RF Power Amplifier
Power, efficiency, linearity: pick any three such a choice just about summarizes the challenges presented to the power amplifier (PA) designer in 2005. As wireless systems pack more data into limited bandwidths, power amplifiers carry a bigger burden but are still required to be efficient. Simultaneously, in order to maintain high information throughput from an ever-increasing number of users, the specifications on adjacent channel interference keep getting more stringent, and more efficient modulation systems keep demanding more dynamic range and higher instantaneous signal bandwidth. Meeting these challenges has evolved into an interesting blend of old and new techniques in device technology, PA architecture, and an ever-increasing intrusion of high speed digital signal processing (DSP) into the very traditional analog preserve of RF power engineering.
From the PA viewpoint, things did not quite go to plan for GaAs fabrication shareholders. Two new transistor technologies emerged to dominate both ends of the mobile phone PA markets: silicon laterally diffused metal oxide semiconductor (LDMOS) for high power base stations and GaAs heterojunction bipolar transistor (HBT) for handsets. Neither technology would have attracted backers in the early 90s to dominate the scene in the way they have done; their emergence is an interesting case study. At the high power end, cost matters most, and although LDMOS needed careful stretching to meet performance goals at 2 GHz, the lavishly over-performing GaAs FET technology appeared to be more expensive on a dollar-per-watt basis. In the battery-powered environment of the mo- problem, which all carry the name of their accredited inventors: Kahn, Doherty and Chireix.2C4 It is probably a fair statement to say that just about any company or research group involved with PA design is putting major R&D efforts into at least one of these techniques at the present time.