高频小信号放大器的特点：放大高频小信号(中心频率在几百kHz到几百MHz，频谱宽度在几kHz到几十MHz的范围内)的放大器。通过的频带和中心频率之比是很小的（窄带），一般都采用选频网络组成谐振或非谐振放大器。 普通调幅无线电广播所占带宽应为kHz，电视信号的带宽为Mz左右。,3.1 概述,高频小信号放大器,谐振放大器（窄带）,非谐振放大器（宽带）,LC集中滤波器,石英晶体滤波器,陶瓷滤波器,声表面波滤波器,（调谐与非调谐）,高频小信号放大器的分类,本章重点讨论晶体管单级窄带谐振放大器。,3.1 概述,高频小信号放大器的主要质量指标,1) 增益：（放大系数）,电压增益：,分贝表示：,功率增益：,2) 通频带：,3.1 概 述,高频小信号放大器的主要质量指标,3) 选择性, 矩形系数：表示与理想滤波特性的接近程度。,：从各种不同频率信号的总和（有用的和有害的）中选出有用信号，抑制干扰信号的能力称为放大器的选择性。选择性常采用矩形系数和抑制比来表示。,3.1 概 述,高频小信号放大器的主要质量指标, 抑制比：表示对某个干扰信号fn 的抑制能力，用dn表示。,3) 选择性,3.1 概 述,高频小信号放大器的主要质量指标,4) 工作稳定性：指放大器的工作状态(直流偏置)、晶体管参数、电路元件参数等发生可能的变化时，放大器的主要特性的稳定。,不稳定状态有增益变化，中心频率偏移，通频带变窄，谐振曲线变形，极端情况是放大器自激（主要由晶体管内反馈引起），使放大器完全不能工作。,3.1 概 述,分析的方便，将把稳定性问题及其改善放至以后讨论。,低频小信号模型,高频小信号模型,高频小信号放大器的主要质量指标,4) 工作稳定性：指放大器的工作状态(直流偏置)、晶体管参数、电路元件参数等发生可能的变化时，放大器的主要特性的稳定。,End,3.1 概 述,End,高频小信号放大器的分析方法,晶体管工作在线性区（放大区），可看成线性元件，可用双端口理论中有源四端网络参数微变等效电路来分析。,3.1 概 述,3.2.1 形式等效电路（网络参数等效电路）,3.2.2 混合等效电路,3.2.3 混合等效电路参数与 形式等效电路参数的转换,3.2.4 晶体管的高频参数,3.2 晶体管高频小信号等效电路与参数,因为放大器由信号源、晶体管、并联振荡回路和负载阻抗并联组成，采用导纳分析比较方便，为此, 引入晶体管的y（导纳）参数等效电路。,晶体管工作在线性区，可看成线性元件，可用有源四端网络参数微变等效电路来分析。,3.2.1 形式等效电路,Admittance,Occasionally we find that the reciprocal of impedance is a more convenient quantity. In this spirit, we define the admittance Y of a circuit element as the ratio of phasor current to phasor voltage (assuming that the passive sign convention is satisfied):,The real part of the admittance is the conductance G, and the imaginary imdinri part of the admittance is the susceptance sseptns B. Thus,Equation 22 should be scrutinized carefully; it does not state that the real part of the admittance is equal to the reciprocal of the real part of the impedance or that the imaginary part of the admittance is equal to the reciprocal of the imaginary part of the impedance!Admittance, conductance, and susceptance are all measured in siemens.,Admittance,The equivalent admittance of a network consisting of a number of parallel branches is the sum of the admittances of the individual branches.,two-port,A general two-port with terminal voltages and currents specified. The two-port is composed of linear elements, possibly including dependent sources, but not containing any independent sources.,the two-port，which the voltage and current at the input terminals are V1 and I1, and V2 and I2 are specified at the output port. The directions of I1 and I2 are both customarily selected as into the network at the upper conductors (and out at the lower conductors).,Since the network is linear and contains no independent sources within it, I1 may be considered to be the superposition (叠加) of two components, one caused by V1 and the other by V2. When the same argument is applied to I2, we may begin with the set of equations,where the ys are no more than proportionality constants, or unknown coefficients, for the present. However, it should be clear that their dimensions must be A/V or S. They are therefore called the y parameters, and are defined by Eqs. 5 and 6.,Admittance Parameters,The y parameters, are represented concisely as matrices. Here, we define the (2 x 1) column matrix I,the (2 x 2) square matrix of the y parameters,and the (2 x 1) column matrix V,Thus, we may write the matrix equation I = yV, orand matrix multiplication of the right-hand side gives us the equalityThese (2 x 1) matrices must be equal, element by element, and thus we are led to the defining equations, 5 and 6.,a physical meaning to the y parameters is through a direct inspection of Eqs. 5 and 6. Consider Eq. 5, for example; if we let V2 be zero, then we see that Y11 must be given by the ratio of I1 to V1. We therefore describe Y11 as the admittance measured at the input terminals with the output terminals short-circuited (V2 = 0). Y11 is best described as the short-circuit input admittance.,Admittance Parameters,Alternatively, we might describe Y11 as the reciprocal of the input impedance measured with the output terminals short-circuited, but a description as an admittance is obviously more direct. It is not the name of the parameter that is important. Rather, it is the conditions which must be applied to Eq. 5 or 6, and hence to the network, that are most meaningful; when the conditions are determined, the parameter can be found directly from an analysis of the circuit (or by experiment on the physical circuit).,Admittance Parameters,Each of the y parameters may be described as a current-voltage ratio with either V1 = 0 (the input terminals short-circuited) or V2 = 0 (the output terminals short-circuited):,Admittance Parameters,Because each parameter is an admittance which is obtained by short- circuiting either the output or the input port, the y parameters are known as the short-circuit admittance parameters.The specific name of Y11 is the short-circuit input admittance, Y22 is the short-circuit output admittance, and Y12 and Y21 are the short-circuit transfer admittances.,3.2.1 形式等效电路,