Radio Frequency Integrated Circuit Design - Webs

A Brief Review of Technology
• Pick largest transistor size to give lowest base resistance. This will have
a direct impact on noise. However, on the down side, large size requires
large current for optimal f T . Another negative impact is that junction
capacitances increase with larger transistors.
• Collector shot noise power is proportional to current, but signal power
gain is proportional to current squared, so more current can improve
noise performance if collector shot noise is dominant.
3.11 CMOS Transistors
Bipolar transistors are preferred for RF circuits due to the higher values of g m
achievable for a given amount of bias current. However, for a complete radio
on a chip, it is necessary to use a process that can be used to implement backend
digital or DSP functions. This could be BiCMOS or straight CMOS.
BiCMOS would be preferable, since bipolar can then be used for RF, possibly
adding p-channel MOS (PMOS) transistors for power-control functions.
However, for economic reasons or for the need to use a particular CMOS-only
process to satisfy the back-end requirements, one may be forced to implement
RF circuits in CMOS. For this reason, we give a brief summary of CMOS
transistors. Basic PMOS and n-channel MOS (NMOS) transistors are shown
in Figure 3.14.
Figure 3.14 CMOS transistors.
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