In current communication systems, high-efficiency technology involves Doherty and envelope tracking ET technologies, and high-linear technology includes analogue-predistortion (APD), digital-predistortion (DPD) and feedforward technologies. Combining all these technologies such as “DPD+Doherty” technology or even “DPD+Doherty+ET” technology together, RF power amplifier will be highly efficient and linear. In the following paragraphs, Doherty, DPD and ET technologies are introduced.
Doherty, a very popular high-efficiency technology, mainly solves the problem about the load impedance transformation. The simplest Doherty structure consists of two amplifiers: one is the main amplifier, the other is an auxiliary amplifier. The main power amplifier works in class A and class B, and the auxiliary amplifier works in class C. However, these two amplifiers do not work by turns. In fact, the main amplifier keeps working all the time, but the auxiliary one does not work until the input reaches the setting peak value.
Three different working modes of “DPD+Doherty” technology:
• When the input signal power is low, the auxiliary amplifier will be completely shut off and remains in high resistance mode.
• When the input signal power is high, the auxiliary amplifier will be fully switched on. In this mode, both main and auxiliary amplifiers are working synchronously.
• When the input signal power is at medium level, the PA status is between these two modes mentioned above.
DPD technology generally cooperates with the cut peak technology and Doherty technology, which significantly improves the linearity and efficiency of RF amplifier.
FiberHome adopts “DPD + Doherty” technology to increase efficiency instead of using the traditional class AB power amplifier. For instance, the efficiency of the last stage power amplifier tube can reach as high as 43%, moreover, amplifier module efficiency is close to 40%. Through collaborating with the cut peak and DPD technologies, the Doherty amplifier has been promoted magnificently as required in efficiency and linearity.
“ENVELOPE TRACKING” is the hotspot technology of improving the amplifier efficiency nowadays. How it works? ET amplifier is divided into RF amplifier and envelope amplifier branches. Furthermore, the RF amplifier branch amplifies the phase and amplitude information, and allows larger amplitude distortion.
The envelope branch separates amplitude envelope information by detectors at first, and then controls the PA power supply by changing the power supply voltage based on envelope. Meanwhile, RF branch needs the delay line to synchronize with envelope signal. Currently, FiberHome is researching on the envelope tracking technology, and planning to apply this technology in products by the end of 2011.
Pi Yonghui is wireless product manager at FiberHome Technologies
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