High- efficiency class E power amplifier using SI-GE HBT technology

One of the most important goals of RF circuit designers is improving efficiency and autonomy in handsets for personal communications. Currently, low-level voltage supplies are used to avoid big weight and volume. Besides, autonomy and efficiency are closely linked to the energy consumed by the circuits. Power Amplifier (PA) is one of the circuits, which spends more energy from the battery. Improving the PA efficiency, handset performance is also improved. Previous publications have mentioned the difference of power amplifiers where the circuit output works as I) a current source or II) as a switch. The class E power amplifiers are devices whose output works as a switch achieving great performance and high power efficiency, reaching, in an ideal case, 100 % of efficiency. In this paper, a low-level voltage class E power amplifier is analyzed, designed, simulated, built and characterized, using a silicio-Germains HBT transistor as an active element. The circuit was optimized using a harmonic balanced simulator (Microwave Office ® [1]). 70% power efficiency was measured after an optimizing procedure, although better performance can be expected with an improved switching transistor and more precisely lumped elements. The prototype was designed to operate at 900 MHz and fed with 2.4 V, but it is well known that low-level polarization voltages affect efficiency, linearity, power gain, bandwidth, noise figure, as well as cost in RF circuits. However the class E PA reported in this paper reaches high efficiency, high power gain as well as a good bandwidth, with lowlevel polarization voltage.