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 be 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 amplifier minimises power consumption for variable output power applications, as required in mobile communications systems.