番茄花园-ElectronicCircuitsandApplications.ppt
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1、2019/4/9,Electronic Circuits and Applications,Chapter Three Physical Electronics Of Transistors,Contents Active devices and Control Elements Bipolar Transistors as Control Elements Field-Effect Transistors as Control Elements,3.0 Active devices and Control Elements (1),1. What is an active device有源器
2、件? An active device is a device capable of controlling the flow of electrical energy from a source to a load. If an active device is to be useful for increasing the power level of signal, then the power required at the control inputs must be much less than the power delivered to the load, the balanc
3、e coming from the dc power sources. The combination of the active device and its associated power source then functions as an amplifier, and is said to have power gain. The op-amp is an active device. 2. What is a control element控制器件? A control element is a network element that can be used for the c
4、ontrol of power flow. A control element normally requires at least three electrical terminals, with the v-i characteristic at the output terminal pair being dependent on the voltage or current at the other terminals. The transform is a control element.,3.0 Active devices and Control Elements (2),3.
5、Transistor and classification The transistor is made from semiconductor. It is the most widely used active device in modern electronic circuits. Transistors can be divided into two general categories: Bipolar transistor (BJT) 双极性晶体管 Field-effect transistor (FET) 场效应晶体管,3.1 Bipolar Transistors as Con
6、trol Elements (1),3.1.1 The Physical Basis of Transistor Operation,Injection注入, Diffusion扩散, and Collection收集,A typical bipolar transistor structure is shown as,It consists of a p-type central region, called the base基区, which is sandwiched between two n-type regions, called the emitter发射区 and the co
7、llector集电区. This arrangement is known as an NPN transistor. It is also possible to construct a complementary form, the PNP transistor, by using p-type material for the emitter and collector regions and n-type material for the base. In either case, the bipolar transistor consists of two p-n junctions
8、 that share a common region, the base, between them.,3.1 Bipolar Transistors as Control Elements (2),To understand how the bipolar transistor operates as a control element, we can draw on our understanding of the p-n junction diode developed in Section 7.3. In the diode a forward bias produces a sig
9、nificant current resulting from a net flow of holes and electrons from the regions where they are majority carriers to the region where they are minority carriers. This injection process is reviewed schematically in following figure.,The total terminal current I is given by the sum of the two curren
10、t components arising from the hole flow and the electron flow.,3.1 Bipolar Transistors as Control Elements (3),Under reverse bias, the p-n junction diode is characterized by a small saturation current, arising from the collection of minority carriers from their respective regions by the electric fie
11、ld in the space-charge layer. The magnitude of this reverse saturation current depends on the available concentration of minority carriers, and is small in the diode.,The bipolar transistor works as a control element by combining injection at one of its p-n junctions with collection at the other p-n
12、 junction.,3.1 Bipolar Transistors as Control Elements (4),In the normal gain or the active gain region放大区 of operation, the emitter-base junction is maintained in forward bias, and the collector-base junction is held in reverse bias. By doping the emitter region much more heavily than the base, mos
13、t of the injection of minority carriers is made to occur into the base side of the junction. Thus, under forward bias conditions, there is a large buildup of minority carriers ( electrons in an NPN transistor, holes in a PNP transistor) on the base side of the emitter-base junction. While this build
14、up of electron concentration is taking place at the emitter end of the base region, the reverse-biased collector-base junction keeps the concentration of minority electrons very low at the collector end of the base region. This combination of a forward bias at the emitter-base junction and a reverse
15、 bias at the collector-base junction thus establishes a large concentration gradient of minority carriers across the base region. Normal thermal motions, therefore, produce a diffusive flow of minority electrons through the base region, from the emitter end, where they are in excess, to the collecto
16、r end, where they are swept across the collector-base junction into the collector region.,3.1 Bipolar Transistors as Control Elements (5),3.1 Bipolar Transistors as Control Elements (6),Above figure illustrates this flow of electrons from the emitter to the base by injection注入, across the base by di
17、ffusion扩散, and into the collector by collection收集. Nearly all the electrons entering the base region from the emitter reach the collector. A small fraction, however, recombine复合 with holes to form complete covalent bonds. Because of this recombination process, and because of the injection of holes f
18、rom the base to the emitter, some holes must be supplied to the base via the base terminal. These holes cannot be supplied from the collector because they are the minority carrier there, and are few in numbers. In summary, the collector-base junction behaves as a reverse-biased diode whose saturatio
19、n current is controlled by the injection of electrons at the emitter-base junction. The collector current is independent of the collector-base voltage, provided that a reverse bias is established at the collector-base junction. Thus, the basic property of a control element, in this case the dependen
20、ce of the output (collector) current on an input variable (emitter current or emitter-base voltage) has been demonstrated.,3.1 Bipolar Transistors as Control Elements (7),3.1.2 Circuit Symbols and Terminal Variables for Bipolar Transistors,1. NPN bipolar transistor,2. PNP bipolar transistor,The only
21、 difference between NPN and PNP device symbols is the direction of the arrow on the emitter lead, which indicates the actual direction of forward current in the emitter-base junction. The reference directions for the terminal currents are all defined entering the device, irrespective of the directio
22、n of positive current flow. Thus at least one of the terminals current must be algebraically negative.,The voltage subscripts denote the terminals between which the voltage is measured, with the first subscript indicating the positive reference terminal and the second the negative reference terminal
23、.,3.1 Bipolar Transistors as Control Elements (8),3.1.3 Regions of Transistor Operation,Since the transistor has three terminals, there are six possible ways to connect it in a given circuit.,There are four regions of transistor operation.,1. Active-Gain region放大区 E-B junction is forward biased C-B
24、junction is reverse biased,2. Saturation region饱和区 E-B junction is forward biased C-B junction is forward biased,3. Cutoff region截止区 E-B junction is reverse biased C-B junction is reverse biased,4. Reverse region反向区 E-B junction is reverse biased C-B junction is forward biased,3.1 Bipolar Transistor
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