7.3.6 Effect of the Cascode on Noise Figure

Chapter 7.3.6 Effect of the Cascode on Noise Figure

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

  • 7.3 Noise in Amplifiers 203similar to the bipolar case. Finally, the gate induced noise power is inversely pro-portional to gm, δωα»´2222,45ggsmLno ikTCvgRI (7.77)hence, the noise figure will also improve with the increasing current, unlike the noise figure due to the base shot noise in a bipolar circuit, which degrades with an increasing current. Thus, with CMOS circuits with the simple models, the noise figure generally improves with an increasing bias current, and an optimum point is hard to identify. However, with the full models as used in a simulator, an opti-mum point will actually be seen due to other effects beyond the simple models. For example, as mentioned earlier, the effective value for g used to determine the drain channel noise increases with bias current. As well, since the signal does not keep in-creasing with increasing current, for example, when the current density approaches or exceeds that for the maximum fmax, the noise figure will degrade. 7.3.6  Effect of the Cascode on Noise FigureAs discussed in Section 7.1.3, the cascode transistor is a common-base amplifier with a current gain close to 1. By Kirchoff’s current law (KCL) of the dotted box in Figure 7.26, ic2 = ie2 – ib2 » ie2. Thus, the cascode transistor is forced to pass the current of the driver on to the output. This includes signal and noise current. Thus, to a first order, the cascode can have no effect on the noise figure of the amplifier. However, in reality it will add some noise to the system. For this reason, the cascode LNA can never be as low noise as a common-emitter amplifier. Figure 7.26  A cascode LNA showing noise sources.