7.3.2 Noise Figure of the Common-Emitter Amplifer

Chapter 7.3.2 Noise Figure of the Common-Emitter Amplifer

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 188 LNA Designwhere ==224and2bnbcncvkTriqI . Now if the input is open circuited in Figure 7.12(b), then only in can have any effect on the circuit. In this case the output noise is: 2222on_totnmii Z gπ= (7.48)Similarly for the model in Figure 7.12(a): π=+22222on_totbnmcniiZ gi (7.49)Now solving (7.48) and (7.49) for in gives: 22222CnBmqIiqIYgπ=+ (7.50)where ==222and2.bnBcnciqIiqI7.3.2  Noise Figure of the Common-Emitter AmplifierNow that the equivalent input referred noise model has been derived, it can be ap-plied to the results in Chapter 2 so that the optimum impedance for noise can be found in terms of transistor parameters.The input-referred noise current has two terms. One is due to base shot noise and one is due to collector shot noise. Since collector shot noise is present for both vn and in, this part of the input noise current is correlated with the input noise volt-age, but the other part is not. Thus: iY2222cmqIgπ=C (7.51) 22uBiqI= (7.52)Likewise, in the case of the vn: 222cmqIvg=C (7.53) 24ubvkTr= (7.54)The correlation admittance Yc can be determined (see Section 2.2.5). It will be assumed that at the frequencies of interest the transistor looks primarily capacitive. 22222CcmcCcmqIYigYYj CqIvgπππω==== (7.55)