WiMAX Amplifiers And Their Application

With the demand for wireless personal communications growing exponentially, complex modulation schemes have been deployed that offer the spectral efficiency to support the demand with fixed spectrum. Modulation formats of this type typically present difficulties for power amplification due to the statistics of the envelope of the signal, often referred to as peak-to-average power ratio or crest factor. Most client base stations employ Class A and Class AB amplifiers. The WiMAX era is erupting as several companies have already announced successful trails for 802.16-2004 (TDD) compliance. Most of the WiMAX base stations that are deployed have gaps in the coverage. In order to ensure quality of service (QOS), higher-power base stations are being developed to close the gaps and extend coverage. Examples of high-power base station transmitters with power averages of .9 watts and 5 watts are shown in Figure 1 and Figure 2 respectively.

Since the semiconductor era uses III-V and IV elements, the RF amplifier technology has generated an array of devices. From Si-bipolar, to GaAs MESFET, to GaAs HBT, to silicon LDMOS, to GaAs HFET, and now wide bandgap devices such as SiC MESFET and, on the horizon, GaN HEMT devices, the choices for power amplifier technology is broad. The MESFET structure has been proven to be one of the best in linearity. While LDMOS structure does not compare in linearity, its power density is very attractive, as well as its high breakdown voltages. However, the silicon breakdown voltage can not match the breakdown voltage of SiC MESFET, which exceeds 100 volts. The advantage of the SiC MESFET is the materials property for high power density per unit area. The low trans-conductance property of SiC material has lower gain. Because of the wide bandgap properties of SiC, gain disadvantages are small in comparison to the high voltage and high output power operations. This paper proposes GaAs MESFET because of its excellent linearity and power gain.