- Buyers Guide
High Voltage HBTs for 3G W-CDMA Base Station Power Amplifiers
A new high voltage, high power InGaP HBT discrete device has been developed that can significantly improve the efficiency of power amplifiers (PA) in cellular base stations. This new technology provides superior performance compared to silicon LDMOS. A 30-watt device driven by a predistorted W-CDMA signal has demonstrated 45 percent collector efficiency at greater than 45 dBc adjacent channel power ratio (ACPR). This is a significant improvement in efficiency over published LDMOS results. Efficiency is an extremely important characteristic as it drives the size and cost of the PA and associated cooling equipment required to remove the excess heat generated. Ongoing electrical cost savings due to the related system efficiency improvement provides another significant advantage offered by InGaP HBT technology.
Base station power amplifier requirements for W-CDMA applications pose significant challenges to existing semiconductor device technologies. The RF power amplifier must provide highly linear operation in order to ensure minimal band-to-band cross talk and out-of-band spurious signal generation. Power amplifier designers are further challenged by the requirement that the peak-to-average power ratio (PAR) of an unclipped W-CDMA signal be in excess of 9 dB. Typically, device efficiency must be compromised to realize the required linear amplification. Even with the application of crest reduction and digital predistortion algorithms, simultaneously achieving the required ACPR and high power-added efficiency (PAE) is a challenge.
The TriQuint TGH29 series of packaged prematched discrete power transistors provides an unparalleled combination of power, efficiency and linearity, specifically targeted for use in W-CDMA base station applications. These devices utilize the company’s proven high reliability, high voltage InGaP HBT technology to realize 8 to 10 points greater efficiency when compared to similar silicon LDMOS devices.
Several design and production enhancements contribute to the success of the TGH29 line including an HBT unit cell that incorporates integrated emitter ballast resistors to minimize thermal runaway. The device design also employs thermal shunts to effectively remove heat from the active region of the transistor. The devices are 100 percent screened to a minimum 68 V breakdown and are compatible with existing 28 V base station power supply voltages. The package also incorporates a proprietary input prematch design that utilizes low cost, passive GaAs MMIC technology to monolithically integrate optimized matching elements onto a single die. This approach improves manufacturability over chip and wire solutions resulting in uniform device-to-device performance and repeatability. The TGH series HBTs are also surface passivated to ensure reliable operation in non-hermetic environments and currently utilize industry standard base station transistor packages.
An important feature of InGaP high voltage HBT technology is high linearity when operated under a near class-B bias condition. The low quiescent current results in high PAE while simultaneously maintaining good linearity performance. Test results for a TGH2910-FL 10 W driver amplifier evaluation board biased at a near class-B quiescent point of 28 V and 30 mA are plotted in Figure 1. The stimulus is a single channel, unclipped W-CDMA (64 DPCH), 3.84 MHz channel bandwidth, 2140 MHz carrier frequency and a 9.6 dB peak-to-average ratio. For an average output power of 30.5 dBm, the amplifier is capable of 30 percent collector efficiency at greater than 45 dBc ACPR. The upper and lower channel ACP levels are symmetric, indicating minimal memory effect, and greater than 45 dBc ACPR is maintained as the input drive power is reduced.
Similar performance characteristics have been observed for higher power prematched devices. Evaluation board data for a TGH2930-FL 30 W prematched power device biased at a near class-B quiescent point is shown in Figure 2. Under the same W-CDMA stimulus conditions previously described, the amplifier demonstrated 28 percent collector efficiency with better than 45 dBc ACPR at an average output power of 34.2 dBm.
The TGH series of prematched HBT power amplifiers also demonstrate superior performance with crest reduction and digital predistortion. Experimental single and four-carrier W-CDMA results for a TGH2930-FL 30 W driver amplifier utilizing commercially available crest factor reduction and digital predistortion algorithms from PMC-Sierra are shown in Figures 3 and 4. Crest reduction circuitry was used to reduce the peak-to-average ratio of the W-CDMA signal to 5.5 dB (EVM < 12 percent). This signal serves as the input to the digital predistortion system and HBT power amplifier running closed loop. For single carrier W-CDMA, the combination of crest reduction and predistortion enables the amplifier to demonstrate 42 percent collector efficiency and 40 dBm output average power with better than 50 dBc ACPR. As evident in Figures 3 and 4, the application of digital predistortion yields a greater than 10 dB improvement in the adjacent channel power ratio. Data was also collected for a two-carrier stimulus utilizing the same experimental conditions, signals and evaluation board. The results are summarized in Table 1.
The TGH series of power amplifier devices are manufactured in an ISO 9001:2000 facility with 100 percent lot traceability and statistical process controls. The die are 100 percent DC tested. The robust intrinsic nature of the GaAs material along with all-gold metallization results in an extremely rugged device. TriQuint’s InGaP HBT exceeds both 106 hours mean-time-to-failure (MTTF) at 150°C junction temperature and 10,000 hours MTTF at 250°C. Devices are currently being sampled to select lead customers. General availability of packaged discrete power transistors is planned for early 2005.