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1、Index Pre-reading 4. 1 Text 4. 2 Reading materials 4. 3 Knowledge about translation 4. 4 Exercises 4. 5 课文参考译文课文参考译文 4. 6 阅读材料参考译文阅读材料参考译文Pre-reading Read the following passage, paying attention to the question. 1) What is a semiconductor diode? 2) What difference is between the forward biased and r
2、everse biased? 3)Can a transistor be used to amplify a signal? 4)What is a CMOS circuit?4. 1 Text 4.1.1 Semiconductor diode 4.1.2 NPN bipolar transistor 4.1.3 MOS Transistors 4.1.4 Ideal Operational Amplifier 4.1.1 Semiconductor Diode A semiconductor diode (refers to diode in short) is the simplest
3、possible semiconductor device. A diode consists of a PN junction made of semiconductor material. The P-type material is called the anode, while the N-type material is called the cathode (Fig 4.1).Fig4.1 diode A diode is forward biased when the anode is more positive than the cathode (greater than th
4、e turn-on voltage, which is approximately 0.3 V for germanium and 0.7V for silicon). In this condition the internal resistance of the diode is low and a large current will flow through the diode (depending on the external circuit resistance ). The diode is reverse biased when the anode is less posit
5、ive than the cathode. In this case, the internal resistance is extremely high, so perfect diodes can block current in one direction while letting current flow in another direction. Diodes can be used in a number of ways. For example, a device that uses batteries often contains a diode that protects
6、the device if you insert the batteries backward. The diode simply blocks any current from leaving the battery if it is reversed - this protects the sensitive electronics in the device. A diodes behavior is not perfect, as shown in Fig 4.2. When reverse-biased, an ideal diode would block all current.
7、 A real diode lets perhaps 10 microamps through - not a lot, but still not perfect. Fig 4.2 a diodes behavior And if you apply enough reverse voltage (V), the junction breaks down and lets current through. Usually, the breakdown voltage is a lot more voltage than the circuit will ever see, so it is
8、irrelevant. Fig 4.3 various diodes When forward-biased, there is a small amount of voltage necessary to get the diode going. In silicon, this voltage is about 0.7 volts. Though a large forward current can flow through the diode, too much current through the diode in either direction will destroy it.
9、 4.1.2 NPN Bipolar Transistor There are two types of standard bipolar transistors, NPN and PNP, with different circuit symbols (Fig 4.4). The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to mak
10、e from silicon.Fig 4.4 transistor circuit symbols The NPN bipolar transistor consists of an N-type emitter (E), P-type base (B), and N-type collector (C). An amplifier can be built with a transistor. Fig 4.5 shows the two current paths through a transistor. You can build this circuit with two standa
11、rd 5mm red LEDs and any general purpose low power NPN transistor.Fig 4.5 the small current controls the larger current When the switch is closed, a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allo
12、w a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly. The amount of collector current (IC) is directly proportional to the amount of base current (IB) and the collector current (IC) will be less than the emi
13、tter current (IE), since a small base current (IB) must flow to turn on the transistor. The relationship of the currents is IE = IC +IB. The ratio of IC to IB is called the current gain of the transistor and indicates its ability to amplify. This current gain is called beta() and is expressed as , w
14、hen the voltage from C to E(UCE) is held constant.BCII To turn on an NPN bipolar transistor, the base must be more positive than the emitter (about +0.6V for silicon). When the transistor is turned on hard (in saturation), this voltage is about +0.7V and the resistance from C to E is low and may eve
15、n appear almost as a short. When the switch (Fig 4.5) is open no base current flows, so the transistor switches off the collector current and both LEDs are off. The resistance from C to E now is high and may appear as an open. Actually a transistors behavior is not perfect too, a small leakage curre
16、nt (ICBO) from C to B is always present and may cause stability problems for a transistor circuit. The Fig 4.6 displays various bipolar transistors. Fig 4.6 various bipolar transistors 4.1.3 MOS Transistors Presently, the most popular technology for realizing microcircuits makes use of MOS transisto
17、rs. The acronym MOS stands for metal-oxide semiconductor, which historically denoted the gate, insulator, and channel region materials respectively. The MOS transistors can be divided into two classes in terms of carrier: N-channel and P-channel, electrons are used to conduct current in N transistor
18、s, whereas holes are used in P transistors While N devices conduct with a positive gate voltage, P devices conduct with a negative gate voltage. The function of the MOS transistor is controlling a large current with a small voltage. According to the voltage condition, there are two kinds of MOS tran
19、sistor: enhancement and exhausted. The symbols used for enhancement MOS transistors and exhausted MOS transistors are shown in Fig 4.7. MOS transistors are actually four-terminal devices; the fourth terminal is a substrate connection. For digital circuits, the substrate connection of N-channel trans
20、istors is almost always the most negative IC voltage (i.e. ground or USS). Similarly, the substrate connection for P-channel transistors will be assumed to be the most positive IC voltage, which is labeled UDD. This will always be assumed to be the case unless stated otherwise, and therefore substra
21、te connections will not be shown. Fig 4.7 MOS transistors Unlike most bipolar-junction transistor (BIJ) technologies, which make dominant use of only one type of transistor, MOS circuits normally use two complementary types of transistors. Microcircuits containing both N and P transistors are called
22、 CMOS circuits, for complementary MOS. 4.1.4 Ideal Operational Amplifier Of fundamental importance in the study of electric circuits is the ideal voltage amplifier or ideal operation amplifier (Op amplifier). Such a device, in general, has two inputs, v1 and v2, and one output, vo. The relationship
23、between the output and the inputs is given by vo=A(v1-v2), where A is called the gain of the amplifier. Note that since the ideal amplifier input resistance R=, when such an amplifier is connected to any circuit, no current will go into the input terminals. Also, since the output vo is the voltage a
24、cross an ideal source, we have that vo=A(v1-v2), regardless of what is connected to the output for the sake of simplicity, the ideal amplifier having gain A is often represented as shown in Fig 4.8. Fig 4.8 Operational Amplifier We refer to the input terminal labeled “-“ as the inverting input and t
25、he input terminal labeled “+” as the noninverting input. 4. 2 Reading Materials 4.2.1 Audio amplifiers(音频放大电路音频放大电路) 4.2.2 The transistor as a switch4.2.1 Audio Amplifiers(音频放大电路) For those of you who like to experiment with audio circuits and would like a simple amplifier that frees you from having
26、 to figure out the biasing resistors, we have two for you (and they run off 9 Volts too!). One uses an Op-Amp (Fig 4.9 (a) and the other uses a transistor (Fig 4.9(b). Fig 4.9 two audio amplifiers (a) Op-Amp (b) Transistor Amp Both circuits need capacitors on the input and output to block DC while p
27、assing AC. The capacitor values depend on which circuit you use and what signal frequency you are amplifying. Start with a 1F and go from there. Low frequencies may require a bigger value. While Op-Amps normally run off of a dual voltage supply (+U and -U), it is possible to run them from a single v
28、oltage by using two equal value resistors (Rb) to create a separate DC grounding point midway between UCC and actual ground, just for biasing the Op-Amp. The DC ground is connected to actual ground through a by-pass capacitor. The value of Rb is not critical; 10k should work just fine. To minimize D
29、C offset in the output, Rb should have a 1% tolerance. The gain of the amplifier is set by R1 and R2 （AV = R2/R1）. R2 should be at least 2k or bigger so as not to load the Op-Amp too much. If you use a bipolar device such as the venerable 741, the output cant go lower than 2 volts above ground or hi
30、gher than 2 volts below UCC. So with a 9-volt battery, the maximum output swing will be 5 volts: from 2V to 7V. If you want to go rail-to-rail from UCC to Ground, then use a CMOS device like the CA3130; UCC can then be as high as the Op-Amp allows. The CA3130 requires a 100 pF compensation capacitor
31、. If you really want quick and dirty, this one transistor circuit is an oldie but goodie. (See Fig4.9 (b). Note that by connecting the base-bias resistor Rb to the collector you get two benefits: 1) the biasing cannot cause saturation or cut-off and 2) you introduce some negative feedback into the s
32、ignal path, which reduces distortion. Its not as good as the Op-Amp circuit but it does work. As for gain, youll just have to measure it and see. Play around with different values of Rc, and make Rb= 100Rc. The voltage peak of input signal should not exceed 15mV. 4.2.2 The transistor as a switch Bec
33、ause a transistors collector current is proportionally limited by its base current, it can be used as a sort of current-controlled switch. A relatively small flow of electrons sent through the base of the transistor has the ability to exert control over a much larger flow of electrons through the co
34、llector. For the sake of illustration, lets insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp (Fig 4.10). Remember that the controlled current through a transistor must go between collector and emitter. Since its the current through the lamp
35、 that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch are now. We must also make sure that the lamps current will move in the direction of the emitter arrow symbol to ensure that the transistors junction bias will be correct. Fig
36、4.10 the transistor as a switch (a) Switch is off. (b) Switch is on. In this example I happened to choose an NPN transistor. We are faced with the need to add something more so that we can have base current. Without a connection to the base wire of the transistor, base current will be zero, and the
37、transistor cannot turn on, resulting in a lamp that is always off. If the switch is open, the base wire of the transistor will be left floating (not connected to anything) and there will be no current through it (Fig 4.10(a). In this state, the transistor is said to be cutoff. If the switch is close
38、d (Fig 4.10(b), however, electrons will be able to flow from the emitter through to the base of the transistor, through the switch and up to the left side of the lamp, back to the positive side of the battery. This base current will enable a much larger flow of electrons from the emitter through to
39、the collector, thus lighting up the lamp. In this state of maximum circuit current, the transistor is said to be saturated. Of course, it may seem pointless to use a transistor in this capacity to control the lamp. After all, were still using a switch in the circuit, arent we? If were still using a
40、switch to control the lamp - if only indirectly - then whats the point of having a transistor to control the current? Why not just use the switch directly to control the lamp current? There are a couple of points to be made here, actually. First is the fact that when used in this manner, the switch
41、contacts need only handle what little base current is necessary to turn the transistor on, while the transistor itself handles the majority of the lamps current. This may be an important advantage if the switch has a low current rating: a small switch may be used to control a relatively high-current
42、 load. Perhaps more importantly, though, is the fact that the current-controlling behavior of the transistor enables us to use something completely different to turn the lamp on or off. Consider this example (Fig4.11 (a), where a solar cell is used to control the transistor, which in turn controls t
43、he lamp. Or, we could use a thermocouple (Fig 4.11 (b) to provide the necessary base current to turn the transistor on. Fig 4.11 examples of controlling the lamp(a) A solar cell control the lamp (b) a thermocouple control the lamp Even a microphone of sufficient voltage and current output could be u
44、sed to turn the transistor on (Fig 4.12), provided its output is rectified from AC to DC so that the emitter-base PN junction within the transistor will always be forward-biased: Fig 4.12 sound control the lamp The point should be quite apparent by now: any sufficient source of DC current may be use
45、d to turn the transistor on, and that source of current need only be a fraction of the amount of current needed to energize the lamp. Here we see the transistor functioning not only as a switch, but as a true amplifier: using a relatively low-power signal to control a relatively large amount of powe
46、r. Please note that the actual power for lighting up the lamp comes from the battery to the right of the schematic. It is not as though the small signal current from the solar cell, thermocouple, or microphone is being magically transformed into a greater amount of power. Rather, those small power s
47、ources are simply controlling the batterys power to light up the lamp. REVIEW: Transistors may be used as switching elements to control DC power to a load. The switched (controlled) current goes between emitter and collector, while the controlling current goes between emitter and base. When a transi
48、stor has zero current through it, it is said to be in a state of cutoff (fully nonconducting). When a transistor has maximum current through it, it is said to be in a state of saturation (fully conducting). 4. 3 Knowledge about translationto V to V与V-ing的区别 1to V to V又称动词不定式，兼有名词、形容词、副词特点，也保留了动词性，可用
49、做主语、宾语、表语、定语、状语。常用来表示具体的（特别是未来的）一次性动作。 Resistor RF is made variable to be able to adjust sufficient feedback voltage to cause oscillation. 电阻RF是可调的，可调节到有足够的反馈电压以引起电路的电路的振荡。（is made 不译，“电路的”为增补词语。用了两个to V 表示一次性的动作、未来的动作） The electromotive force creates the electric pressure that causes the current to
50、 flow through a conductor. 电动势产生电压，这电压使电流流过一个导体。（to V 作宾语补足语） It is very difficult to measure the passing current in insulators. 测量绝缘体中通过的电流是很困难的。（不定式短语作主语时，尤其是不定式短语比较长时，往往引入it作形式主语，而把不定式短语放在谓语动词的后面。） To describe the motion one must introduce the concept of time. 为了描述运动，必须引入时间的概念。（不定式短语作状语） 英语中有些动词后