Small-signal substrate resistance effect in RF CMOS cascode amplifier

With common-source RF application amplifier, it is well known that the small substrate resistance helps to improve the output resistance as well as the transconductance. This idea can be easily extended to all CMOS transistors in RF applications. However, with cascode amplifier at high frequencies, the maximum available gain, noise figure minimum, and the tuned output impedance are improved by increasing the substrate resistance of the common-gate transistor, so that the range of operational frequency can be extended. These contradicting phenomenons between the common-source and common-gate topology can be explained theoretically, and the supporting measurement results are presented base on a 0.35 μm CMOS technology.