Electronic Design

Efficient Digital RF Power Amp Slashes Basestation Power, Size, And Cost

Blame it on RF power amplifiers. Large and costly, they comprise roughly 40% of a basestation's cost. And since these amplifiers must transmit wide bandwidth signals of various modulation types, linearity is critical.

But linear amplifiers (usually class AB) are very inefficient, leading to considerable power consumption and high heat output. Furthermore, required linearization techniques like feedforward and predistortion are costly and power-hungry, and they add to the inefficiency. Pulse-Wave RF solves the problem with the first highly efficient all-digital RF amplifier for basestation service.

PulseWave RF has created an all-digital "Class M" switching power module that's used to form a classical multicarrier power amplifier (MCPA) for wideband wireless technologies like cdma2000, WCDMA, TD-CDMA, OFDM, and WiMAX. The new Class M amplifier reduces cost and size by 50% and increases efficiency by 40% over existing designs. Full Class M population can reduce overall basestation cost by 20%.

This amplifier's success lies in the unique modulator (see the figure). The modulator inputs are the I and Q IF signals derived from multiplying digital-to-analog converters (DACs) via the baseband circuits. The modulator consists of an upconverter to the final output in the 800- to 900-MHz range, a linearizer, and a digitizer—all on a single silicon-germanium (SiGe) chip. It receives feedback from the switch-mode power amplifier that provides a closed loop to manage linearity.

The modulator output is a constant amplitude pulse train that's a hybrid of pulse-width modulation (PWM) and pulseposition modulation (PPM). A driver operates the gallium-arsenide (GaAs) switchmode amplifier transistors. An external band-pass filter in the signal path recreates the analog signal to the antenna.

These Class M modules are combined by paralleling them to achieve the desired power level. A Class M module contains four 5-W channels that are combined for an output of 20 W. This is the average power level, but the module is capable of a peak power of 100 W. Combining two units with an isolator produces an average output power of 40 W. Additional units can produce up to 160 W average. The switching output stage has an efficiency of 60% to 70%. Factoring in all drivers, modulator, and related circuitry, overall efficiency is 22%—a big increase over traditional basestation power-amplifier systems. Also, a basic Class M module is only 2.25 by 2.75 in. and can crank out up to 100 W peak at 25% efficiency.

Contact the company for pricing. PulseWave RF
www.pulsewaverf.com

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