Analysis of losses in non-ideal passive components in the Class-E power amplifier Dusan Milosevic, Johan van der Tang and Arthur van Roermund Eindhoven University of Technology (TU/e) Department of Electrical Engineering, Mixed-signal Microelectronics (MsM) Group, EH 5.28 P.O. Box 513, 5600 MB Eindhoven, The Netherlands phone: +31 40 247 3393, fax: +31 40 245 5674 email: d.milosevic@tue.nl Abstract— This paper analyzes some of the losses that oc- cur in non-ideal passive components employed in the Class- E power amplifier. A complete analysis of the Class-E oper- ation in the presence of the switch on-resistance and shunt capacitor ESR has been performed and an analytic depen- dence of the output efficiency on the parasitic resistances has been derived. New design equations for modified circuit ele- ments are proposed. An overview of other sources of losses in the Class-E circuit is given and the dominant ones are dis- cussed. Keywords— Power amplifier, class E operation, efficiency, passives, parasitics I. I NTRODUCTION Class-E power amplifiers were first proposed by the two Sokals in 1975. [1]. They belong to the switching type power amplifiers (PA), thus offering higher theoretical effi- ciency than conventional PA configurations. In addition to that, the Class-E PA brought a unique feature into the fam- ily of switching type amplifiers: the so-called soft switch- ing, which enabled spectacular efficiencies to be achieved, infeasible with other topologies. This intriguing circuit has aroused a lot of interest in the scientific circles, and many detailed in-depth studies have been published, par- ticularly in the late seventies and during the eighties. Some of the classic early days work include [2],[3],[4],[6]. To- day, with the rapid development of wireless communica- tion systems worldwide, Class-E PAs are becoming more interesting than ever. Their high efficiency makes them attractive for utilization in various portable radio transmit- ting devices. Through saving the battery power, higher efficiency is reflected in longer operation time and, possi- bly, smaller, lighter and more reliable handset. These im- portant issues indicate that there is a strong interest to go further in the research of this unusual, but yet promising type of PAs. This is particularly true if we keep in mind that the semiconductor technology is constantly progress- ing, offering better, faster and more reliable devices than ever. The Class-E PA is, like any other circuit, followed by the real world difficulties that spoil the performance and cause various undesired effects. The goal in this paper is to analyze losses in the Class-E circuit which are caused by the presence of the parasitic resistances in the switch and shunt capacitor. These losses may be small in com- parison with some other types of losses in the circuit, par- ticularly those associated with the active device, but still are not negligible. Moreover, it is interesting to estimate the theoretical limit for the efficiency as a function of these parasitics. The paper is organized as follows. In the second sec- tion, the analysis of the conventional (ideal) Class-E op- eration is performed. In the third section, we repeat the analysis for the cases when the circuit includes parasitic resistances; one parasitic effect is analyzed at a time. II. THE CLASS E POWER AMPLIFIER In this section we will perform an analysis of the Class- E operation and of losses that occur in some of the non- ideal passive components employed in the circuit. The equivalent circuit diagram of the Class-E PA is shown in figure 1. The circuit consists of an active device (which is shown as a switch), shunted by a capacitor C, an RF choke (RFC), a series LC resonator L 0 C 0 , an excessive reactance X and a load resistance R L . The parasitic switch inductance and electrical series resistance are denoted as L sw and r on , respectively, while r c represents the electrical series resistance of the shunt capacitor. In order to simplify the analysis, it is necessary to make certain assumptions. These assumptions are the following : 1. The transistor is modeled as an ideal switch, i.e. a short circuit in the ON state and an open circuit in the OFF state, with instant switching action. 244