Powering a WiMAX base station is not a trivial matter. The wireless application occupies the 3.5-GHz band among others, which is too high in frequency for traditional RF transistors and too high in power for most microwave devices. In the 2.5 GHz WiMAX band, power-amplifier (PA) designers have been able to choose from both silicon LDMOS FETs and compound semiconductor devices such as GaAs FETs. But the silicon devices have lacked the power, linearity, and efficiency of their higher-priced GaAs counterparts at higher frequency. That is, until Freescale Semiconductor developed a "WiMAX-worthy" LDMOS device capable of the power, efficiency, and linearity needed for 3.5-GHz WiMAX PA applications.
These new LDMOS FETs have demonstrated performance as high as 3.8 GHz. But bandwidth alone is not enough to satisfy WiMAX requirements at 3.5 GHz. WiMAX employs 64-state quadrature amplitude modulation (64QAM) and orthogonal frequency-division multiplexing (OFDM), which together demand exceptionally high linearity. WiMAX systems also require good error-vector-magnitude (EVM) performance as a benchmark of modulation fidelity.
For improved linearity, most power devices are used "backed off" from maximum power levels. When backed off, the new LDMOS devices from Freescale provide higher average power than many comparable GaAs devices. The new Freescale LDMOS lineup includes the models MRF7S38010H, MRF7S38040H, and MRF7S38075H (see figure), with 10, 40, and 75 W of peak output power, respectively. These devices provide average power levels of 2, 8, and 16 W from 3.4 to 3.8 GHz when tested using the 7-MHz-wide WiMAX IEEE 802.16 signal (see table).
The cost of these new LDMOS devices is about 30 to 80 percent less than compound semiconductor devices with comparable performance. Over the cost of an entire WiMAX system, the savings by using LDMOS can be quite large.
As impressive as this increase in LDMOS frequency range might be, Freescale has chosen to pursue parallel development paths to serve WiMAX, devoting considerable resources to producing GaAs PHEMT devices that meet similarly demanding requirements. Freescale is also developing GaN technology for high-frequency, high-power applications through 6 GHz.
The three new LDMOS devices operate-at +28 VDC, are internally matched, and housed in a low-thermal-resistance package with integrated electrostatic-discharge (ESD) protection. Freescale Semiconductor; (800) 521-6274, Internet: www.freescale.com/rf.