RF power amplifiers-classes A through S

With at least ten lettered classes of RF power amplifiers and several combinations of those classes, it is not surprising that many RF engineers are confused about power amplifiers. The complexity of the subject is compounded by the fact that the RF power transistor acts as a high-resistance current source or as a low-resistance switch, or - in some amplifiers - as a high-resistance current source during part of the" on" interval and as a low-resistance switch during another part of the" on" interval (mixed-mode operation). Inspection of the circuit topology is not sufficient to define unambiguously the transistor operating mode or the amplifier class of operation. In this paper the authors present the facts to clarify matters.<>