Variable-load constant efficiency power amplifier for mobile communications applications

A SiGe HBT Class-B power amplifier has been developed with a voltage-controlled variable load, for constant efficiency also at low-power levels. The output matching network includes varactor diodes that can change the load impedance at the collector of the transistor. The load line can therefore he adjusted to provide a variable output power saturation level shifting the peak of the efficiency to the desired power level. Constant efficiency over a wide output power range is obtained. This amplifer minimises power consumption for variable output power applications as required in mobile communications systems.