Analysis of Small Signal Amplifiers in the Mid-Frequency Band.doc

Analysis of Small Signal Amplifiers in 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 5.1TerminalBJTFET1BaseGate2CollectorDrain3EmitterSourceFig. 5. 1 transconductance modelAs previously explained, the input resistance is virtually an open circuit for the FET, and it is equal to for the BJT. The transconductance model is a simplified version of an equivalent circuit known as the hybrid- 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 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 have Terminal voltage gain: (5.1) Voltage gain referred to Source: (5.2)Current gain: (5.3)Input resistance: (5.4)Output resistance: This is the internal resistance of the amplifier as seen by the load. It can be found by setting, and applying a voltage (in principle) to the output terminals with the load removed. With reference to Fig. 5.2(b), the output resistance is then (5.5) Fig. 5. 2 output resistance computationIt will also be seen from these definitions that (5.6) and that (5.7)Amplifier ConfigurationsThe 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 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. Voltage 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. 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 occurring 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 the common 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, or, is shown, but this is effectively short-circuited to signal currents by capacitor or. The DC blocking capacitors C are assumed to be short circuits to signal currents also. The DC supply, or , is also assumed to be a short circuit to signal currents, and in practice this may be achieved by connecting a large capacitor across the supply. In the equivalent circuit, therefore, appears in parallel with, and appears in parallel with, the latter combination being shown as, 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. The input resistance is seen to be (5.8)Note that the signal source resistance is not included in the input resistance.Voltage Gain: Referring to Fig. we find that it is convenient to let (5.9) For the output loop it is seen that (5.10)and by inspection, , so that the voltage gain is (5.11)Current Gain:As defined in Section 5.2, the current gain is Equation (5.11) for voltage gain may, therefore, be substituted in this to give（将式(5.11)代入上式可得） （5.12） For the FET the input resistance is , and since the signal current in , is negligible, the current gain of the common-source amplifier is not a meaningful quantity. For the BJT, and since then.Output Resistance: By inspection of the circuit of Fig. the output resistance is (5.13) Note that 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。这是一跨导模型，现用更一般的形式重画如图5.1所示。其终端分别用1、2、3符号表示，这些数字与表5.1 所示各电极相对应。 如前所述，对场效应管(FET)来说，输入电阻实际上是开路的。而对晶体管(BJT)来说，它等同于。跨导模型是通称为混合型电路的等效电路的简化，其考虑了可影响晶体管高频性能的各种等效电路元件。在本课中，高频和低频的频效均不予讨论，但这一在中频段的简化分析是有用的，该频带是一般应用所感兴趣的频率范围。定义不管晶体管的连接方式如何，放大器性能都能用一通用形式来定义。正确规定这些定义中电压和电流的极性是很有必要的，这些极性如图5.2所示，从图5.2中我们得出：终端电压增益： 电源电压增益： 电流增益： 输入电阻： 输出电阻：输出电阻是从负载端看的放大器的内电阻。将短路，移开负载后，在输出端施加一电压(原理上)则可求得输出电阻，参看图5.2(b)，则输出电阻为： 由定义还可看出： 及 放大器组态 从图5.1所示的晶体管等效电路可看出：晶体管有三个端头，而在如图5.2所示的更加通用的方框图中，放大器有四个端头，两个输入和两个输出。这就意味着晶体管必须有一个端头是为输入和输出所共用的。晶体管的三个终端中任何一个都可作为共用终端，这就产生了三个截然不同的放大器组态。当使用晶体管时，组态可分为共射极、共集电极和共基极。当使用场效应管时，基本组态是共源极、共漏极和共栅极。 每种组态都有其特定的特征及最佳应用，本课的目的就是要详细地分析这些特征。因而，下面对每个放大器组态的主要特征做一概述，这将是很有用的。 CE和CS放大器：这种组态以端点3为公共端，是通用放大器中使用最为广泛的线路。电压增益范围中至高，CE电路的电流增益也是中至高。对于CS放大器来说电流增益无多大意义，因而无此提法。这是因为场效应管(FET)的栅极输入呈现一个极高的电阻，因此，输入电流可以忽略不计。相比之下，CE放大器的输入电阻值由低至中。CS和CE放大器均有中至高的输出电阻。 CC和CD放大器：其为共用端2组态，主要用作信号源与低阻抗负载之间的缓冲放大器。作为缓冲放大器，该线路可允许信号传输给负载，又可防止负荷直接影响信号。该放大器组态的主要特征是，高至极高的输入电阻，低至极低的输出电阻，以及具有一个接近但总是小于1的电压增益。输出电压“跟随”输入电压，因此，该电路也称为射极跟随器和信号源跟随电路。 CB和CS放大器；其为共用终端1组态，一般在输入和输出线路之间需要很好隔离时使用。在高频状态尤其需要该线路来防止放大器线路中发生振荡。该线路的主要特征是中至高的电压增益，低的输入电阻和高的输出电阻。CE和CS放大器 基本的共射放大器(CE)电路和共源放大器(CS)电路如图(略)所示。“公共”的意思是发射极或源极的端头为输入和输出信号电路所共用。如图所示，每个电路中均有自偏置电阻，和，但其信号电流被电容或有效地短路了。直流隔离电容C被认为将信号电流短路了。直流供电电源或，也被认为对信号电流是短路的，在实际中，只将一大电容与电源并联便可实现。因此，在等效电路中，与并联，与并联，后一并联电阻在等效电路图(略)中表示为。 由等效电路图(略)可以得出所有重要的电路特征。输入电阻： 由图可知，输出电阻为：注意信号源电阻，未包括在输入电阻中。电压增益： 由图可以看出令 是方便的。 对于输出回路，可以得出： 经观察，所以电压增益为：电流增益： 如前所定义，电流增益为： 因此，电压增益公式(511)代人上式中，则得到：对本场效应器(FET)来说，输入电阻是，由于中的信号电流可以忽略不计，这样共源放大器的电流增益定量计算没有意义了。对于晶体管(BJT)来说，由于 因