MMIC power amplifier applications of heterojunction bipolar transistors (HBTs)

This paper compares the performance of SiGe and GaAs HBT power amplifiers for wireless handset applications. To make a fair comparison, we have designed and characterized monolithic SiGe power amplifiers and compared their performance with similarly designed commercial GaAs power amplifiers for both cellular dual-mode (CDMA/AMES) and PCS CDMA handsets. The designed SiGe cellular power amplifier, at 824-849 MHz, satisfies both CDMA and AMPS requirements in output power, linearity and efficiency. At Vcc = 3V, the power amplifier shows excellent linearity (1st ACPR 12:1) up to Vcc = 44V but died instantly as Vcc > 4.5 V, due to their low breakdown voltages. We also observed inter-modulation spurs (-22 dBc) appeared in CDMA outputs at two specific tuning angles, but with no spurs appeared in AMPS outputs at any tuning angle. The possible mechanism for generating those output spurs will be discussed as well. In addition, We also designed and characterized a monolithic SiGe power amplifier for PCS (1850-1910 MHz) CDMA handset applications. At Vcc = 3.5 V, the SiGe PA satisfies the linearity requirement up to maximum power output 28 dBa with a comparable gain (23-26 dBm), but has a relatively low PAE (25%) compared with that of GaAs counterparts at the high output power end.