三、網(wǎng)絡(luò)元件與偏置電路對(duì)噪聲的影響Theinfluencesofnet

1、三、網(wǎng)絡(luò)元件與偏置電路對(duì)噪聲的影響 The influences of net components & bias circuits on noise1、串、并聯(lián)網(wǎng)絡(luò)對(duì)噪聲的影響 The influence of series、parallel network on noise a.并聯(lián)耦合網(wǎng)絡(luò) b.串聯(lián)耦合網(wǎng)絡(luò) parallel coupling network series coupling network圖中： ：源阻抗， ； Source impedance sZsssjXRZ ：并聯(lián)網(wǎng)絡(luò)阻抗， ；Parallel network impedance ：串聯(lián)網(wǎng)絡(luò)阻抗， ；Ser

2、ies network impedance ： 源電阻熱噪聲電壓， ； Thermal noise voltage of source resistance ：并聯(lián)阻抗實(shí)部噪聲電流； The real part of the noise current of parallel impedance. ：串聯(lián)阻抗實(shí)部噪聲電壓； The real part of the noise voltage of series impedance. 、 ：放大器噪聲模型；Noise model of a amplifier ：放大器電壓增益；Amplifiers voltage gain ：輸出噪聲電壓。 O

3、utput noise voltagecscscsjXRZcpZcsZcpcpcpjXRZnsEfkTREsns 42cpIcsEnEnIvAnoE（1）并聯(lián)網(wǎng)絡(luò) Parallel network 各噪聲源在輸出端開路時(shí)的分量為 The components of each noise source on output end (open circuit) are總輸出噪聲均方值為 The total output noise mean square value is (3.1.75)vcpscpnsEnoAZZZEEns)(vcpscpscpvcpscpInoAZZZZIAZZIEcp)/(

4、)(vnEnoAEEn)(vcpscpsnvcpsnInoAZZZZIAZZIEn)/()(22)(2)(2)(2)(2vInoEnoInoEnonoAEEEEEnncpns22222222vcpscpsnncpscpscpcpscpnsAZZZZIEZZZZIZZZE網(wǎng)絡(luò)電壓傳輸增益為 The network voltage transmission gain is (3.1.76)等效輸入噪聲為 The equivalent input noise is (3.1.77) (3.1.78)表明 Showing that 若欲減小并聯(lián)網(wǎng)絡(luò)對(duì)噪聲的影響，需使并聯(lián)網(wǎng)絡(luò)阻抗的直流電阻 盡可能增大，

5、同時(shí)使 上的噪聲電流 盡可能減小。 To reduce the influence of parallel network on noise, the DC resistance of the parallel network impedance should be increased as large as possible,and the noise current on the decreased as small as possible.vcpscpsvcpscpsssovAZZZVAZZZVVVK222222222)(1sncpcpsnnsvnoniZIIZZEEKEE2222222

6、22)()1(1nssncpnscpsnnsniEZIIEZZEEEFcpRcpRcpIcpRcpRcpI 中包含兩個(gè)部分 The includes two parts： 熱噪聲電壓，它與 成反比； The thermal noise voltage, which is inversely proportional to the . 流過 的低頻噪聲電流， 它與流過 的直流電平成 正比。 The low-frequency noise current flowing through the , which is directly proportional to the DC level of

7、the . 故欲減小 ，還需盡量減小 上的直流壓降。 Therefore, the DC voltage drop on the has to be reduced to reducethe .（2）串聯(lián)網(wǎng)絡(luò) Series network 先求出各噪聲源在輸出端開路時(shí)輸出的分量分別為： The output components of each noise source (open circuit) are cpIcpRcpRcpRcpRcpIcpRcpRcpRcpIcpIcpR 得總輸出噪聲電壓均方值為 Then the total output noise voltage mean squ

8、are value is電壓傳輸增益為 The voltage transmission gain is等效輸入噪聲為 The equivalent input noise is (3.1.79) vnsEnoAEEns)(vcsEnoAEEcs)(vnEnoAEEn)(vcssnInoAZZIEn)()(2222222)(vcssnncsnsnoAZZIEEEEvsvsssovAVAVVVK22222222)(cssnncsnsvnoniZZIEEEKEE表明 Showing that: 欲減小串聯(lián)網(wǎng)絡(luò)對(duì)噪聲的影響，應(yīng)使 盡可能小， 并盡量減小 上的噪聲電壓 ， 上的直流壓降也應(yīng)盡量減小。

9、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 theseries network on the system noise. （3）并串聯(lián)網(wǎng)絡(luò)（導(dǎo)出過程留在習(xí)題課中解決） The noise analysis of Parallel-Series Networks ( The derivation will be solved in exercises-lesson ) (3.

10、1.79)csEcsRcsRcsRcsEcsRcsRcsR22222222211Z)ZZ(ZIZI)ZZ)(EE(EEscpscsnscpcpsncsnsni（4）串并聯(lián)網(wǎng)絡(luò) The noise analysis of Series-Parallel Networks (3.1.80) 上二式表明 : 若欲減小網(wǎng)絡(luò)對(duì)噪聲的影響，對(duì)并聯(lián)網(wǎng)絡(luò)，要盡可能提高并聯(lián)阻抗的數(shù)值；對(duì)串聯(lián)網(wǎng)絡(luò)，要盡可能減小串聯(lián)電阻抗的數(shù)值。同時(shí)，兩者都應(yīng)盡可能減小網(wǎng)絡(luò)阻抗上的壓降，以減小低頻噪聲。 To reduce the influence of a network on noise, the parallel impe

11、dance 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 on the network impedance of the two should be decreased to reduce the low-frequency noise. 22222222)()1 (cssncpcpcssn

12、csnsniZZIIZZZEEEE2、偏置電路噪聲的影響和元件的選取 The influence of bias circuit noise & components selection 偏置電路對(duì)放大器噪聲的影響不可忽視，偏置元件的熱噪聲和低頻噪聲通過放大器放大可使系統(tǒng)噪聲特性變壞。 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 sys

13、tems noise characteristics when they pass through the amplifier.（1）晶體管共發(fā)放大器電路及其噪聲模型 The common-emitter circuit of a transistor & its noise model圖中： 、 、 、 ：上下偏置電阻及其噪聲電壓； Up-down bias resistances and their noise voltages 、 ：發(fā)射極反饋電阻及其噪聲電壓； Emitter feedback resistance and its noise voltage 、 ：發(fā)射極電阻及

14、其噪聲電壓； Emitter resistance and its noise voltage ：集電極電阻；Collector resistance ：基極耦合電容；Base coupling capacitance ：發(fā)射極旁路電容；Emitter shunt capacitance（2） 噪聲分析 Noise analysis 依對(duì)串并聯(lián)網(wǎng)絡(luò)的噪聲分析 (3.1.80) 及晶體管共發(fā)放大器等效電路可知:1R2R1E2EfRfEeReEcRcCeC According to the noise analysis (3.1.80) of series-parallel network and

15、 theequivalent circuit of the transistor common-emitter amplifier, we know that 偏置電阻 、 與源電阻并聯(lián)，阻值越大對(duì)噪聲影響越小，當(dāng)滿足 時(shí)，偏置電阻對(duì)噪聲的影響可略； 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 influen

16、ce on noise can be omitted. 為使偏置電阻有良好的溫度穩(wěn)定性， 取值要低，一般需使其保證流過 的電流為基極電流的 倍。To obtain good temperature stability of the bias resistance, the should be low enough. Usually, it should be ensured that the current flowing throughthe is times of that passing the base. 綜上二條，低噪放大器的設(shè)計(jì)應(yīng)兼顧上述兩方面要求。nnsoptIERRR21/1R

17、2R10521/ RR21/ RRnnsoptIERRR21/1R2R10521/ RR21/ RR As a summary: the design of low noise amplifier should satisfy the above two point. （3）實(shí)際電路 A practical circuit 為減小偏置電阻對(duì)放大器噪聲的影響，可采用“無噪聲偏置電路”如下： To reduce the influence of bias resistance on amplifiers noise, the following “non-noise bias circuit ”

18、can be adopted:a、無噪聲偏置電路 b、噪聲等效電路 Non-noise bias circuit Noise equivalent circuit 電路等效輸入噪聲均方值及噪聲系數(shù)的推導(dǎo)可留做作業(yè)或試題 這種電路在滿足一定數(shù)量關(guān)系時(shí)可減小偏置電路對(duì)噪聲的影響。由共發(fā)放大器等效噪聲分析結(jié)論可知其數(shù)量關(guān)系為： This kind of circuits can reduce the influence of the bias circuit on the noise, when certain magnitude relation is satisfied. From the co

19、nclusion of theequivalent noise analysis of the common-emitter amplifier, we know therelation is (3.1.81) 稱為低噪偏置電阻 Low noise bias resistance (3.1.82) 稱為低噪偏置電流 Low noise bias current 式中: 是 、 和 的并聯(lián)阻抗： where : are the parallel impedance of 、 and .22222nnsoptbgcIEEZRR22222)(nbgbgcIZREEICbZ1R2RCbZ1R2R (3

20、.1.83) 是 、 上的噪聲電壓 、 在 兩端的合成電壓。 is the synthetic voltage of the noise voltages and on and between the two ends of . (3.1.84)212121211/RCRjRRRRCjRRZbbE1E2E21212222122121/1/1/1/CjRRCjRECjRRCjREEbbZ1R2R1E2EbZ1R2RbE22212222121222221)(RRCRRRERE分析 Analysis: 由(3.1.81)、(3.1.82)兩式知: 為減小偏置電路對(duì)噪聲的影響，需使 大， 、要小。又由

21、(3.1.84)可知:由于在偏置電路中并聯(lián)了電容 C ，使 比不并聯(lián)電容時(shí)要小， 當(dāng) C 充分大時(shí)，可使 很小。這意味著偏置電路的熱噪聲和低頻噪聲都很小。 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 la

22、rge enough, the can be very small, which means that both the thermal noise and low frequency noise in the bias circuit are small. bgZRgE2bE2bEbEbgZRgE2bE2bEbE 當(dāng) C 充分大時(shí), 很小。為滿足 (3.1.81)、 (3.1.82) 兩式的要求,可使 足夠大, 此時(shí)兩個(gè)條件變?yōu)? When C is large enough, is quite small. To satisfy Eqs.(3.1.81) & (3.1.82), t

23、he can be chosen large enough, and then the two conditions become (3.1.85) (3.1.86) 在實(shí)際電路中，由于 兩端直流壓降很小，故低頻噪聲可略，而只要 足夠大， 中的熱噪聲電流均方值就很小。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,.CCjZb11nngIERgRCCjZb11gR2222)(ngbgIREEgngRfkTI 42gRgRgRgRgRgRgngRfkTI 42另