Adaptive Power Control

Cellular phone power amplifiers usually operate in strongly fluctuating environments. The output power varies over a wide range to compensate for fluctuations in propagation path loss. The collector load impedance varies due to fluctuations in antenna body-effects [14, 15] and it varies as a function of the transmitting frequency due to the narrow bandwidth of miniaturized antennas. In addition, the supply voltage changes due to charging and de-charging of the battery. At high output power and extreme operating conditions the power transistor suffers from avalanche, self-heating and distortion due to saturation. Usually, large margins are built-in, by design and technology, in order to prevent the transistor from avalanche breakdown, thermal run-away, and severe clipping. Inevitably, these large margins deteriorate RF performance and come with additional costs in packaging, chip area, and IC-technology.