1中频段小信号放大器

Words and Expressionselements 元件 performance 性能mode 模式、模态 observe 遵守，转译规定load 负载 configuration 组态examine 分析 specified 指定，转译成定义与前呼应unity 1 across 跨过，转译为并联quantity 数量，量small signal amplifier 小信号放大器 mid —frequency band 中频带 bipolar junction transistor (BJT) 双极性晶体管 field effect transistor(FET) 场效应管 transconductance model 跨导模型 hybrid-π model 混合π型模型general way （manner ） 一般形式，通用形式 block diagram 框图frequency effects 频效 internal resistance 内阻voltage gain 电压增益 current gain 电流增益input resistance 输入电阻 output resistance 输出电阻configuration 组态 common base 共基极common collector 共集电极 common source 共源极common gate 共栅极 common drain 共漏极common emitter 共射极 buffer amplifier 缓冲放大器self-bias resistor 自偏电阻 in parallel with 和……并行，和……并联 DC supply 直流供电电源 parallel circuit 并联电路source follower circuit 信号源跟随电路 emitter follower 发射极输出器，射极跟随器 DC blocking capacitor 直流耦合电容器，隔直流电容Unit1 Analysis of Small Signal Amplifiersin the Mid-Frequency BandIntroductionIn the other lesson, a single small signal equivalent circuit was developed, applicable to both bipolar junction transistors and field effect transistors . This is the transconductance model , and it is redrawn in a more general way in Fig. 5.1, where terminal designations 1, 2 and 3 are used, the numbers corresponding to the various electrodes, as shown in the table 5.1. Table 5.1TerminalBJT FET 1Base Gate 2 Collector Drain13V 13V g m 0R 23-∙∙∙3Emitter SourceFig. 5. 1 transconductance modelAs previously explained, the input resistance is virtually an open circuit for the FET, and it is equal to be r for the BJT. The transconductance model is a simplified version of an equivalent circuit known as the hybrid-π model, which takes into account equivalent circuit elements which affect the high frequency performance of the transistor. In the present lesson, frequency effects at both high and low frequencies are ignored, but the simplified analysis is useful over a range of frequencies termed the mid-frequency band, which is the frequency region of interest for general-purpose applications.General DefinitionsIrrespective of the mode of connection of the transistor, amplifier properties can be defined in a general manner . It is essential to observe （观察、观测、遵守） correctly the polarities of voltages and currents in these definitions, and these are shown in Fig. 5.2. Referring to Fig. 5.2(a), we haveTerminal voltage gain: iL v V V A = (5.1) V oltage gain referred to Source: sL vs V V A = (5.2) Current gain: iL i I I A = (5.3) Input resistance: i i i I V R =(5.4) Output resistance: This is the internal resistance of the amplifier as seen by the load （负载）. It can be found by setting 0=s V , and applying a voltage (in principle) to the output terminals with the load L R removed. With reference to Fig. 5.2(b), the output resistance is then00I V R = (5.5) Network Output Input i I sR s V i V L V L R L I ---+++ Network Output Input s R 0=s V i V 0V 0I --++Fig. 5. 2 output resistance computation It will also be seen from these definitions thatLi v i R R A A ⋅= (5.6) and that is i v vs R R R A A +⋅= (5.7) Amplifier Configurations The transistor equivalent circuit, Fig. 5.1, is seen to have three terminals, while the rather more general block diagram , Fig. 5.2, shows the amplifier with four terminals, two inputs and two outputs. This means that one of the transistor terminals must be common to input and output. Any one of the three transistor terminals may be made the common terminal and this gives rise to three distinct amplifier configurations . When the BJT is used, the configurations are known as Common Emitter (CE), Common Collector (CC), and Common Base (CB). When the FET is used, the corresponding configurations are the Common Source (CS), the Common Drain (CD), and the Common Gate (CG).Each circuit configuration has properties best suited to certain applications, and the purpose of this lesson is to examine （测试、调查、分析） these properties in detail. At this point, however, it will be useful to have a summary of the main features of each amplifier configuration.CE and CS Amplifiers. This configuration, common terminal 3, is the most widely used for general purpose amplifier applications. V oltage gains range from moderate to high and the current gain of the CE circuit is also moderate to high. Current gain has little meaning for the CS amplifier and is not specified （指定）. This is because the gate input of the FET presents an extremely high resistance and therefore the input current is negligible. The input resistance of the CE amplifier by contrast has values ranging from low to moderate. Both the CS and the CE amplifiers have moderate to high output resistances.CC and CD Amplifiers. The common terminal 2 configuration is used mainly as a buffer amplifier between a signal source and a low impedance load. As a buffer amplifier, the circuit allows the signal to be transferred to the load while preventing the load from directly affecting the source. The main characteristics of this amplifier configuration are, high to very high input resistance, low to very low output resistance, and a voltage gain which is close to, but always lower than, unity . The output voltage "follows" the input voltage, and for this reason the circuits are also known as Emitter Follower , and Source Follower circuits .CB and CG Amplifiers. The common terminal 1 configuration is used where very good isolation is required between input and output circuits. This is particularly required at high frequencies to prevent oscillations occuring in amplifier circuits. The main circuit properties of this configuration are, moderate to high voltage gain, low input resistance, and high output resistance.CE and CS amplifiersThe circuit for the basic common emitter (CE) amplifier is shown in Fig. (omitted), and for thecommon source (CS) amplifier in Fig. (omitted). By "common" is meant that the emitter, or source, terminal is common to both input and output signal circuits. In each of the circuits a self-bias resistor , E R or st R , is shown, but this is effectively short-circuited to signal currents by capacitor E C or s C . The DC blocking capacitors C are assumed to be short circuits to signal currents also. The DC supply , CC V or DD V , is also assumed to be a short circuit to signal currents, and in practice this may be achieved （实现） by connecting a large capacitor across （跨过，并联） the supply. In the equivalent circuit, therefore, 2R appears in parallel with L R , and A R appears in parallel with B R , the latter combination being shown as 1R , in the equivalent circuit of Fig. (omitted).All of the important circuit properties can be derived from the equivalent circuit of Fig. (omitted).Input Resistance:By inspection of Fig （由图可见，得）. the input resistance i R is seen to be1R R i =∥1r (5.8)Note that the signal source resistance s R is not included in the input resistance.Voltage Gain:Referring to Fig （由图可知）. we find that it is convenient to let （为方便起见令）0r R P =∥2R ∥L R (5.9) For the output loop it is seen thatP m L R V g V 13-= (5.10) and by inspection （由图可见）, 13V V i =, so that the voltage gain isP m iL v R g V V A -==(5.11) Current Gain: As defined in Section 5.2, the current gain isLi v i R R A A = Equation (5.11) for voltage gain may, therefore, be substituted in this to give （将式(5.11)代入上式可得）Li P m i R R R g A -= （5.12） For the FET the input resistance is 1R R i =, and since the signal current in 1R , is negligible, the current gain of the common-source amplifier is not a meaningful quantity （数量，量quality 质量）. For the BJT, be i r R ≈ and since β=be m r g then L P i R R A /⋅-=β.Output Resistance:By inspection of the circuit of Fig. the output resistance 0R is00r R =∥2R (5.13) Note that L R . is not included in the output resistance .中频带小信号放大器的分析引言在其他课文中，已建立了单个小信号等效电路，它既适用于双极性晶体管也适用于场效应管。