Analysis of Current Efficiency for CMOS Class-B LC Oscillators

This paper focuses on the study of current efficiency in CMOS class-B LC oscillators. Exact expressions for current efficiency have been derived for the class-B oscillator. Theoretically, to achieve optimal figure-of-merit (FoM), the current efficiency in class-B oscillators is between 0.60 and 0.85, which revises the common assumption that current efficiency in all class-B oscillators is equal to 2/pi. According to the theoretical analysis, the character of the tail capacitance and the boundary between current-limited regime and voltage-limited regime are discussed as well. A closed-form FoM expression is given with comparisons among different oscillators (including the traditional class-B, the AC-coupled class-B and the class-C), which show that the current efficiency of the two kinds of class-B oscillators is identical and the current efficiency of class-C oscillators is between 0.85 and 1. It is also shown that the main difference between the performances of these oscillators lies in voltage efficiency rather than current efficiency.