Date of Award
MS in Electrical Engineering
Vladimir Ivanov Prodanov
In modern wireless communication systems, a base station typically serves a few hundred users within its cell coverage. To combat the near-far problem – the situation where a nearby user’s strong cellular signal masks the cellular signal of a faraway user – base stations continually enforce power control. That is, nearby users must lower their transmit power. In CDMA technology, power control can be as large as 70-80dB. At low power outputs, this greatly impacts the performance of the RF power amplifier (PA) in the cellular device. For small RF drives, the magnitude of the output RF current approaches the magnitude of the DC current and thus the efficiency suffers. Operating the RF PA in class C operation improves the efficiency, but results in poor linearity.
Several methods of so-called dynamic biasing have been proposed. These strategies entail lowering the bias of the PA as the RF drive increases. The proposed methods, however, fail to explain how to achieve linearity and low third-order intermodulation distortion. Additionally, the methods utilize open-loop implementations.
This work presents a novel dynamic biasing topology that results in a much improved linear class C PA. The topology utilizes a closed loop that cleverly senses the operating conditions of the "power device." Particularly, the loop operates on the principle of keeping the conduction angle remarkably constant and thereby ensuring linearity. The work details a thorough design methodology that should provide assistance to a designer wanting to implement the topology in an RF integrated circuit. Agilent ADS simulations and laboratory results from a functional PCB prototype bring merit to the topology.