三,网络元件与偏置电路对噪声的影响.ppt

1、三、网络元件与偏置电路对噪声的影响 The influences of net components the series impe- dance should be decreased as small as possible for a series network. Meanwhile, the voltage drop on the network impedance of the two should be decreased to reduce the low-frequency noise.,2、偏置电路噪声的影响和元件的选取 The influence of bias circ

2、uit 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 bias 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.

3、 （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,图中： 、 、 、 ：上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 r

4、esistance ：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 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:,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

5、、 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； 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

6、good temperature stability of the bias resistance, 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。,As a summary: the design of low noise amplifier should satisfy the above two point. （3）实际电路 A pr

7、actical 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, 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

8、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 know the relation is (3.1.81) 称为低噪偏置电阻 Low noise bias resistance

9、(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 .,(3.1.83) 是 、 上的噪声电压 、 在 两端的合成电压。 is the synthetic voltage of the noise voltages and on and between the two ends of . (3.1.84),分析 Analysis: 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

10、)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 capacitance C is smaller than that without C. When C is large e

11、nough, the can be very small, which means that both the thermal noise and low frequency noise in the bias circuit are small., 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 th

12、en 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 lo

13、ng as the is large enough,.,另外，由于 很小，所以 就更小。 Besides, the is even smaller, because is quite small. 满足上述条件的偏置电路称为“无噪声偏置电路”。 The bias circuit satisfying above conditions is called “non-noise bias circuit”. 电路图中的发射极阻容网络 及基极中的耦合电容 都是与源串联的，为减小其对放大器噪声的影响，应使 Both the emitter RC network and base coupling capacitor are in series with the source. To reduce its influence on amplifier noise, the should make following Eqs. hold (3.1.87) (3.1.88),因此, 与 应选得较大, 要小。 Therefore, both and should be larger, and smaller. 同样， 也串在源上，其值也不宜过大，为提高放大器输 入负载而加大射