The Doherty Amplifier: New After 70 Years
By Freescale Semiconductor, RF Division

The Doherty amplifier architecture has in less than 5 years become the “amplifier of choice” for new wireless transmitters after essentially laying dormant since W.H. Doherty first described it in 1936. The Doherty’s obscurity is directly attributable to the predominant modulation schemes (AM and FM) employed in communication systems over the years, which do not possess high peak-to-average ratios (PARs). The resurgence of interest in the concept is based on its very high power-added efficiency when amplifying input signals with high PARs – precisely the type exhibited by WCDMA, CDMA2000, and systems employing Orthogonal Frequency Division Multiplexing (OFDM), such as WiMAX and the upcoming Long-Term Evolution (LTE) enhancement to the UMTS wireless standard.

In fact, when properly designed, a Doherty amplifier can produce increases in efficiency of 11% to 14% when compared to standard parallel Class AB amplifiers that have traditionally been employed in wireless base station transmitters. Since the transmitter accounts for a high percentage of overall system power consumption, the cost savings delivered by the Doherty amplifier’s efficiency can reduce base station annual electricity costs. Thus its appeal for wireless base station manufacturers and wireless service providers.

While the intrinsic high efficiency of the Doherty architecture makes it desirable for current and next-generation wireless systems, it presents unique challenges from a design perspective. The linearity and output power of the Doherty architecture are slightly less than exhibited by a dual Class AB amplifier, and it can produce higher distortion as well. Fortunately, the advancements in analog and digital predistortion and feed-forward linearization techniques can dramatically reduce the Doherty’s distortion. In addition, careful amplifier design can mitigate its inherently lower linearity. The remaining challenge is to create RF power transistors that can accommodate the requirements of the two types of amplifiers employed by the Doherty architecture and produce optimum RF output power over a wide array of signal conditions.

A Doherty overview
A “classic” Doherty amplifier (Figure 1) employs two amplifiers. The carrier amplifier is biased to operate in Class AB mode and the peaking amplifier is biased to operate in Class C mode. The input signal is split by a power divider equally to each amplifier with a 90-deg. difference in phase. After the signals are amplified, the signals are recombined with a power combiner. Both amplifiers operate when the input signal peaks, and are each presented with the load impedance that enables maximum power output. However, as the input signal decreases in power, the Class C peaking amplifier turns off and only the Class AB carrier operates. At these lower power levels, the Class AB carrier amplifier is presented with a modulated load impedance that enables higher efficiency and gain. The result is an extremely efficient solution for amplifying the complex modulation schemes employed in current and emerging wireless systems.

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