Leadless, Multi-band, High Performance GaAs HBT Driver Amplifiers for Cellular Base Station Infrastructure
As cellular system manufacturers begin to roll-out advanced transceiver and multi-channel power amplifier (MCPA) designs destined for the emerging 2.5G and 3G markets, a new series of robust driver power amplifiers is introduced offering users a lower cost, high performance platform approach to base station infrastructure amplification needs.
The RF Micro Devices“ RF3800 series of driver amplifiers provides up to +37 dBm of output power (P1dB), high power efficiency (> 35 percent at P1dB), high linearity (+50 dBm OIP3) and higher gain (14 to 20 dB) under linear class AB operation, as compared with alternate Si-LDMOS discrete transistors commonly used in similar infrastructure power amplifier applications. Table 1 lists the RF3800 series amplifiers’ RF performance, while Table 2 shows a comparison with Si-LDMOS driver amplifiers for base station applications. Utilizing a mature, highly uniform GaAs heterojunction bipolar transistor (HBT) technology, this high yielding semiconductor process provides a lower cost, multi-band platform design approach for infrastructure applications ranging from 450 to 2200 MHz for WCDMA platforms.
The series of HBT amplifiers features a higher breakdown voltage, which enables an 8 V collector bias. The higher bias voltage provides higher output power, higher efficiency and higher linearity. To further enhance ruggedness and to mitigate temperature variation, the RF3800 series is designed incorporating both base and emitter ballasting to prevent thermal runaway at the 8 V bias level. The power performance of the RF3802 (860 to 960 MHz band) is shown in Figure 1. The phase variation of the amplifier maintains between –1° and 1°, which is highly desirable in typical GSM applications. Compared to low power Si-LDMOS discrete transistors, the RF3800 amplifiers exhibit superior linearity and CDMA performance. Figure 2 shows the two-tone measurements of the RF3802 exhibit higher than 50 dBm OIP3 at more than 30 dBm output power. In addition, RFMD’s proprietary active-bias architecture is incorporated to lower the quiescent bias current for optimum thermal performance, while maintaining a high third-order intercept point. The CDMA2000 performance of the RF3802 is displayed in Figure 3 with associated efficiency. The amplifier demonstrates 31 dBm CDMA2000 output power at –50 dBc ACPR, at only –5 dB back-off from the 36 dBm P1dB. Lower back-off means that the amplifier can be operated at a higher efficiency with lower junction temperature (Tj) in the harsh base station environment.
The RF3800 series is assembled in a thermally enhanced aluminum nitride (AlN) LCC-8 package and provides a more robust option to lower power (< 2 W P1dB) GaAs MMIC plastic-packaged pre-driver amplifiers and is a cost-effective alternative to higher power (> 10 W P1dB) flange-mount Si-LDMOS transistors. The LCC-8 leadless package is footprint compatible with the industry standard SOIC-8 and requires minimal PC board artwork changes for users seeking more head-room from their pre-driver amplifiers than what is typically seen in the leaded slug plastic packages. This AlN LCC-8 package is inherently lead(Pb)-free and eliminates the need for costly re-qualification efforts, which are necessary to conform to the European mandated “Restricted use of Hazardous Substances” (RoHS) compliance. High thermal dissipating material allows for direct attachment methods, which helps reduce lead co-planarity, a challenge in the high power amplifier assembly processes. The high thermal conductivity of AlN allows the packaging of devices with dissipating power levels of 8 W or above. As observed in Figure 4, the measured thermal performance of the AlN LCC-8 package (Rth = 14.5°C/W) out-performs all of the similar “high power” packages typically available for pre-driver amplifiers of less than 2 W P1dB, such as plastic SOIC-8 slug, QFN-16 slug and a metal-base ceramic leaded package. All package designs compared in this analysis were evaluated using a high thermal-based epoxy to eliminate the effect of the die attach material from the evaluation. Another significant advantage of the AlN package is its ability to utilize high thermal dissipating die attach techniques, such as eutectic die attach. This attachment method further reduces the junction temperature (Tj) under RF drive conditions and enhances the reliability of the amplifier assembly (see Figure 5). The AlN package incorporates a hermetic seal and is rated MSL 1 at level 260, as compared with alternate plastic packages with rated MSL levels of 3 or higher. This eliminates the concern of “pop-corning” in assembly and provides a highly reliable package for base station infrastructure applications. Lastly, for high volume contract assembly capability, the AlN LCC-8 package is tape-and-reel ready, accommodating automated assembly techniques
As more demanding voice and data cellular handset applications push infrastructure manufacturers to begin new transceiver developments to manage growing capacity needs, the RF3800 series of driver amplifiers provides engineers with a rugged, cost-effective platform design for their highly linear amplifier requirements. The use of a high volume GaAs HBT fabrication process combined with a low cost thermally enhanced AlN LCC-8 package provides potential cost savings of greater than 30 percent, as compared to alternate Si-LDMOS transistors that commonly utilize a costly metal-base ceramic leaded package. Further cost savings are realized when these platform amplifiers are used within multiple transceiver or high power amplifier designs (RF3805 for DCS, PCS, and UMTS frequency bands, for example), providing users a lower per-unit cost through economies-of-scale.
RF Micro Devices,