MMIC power amplifier applications of heterojunction bipolar transistors (HBTs)

This paper compares the performance of SiGe and GaAs HBT power amplifiers f or wireless handset applications. To make a fair comparison, we have design ed and characterized monolithic SiGe power amplifiers and compared their pe rformance with similarly designed commercial GaAs power amplifiers for both cellular dual-mode (CDMA/AMPS) and PCS CDMA handsets. The designed SiGe ce llular power amplifier, at 824-849 MHz, satisfies both CDMA and AMPS requir ements in output power, linearity and efficiency. At Vcc = 3 V, the power a mplifier shows excellent linearity (1st ACPR < -44.1 dBc and 2nd ACPR < -57 .1 dBc) up to 28 dBm for CDMA applications. Under the same bias conditions, the power amplifier also meets AMPS handset requirements in output power ( up to 31 dBm) and linearity (with 2nd and 3rd harmonic to fundamental ratio s lower than -37 dBc and -55 dBc, respectively). At the maximum output powe r level, the worst power-added-efficiencies; (PAE) are measured to be 36% f or CDMA and 49% for AMPS operations. The performance of SiGe cellular power amplifiers is comparable to that of GaAs HBT power amplifiers but with two exceptions: 1) SiGe power amplifier showed a relatively low gain than that of GaAs amplifiers (about 4-6 dB). This may be attributed to the use of lo w-Q inductors (Q < 5) for on-chip impedance matching, imprecise device mode ling and the higher interconnect parasitics; 2) SiGe power amplifiers survi ved severe output mismatch (VSWR > 12:1) up to Vcc = 4V 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. Th e possible mechanism for generating those output spurs will be discussed as well. In addition, We also designed and characterized a monolithic SiGe po wer 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 o utput 28 dBm with a comparable gain (23-26 dBm), but has a relatively low P AE (similar or equal to 25%) compared with that of GaAs counterparts at the high output power end.