
• 例如，如果想驱动一个给定负载并且希望输出的衰减因子不要太小，就要知道放大器的输出电阻应为多大