三网络元件与偏置电路对噪声的影响学习课件PPT学习教案

1、会计学1 三网络元件与偏置电路对噪声的影响学三网络元件与偏置电路对噪声的影响学 习课件习课件 ：并联网络阻抗， ；Parallel network impedance ：串联网络阻抗， ；Series network impedance ： 源电阻热噪声电压， ； Thermal noise voltage of source resistance ：并联阻抗实部噪声电流； The real part of the noise current of parallel impedance. ：串联阻抗实部噪声电压； The real part of the noise voltage of se

2、ries impedance. 、 ：放大器噪声模型；Noise model of a amplifier ：放大器电压增益；Amplifiers voltage gain ：输出噪声电压。 Output noise voltage cscscs jXRZ cp Z cs Z cpcpcp jXRZ ns E fkTRE sns 4 2 cp I cs E n E n I v A no E 第1页/共18页 （1）并联网络 Parallel network 各噪声源在输出端开路时的分量为 The components of each noise source on output end (op

3、en circuit) are 总输出噪声均方值为 The total output noise mean square value is (3.1.75) v cps cp nsEno A ZZ Z EE ns )( v cps cps cpvcpscpIno A ZZ ZZ IAZZIE cp )/( )( vnEno AEE n )( v cps cps nvcpsnIno A ZZ ZZ IAZZIE n )/( )( 22 )( 2 )( 2 )( 2 )( 2 vInoEnoInoEnono AEEEEE nncpns 2 2 22 2 2 2 2 v cps cps nn cps

4、 cps cp cps cp ns A ZZ ZZ IE ZZ ZZ I ZZ Z E 第2页/共18页 网络电压传输增益为 The network voltage transmission gain is (3.1.76) 等效输入噪声为 The equivalent input noise is (3.1.77) (3.1.78) 表明 Showing that 若欲减小并联网络对噪声的影响，需使并联网络阻抗的直流 电阻 尽可能增大，同时使 上的噪声电流 尽可能减小。 To reduce the influence of parallel network on noise, the DC

5、resistance of the parallel network impedance should be increased as large as possible, and the noise current on the decreased as small as possible. v cps cp sv cps cp s s so v A ZZ Z VA ZZ Z V V V K 222222222 )(1 sncpcpsnnsvnoni ZIIZZEEKEE 2 2 22 2 22 2 2 )( )1( 1 ns s ncp ns cpsn ns ni E Z II E ZZE

6、 E E F cp R cp R cp I cp R cp R cp I 第3页/共18页 中包含两个部分 The includes two parts： 热噪声电压，它与 成反比； The thermal noise voltage, which is inversely proportional to the . 流过 的低频噪声电流， 它与流过 的直流电平成 正比。 The low-frequency noise current flowing through the , which is directly proportional to the DC level of the . 故欲

7、减小 ，还需尽量减小 上的直流压降。 Therefore, the DC voltage drop on the has to be reduced to reduce the . （2）串联网络 Series network 先求出各噪声源在输出端开路时输出的分量分别为： The output components of each noise source (open circuit) are cp I cp R cp R cp R cp R cp I cp R cp R cp R cp I cp I cp R 第4页/共18页 得总输出噪声电压均方值为 Then the total out

8、put noise voltage mean square value is 电压传输增益为 The voltage transmission gain is 等效输入噪声为 The equivalent input noise is (3.1.79) vnsEno AEE ns )( vcsEno AEE cs )( vnEno AEE n )( vcssnIno AZZIE n )( )( 2222222 )( vcssnncsnsno AZZIEEEE vsvs s so v AVAV V V K 22222222 )( cssnncsnsvnoni ZZIEEEKEE 第5页/共18页

9、 表明 Showing that: 欲减小串联网络对噪声的影响，应使 尽可能小， 并尽量 减小 上的噪声电压 ， 上的直流压降也应尽量减小。 All the , the noise voltage on the and the DC voltage drop on the should be decreased as small as possible to reduce the influence of the series network on the system noise. （3）并串联网络（导出过程留在习题课中解决） The noise analysis of Parallel-S

10、eries Networks ( The derivation will be solved in exercises-lesson ) (3.1.79) cs E cs R cs R cs R cs E cs R cs R cs R 222222222 11Z) Z Z (ZIZI) Z Z )(EE(EE s cp s csnscp cp s ncsnsni 第6页/共18页 （4）串并联网络 The noise analysis of Series-Parallel Networks (3.1.80) 上二式表明 : 若欲减小网络对噪声的影响，对并联网络，要尽可能提高并联 阻抗的数值；对

11、串联网络，要尽可能减小串联电阻抗的数值。同时， 两者都应尽可能减小网络阻抗上的压降，以减小低频噪声。 To reduce the influence of a network on noise, the parallel impedance should be increased as large as possible for a parallel network; the series impe- dance should be decreased as small as possible for a series network. Meanwhile, the voltage drop

12、on the network impedance of the two should be decreased to reduce the low-frequency noise. 22222222 )()1 ( cssncp cp css ncsnsni ZZII Z ZZ EEEE 第7页/共18页 2、偏置电路噪声的影响和元件的选取 The influence of bias circuit noise & components selection 偏置电路对放大器噪声的影响不可忽视，偏置元件的热噪 声和低频噪声通过放大器放大可使系统噪声特性变坏。 The influence of bi

13、as circuit on an amplifiers noise cannot be ignored. The thermal noise of bias components and low-frequency noise can degene-rate systems noise characteristics when they pass through the amplifier. （1）晶体管共发放大器电路及其噪声模型 The common-emitter circuit of a transistor & its noise model 第8页/共18页 图中： 、 、 、 ：上

14、下偏置电阻及其噪声电压； Up-down bias resistances and their noise voltages 、 ：发射极反馈电阻及其噪声电压； Emitter feedback resistance and its noise voltage 、 ：发射极电阻及其噪声电压； Emitter resistance and its noise voltage ：集电极电阻；Collector resistance ：基极耦合电容；Base coupling capacitance ：发射极旁路电容；Emitter shunt capacitance （2） 噪声分析 Noise

15、analysis 依对串并联网络的噪声分析 (3.1.80) 及晶体管共发放大器 等效电路可知: 1 R 2 R 1 E 2 E f R f E e R e E c R c C e C 第9页/共18页 According to the noise analysis (3.1.80) of series-parallel network and the equivalent circuit of the transistor common-emitter amplifier, we know that 偏置电阻 、 与源电阻并联，阻值越大对噪声影响越小， 当满足 时，偏置电阻对噪声的影响可略；

16、 Bias resistances 、 are in parallel with source resistance. The larger the resistance is, the smaller the influence on noise is. When fulfilling the condition of , the influence on noise can be omitted. 为使偏置电阻有良好的温度稳定性， 取值要低，一般 需使其保证流过 的电流为基极电流的 倍。 To obtain good temperature stability of the bias re

17、sistance, the should be low enough. Usually, it should be ensured that the current flowing through the is times of that passing the base. 综上二条，低噪放大器的设计应兼顾上述两方面要求。 nnsopt IERRR 21/ 1 R 2 R 105 21/R R 21/R R nnsopt IERRR 21/ 1 R 2 R 105 21/R R 21/R R 第10页/共18页 As a summary: the design of low noise amp

18、lifier should satisfy the above two point. （3）实际电路 A practical circuit 为减小偏置电阻对放大器噪声的影响，可采用“无噪声偏置 电路”如下： To reduce the influence of bias resistance on amplifiers noise, the following “non-noise bias circuit ” can be adopted: a、无噪声偏置电路 b、噪声等效电路 Non-noise bias circuit Noise equivalent circuit 第11页/共18

19、页 电路等效输入噪声均方值及噪声系数的推导可留做作业或试题 这种电路在满足一定数量关系时可减小偏置电路对噪声的 影响。由共发放大器等效噪声分析结论可知其数量关系为： This kind of circuits can reduce the influence of the bias circuit on the noise, when certain magnitude relation is satisfied. From the conclusion of the equivalent noise analysis of the common-emitter amplifier, we k

20、now the relation is (3.1.81) 称为低噪偏置电阻 Low noise bias resistance (3.1.82) 称为低噪偏置电流 Low noise bias current 式中: 是 、 和 的并联阻抗： where : are the parallel impedance of 、 and . 222 2 2 nnsoptbg c IEEZRR 2 2 222 )( nbgbg c IZREEI C b Z 1 R 2 R C b Z 1 R 2 R 第12页/共18页 (3.1.83) 是 、 上的噪声电压 、 在 两端的合成电压。 is the sy

21、nthetic voltage of the noise voltages and on and between the two ends of . (3.1.84) 2121 21 21 1 / RCRjRR RR Cj RRZb b E 1 E 2 E 2 12 1 2 2 2 21 2 2 1 2 1 / 1 / 1 / 1 / Cj RR Cj R E Cj RR Cj R EEb b Z 1 R 2 R 1 E 2 E b Z 1 R 2 R b E 2 2 2 1 222 21 2 1 2 2 2 2 2 1 )(RRCRR RERE 第13页/共18页 分析 Analysis:

22、由(3.1.81)、(3.1.82)两式知: 为减小偏置电路对噪声的影响，需使 大， 、 要小。又由(3.1.84)可知:由于在偏置电路中并联了电容 C ，使 比不并联电容时要小， 当 C 充分大时，可使 很小。这意味着 偏置电路的热噪声和低频噪声都很小。 To reduce the influence of the bias circuit on the noise, the should be large, the and be small. From (3.1.84), we know that the in the bias circuit with a parallel capaci

23、tance C is smaller than that without C. When C is large enough, the can be very small, which means that both the thermal noise and low frequency noise in the bias circuit are small. bg ZR g E 2 b E 2 b E b E bg ZR g E 2 b E 2 b E b E 第14页/共18页 当 C 充分大时, 很小。为满足 (3.1.81)、 (3.1.82) 两式的要求,可使 足够大, 此时两个条件

24、变为: When C is large enough, is quite small. To satisfy Eqs.(3.1.81) & (3.1.82), the can be chosen large enough, and then the two conditions become (3.1.85) (3.1.86) 在实际电路中，由于 两端直流压降很小，故低频噪声可 略，而只要 足够大， 中的热噪声电流均方值 就很小。In practical circuits, the low frequency noise can be omitted, because the DC voltage drop on the two ends of the is small. The mean square value of the thermal current of the will be very small, so long as the is large enough,. CCjZb11 nng IER g R CCjZb11 g R 2222 )( ngbg IREE gng RfkTI 4 2 g R g R g R g R g R g R gng RfkTI 4 2 第15页/共18页 另外，由于 很小，所以 就更小。 Besides, the