Electronic Design

Silicond-Germanium Amplifiers Boost 3G WCDMA And 802.11a Radios

Making an integrated power amplifier for microwave radio has never been easy. In the past, most manufacturers settled on gallium-arsenide (GaAs) amplifiers to wring out the most performance. But their high cost keeps the bill of materials (BOM) of cell phones, wireless local-area networks (WLANs), and other wireless products high. A proven solution is the use of silicon-germanium (SiGe) semiconductor materials that offer GaAs performance but can be made in silicon fabs, thereby greatly reducing costs.

Recently, SiGe Semiconductor expanded its power-amplifier (PA) product line to include wideband code-divsion multiple access (WCDMA) 3G cell phones and the higher-speed 802.11a WLAN products. One new entry, the RangeCharger SE5120, targets the growing WCDMA 3G cell-phone market. This three-stage linear amplifier operates in the 1920- to 1980-MHz range. It features a peak output power of +28 dBm with a linearity of −38 dBc, and power-added efficiency runs above 40%.

Integrated in the device are the amplifier, regulator, matching circuitry, and optional power detector. Also included are bias and gain control circuitry as well as on-chip electrostatic discharge and load mismatch protection. It can sustain a voltage standing-wave ratio mismatch up to 10:1. Separate power pins to the control and amplifier cells help designers regulate the amp's VCC and improve efficiency at lower power levels. Its sleep mode uses a standby current of 2 µA.

The SE5120 comes in an eight-pin QFN package that measures 3 by 3 by 0.9 mm. Pricing in 100,000-unit lots is $1.50. The company claims that due to all of the integrated circuitry, a system BOM reduction of up to $2.80 is possible.

Another new SiGe PA is the SE2535 (see photo). Designed for the 802.11a 5-GHz Wi-Fi band, it solves the wide bandwidth problems associated with orthogonal frequency-division multiplexing (OFDM), the modulation method used in 802.11a. An error vector magnitude (EVM) of less than 2.5% at 18 dBm can be achieved.

This chip houses a linear, three-stage, 27-dBm power maplifier that incorporates a power detector and all of the analog biasing and matching circuitry into a standard QFN package measuring just 4 x 4 x 0.9 mm. The power detector adjusts the operating point to accommodate the peak-to-average power ratio, saving power while maintaining performance. The current drain is 160 mA for +18 dBm output.

The SE2535 meets the 802.11a harmonic requirements of worldwide standards bodies and eliminates the need for external bandpass filtering. A power detector is fully integrated. All of these features translate into a smaller form factor and a lower BOM cost. Available now, the SE2535 costs $1.75 in 100,000-unit quantities.

SiGe Semiconductor

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