SOI-technology, monolithic RF chip optimises Doherty amplifier performance

June 04, 2014 // By Graham Prophet
Peregrine Semiconductor's UltraCMOS Monolithic Phase & Amplitude Controller (MPAC) addresses a problem that has prevented, Peregrine says, the Doherty configuration from achieving its potential performance in communications-infrastructure transmitter stages.

Through a digital interface, MPAC enables alignment of the phase and amplitude between the Doherty amplifier's carrier and peaking paths. Designed for wireless infrastructure applications, MPAC improves system performance, lowers costs, increases reliability and provides maximum tuning flexibility.

“Doherty architectures have long encountered difficulties in optimising performance, which leads to higher manufacturing and engineering costs,” says Duncan Pilgrim, Peregrine’s vice president of marketing. “MPAC provides an ideal solution for this RF challenge and does so monolithically through intelligent integration—something gallium arsenide (GaAs) technologies could never achieve. Peregrine is introducing a superior UltraCMOS product that will disrupt GaAs-based alternatives.”

Pilgrim's assertion is the the MPAC makes the Doherty amp easier to design, and makes it work better. The Doherty configuration attempts to amplify the high peak-to-average signal of today's modulation schemes efficiently by splitting the signal into two paths; one amplifier handles the “base” of the signal – effectively, the carrier – while the other is appropriately termed the “peaking” amplifier. Implemented as intended, this can improve power efficiency (RF power added relative to DC power consumed) but the phase and amplitude between the two paths must be matched exactly – today, this is a matter of high-cost, low tolerance passive components and manual tuning, with no option to correct the setup as conditions (such as temperature) change, or components age.

While widely used in the wireless infrastructure industry, Doherty amplifiers require a considerable engineering investment to manually implement and optimise. Any mismatch or misalignment in phase and amplitude between the Doherty architecture’s carrier and peaking paths can quickly contribute to higher costs and degradation of the system’s overall performance. Even after optimising with discrete components, the system remains inflexible to manufacturing variances from power amplifier assemblies.

MPAC solves this engineering challenge with the industry’s first monolithic solution to maximise asymmetric and symmetric Doherty amplifier performance. MPAC also integrates Peregrine’s industry-leading digital step attenuators and phase shifter products onto a single