Freescale claims its advanced Doherty alignment module (ADAM) improves upon symmetric Doherty amplifiers. Leonard Pelletier from Freescale explains why. Tell us what you think in comments section, below.
In cellular infrastructure power amplifier (PA) equipment, the Doherty circuit configuration has been used almost exclusively for the past eight years—since the creation of very effective digital pre-distortion (DPD) systems.
In general practice, symmetric Doherty amplifiers are used, which are constructed of two identical sub-amplifiers; the carrier, which in practice is biased in deep class AB and the peaking amplifier, which is biased in class C. The advantage of symmetric Doherty amplifiers is the simple design, as two identical amplifiers are used. In addition, this type of Doherty PA is straight forward to put into production as all components used to create the sub-amplifiers generally come from the same manufacturing lot and therefore vary in the same direction. Symmetric Doherty amplifiers have a theoretical efficiency peak at 6dB output power backoff.
Due to the large peak-to-average ratio of signals used in modern communication systems, power amplifiers are generally operated backed off farther than 6dB, often somewhere around 8dB. Hence, asymmetric Doherty PAs that move the efficiency peak closer to the operating point are of great interest to infrastructure equipment manufactures.
While asymmetric Doherty PAs do improve efficiency, they also have challenges associated with them. In general, they are more non-linear compared to their symmetric counterparts and pose a bigger challenge to DPD systems. In addition, the components used to construct carrier and peaking amplifiers are different, and hence, any variation in these components is independent of one another. Therefore, the asymmetric Doherty PA has proven challenging to put into production without the use of additional production margins and/ or impact on production yield, often diminishing the efficiency advantage.
But not any longer.
Freescale Semiconductor's RF business has developed a novel device that not only incorporates the 3dB input split and the 90 degree delay line, but also allows to tune both these values using digital controlled phase shifters and step attenuators on both outputs of the splitter. By digitally manipulating the phase shifters and attenuators, it is possible to adjust the magnitude and phase of the signals going to each side of the Doherty amplifier and electrically tune the assembly for maximum Doherty performance, regardless of independent variation of the two sub amplifiers.
An additional benefit is that this device can be adjusted in the field, on the fly, in order to optimize the amplifier system for traffic loading, frequency changes, operating temperatures, or any other environmental conditions.
Click on image to enlarge.
This device, which is integrated in a single over-molded plastic package, is named the advanced Doherty alignment module (ADAM). The ADAM chip is available in three slightly different varieties covering the three most popular cellular communication bands between 700 MHz and 2700 MHz. The ADAM device operates off of a single 5V supply voltage, draws 12 mA of current, has a TTL/CMOS/SPI digital interface set up for 3.3V logic systems and comes in a 6 mm QFN surface mount package.
More information can be found here on Freescale.com
Leonard Pelletier works in applications support for Freescale Semiconductor's Radio Frequency Division.