Electronic Design

A Word About RF Power Transistors

MOSFETs used in cellular and other PAs are virtually 100% discrete RF power transistors. The laterally diffused metal-on-silicon (LDMOS) FETs are the most popular type. Freescale Semiconductor is a leader here, with a wide range of devices.

The company’s recent products include the MRF8S9100H/HS, which operates in the 920- to 960-MHz range. It’s designed for class AB or C operations in GSM/EDGE systems. It can deliver 45-W average power, a gain of 19.1 dB, and an efficiency of 44%. The MRF8S18120H/HS is made for the 1800- to 1900-MHz bands, where it can deliver 46-W average power with an efficiency of 42% (see the figure).

But for the higher power levels needed in more rugged applications, gallium-nitride (GaN) transistors are gaining converts. They offer higher power density, higher gate-todrain breakdown voltages, and higher operating voltages. In addition, they can operate at higher junction temperatures. Some deliver up to 100 W in the 1- to 3-GHz range.

Nitronex and NXP offer such devices. BC Systems supplies complete power amplifiers with those power and frequency ranges. GaN devices are seen as gradually replacing traveling-wave-tube (TWT) vacuum tube amps in some high-power microwave applications such as in satellites.

TAGS: Freescale
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