4.11 CMOS Transistors 87only when current is reduced to half of the optimum value or when it is increased by 50% over its optimum value. Figure 4.8 also shows that junction capacitance is roughly proportional to transistor size, while base resistance is inversely propor-tional to transistor size.Detailed guidelines will be given in the design chapters; however, a few general guidelines are provided as follows: Pick a lower current to reduce power dissipation with minimal reduction of fT. As will be further discussed in Chapter 7 for LNA design, the optimal cur-rent density for minimum noise figure is also lower than the current required for highest fT, typically by about a factor of 8, so this is often a good starting point. Pick a larger transistor size to give lower base resistance. This will have a direct impact on noise. However, on the down side, large size requires large current for optimal fT. Another negative impact is that junction capacitances increase with larger transistors. The optimal size for best noise performance in an LNA is further discussed in Chapter 7. Collector shot noise power is proportional to current, but signal power gain is proportional to current squared, so more current can improve noise perfor-mance if collector shot noise is dominant. The optimal current for best noise performance in an LNA is further discussed in Chapter 7.4.11 CMOS TransistorsBipolar transistors have traditionally been preferred for RF circuits due to the higher values of gm achievable for a given amount of bias current. However, much more attention is being paid currently to CMOS for the design of radio circuits. Using CMOS allows for the design of single-chip radios, as CMOS is necessary to implement back-end digital or DSP functions. For this purpose, it is possible to use either BiCMOS or straight CMOS. With BiCMOS it is possible to use the bipolar transistors for RF, possibly adding 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, it is now quite common to implement RF circuits in a CMOS only process. ···Figure 4.13 Transistor small-signal model with noise.