6.9 Power Gain

The load network corresponding to any type of Class-E mode, in order to realize the idealized switching conditions, is tuned to provide inductive or capacitive impedance at the fundamental frequency, thus violating the conjugate matching conditions required for conventional Class-B operation to provide maximum power delivery to the load. This means that generally the output voltage and current waves consist of both incident and reflected components. Besides, the power gain in a switching mode is normally lower than in a conventional mode because it requires higher driving voltage to realize a device voltage-saturation mode. In this case, the ratio of the power gain in a Class-E power amplifier to that in a Class-B power amplifier, for the same output power, is inversely proportional to a squared ratio of their voltage peak factors [36]. For a Class E with one inductor and one capacitor, a maximum operating power gain Gp(E)max of a single-stage bipolar power amplifier can be estimated by where f is the operating frequency, V _th is the threshold voltage, and it is assumed that the effect of a non-zero fall time is negligible [37].

However, it is very important to qualitatively compare the power gains of the Class-B and Class-E power amplifiers as functions of the device and load-network parameters. In this case, a parallel-circuit Class-E mode looks very attractive since it provides the highest value of load resistance compared with other Class-E alternatives. The operating power gain G _p, expressed through the...