RF power amplifiers, classes A through S-how they operate, and when to use each

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 either 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 ofthe circuit topology is not sufficient to define unambiguously the transistor operating mode or the amplifier class of operation. Examples are given of the same circuit topology operating as a Class B, C, or E amplifier, depending on how the transistor is biased and driven, and on the specific L-R-C values in the load network. Modes of operation and applications of each type of amplifier are discussed.