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 < 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 ratios lower than -37 dBc and -55 dBc, respectively). At the maximum output power level, the worst power-added-efficiences (PAE) are measured to be 36% for 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 low-Q inductors (Q < 5) for on-chip impedance matching, imprecise device modeling and the higher interconnect parasitics; 2) SiGe power amplifiers survived severe output mismatch (VSWR > 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.
|Number of pages||6|
|Journal||IEICE Transactions on Electronics|
|State||Published - 1 Jan 2001|
- Power amplifier MMIC