IEEE Microwave Magazine - May 2016 - 47

Doherty Versus Envelope Tracking
The growing demand for energy-efficient wireless devices
has spawned tactics to improve back-off PA efficiency,
including load modulation, envelope tracking (ET), and
digital predistortion (DPD)-all of which are currently
dominating cellular
networks. Dohertybased PAs are excellent at optimizing the
efficiency at the back-off
region [1], [2]. The operation of these PAs is based on
the active-load-modulation
technique, which allows active
devices to operate into optimal
load-impedance trajectories that
maximize the efficiency according to
the amplitude of the input signal.
Because the main and auxiliary
amplification paths of the Doherty PA are
operated in different classes, several techniques (e.g., uneven power splitting and
asymmetrical Doherty PA design) are typically adopted to minimize the imbalances
among the combing signals, thus maintaining improved efficiency. On the other hand,
new research has established multiband and
broadband Doherty PA design techniques [3]-
[5], which would help consolidate the position of
these types of PAs in the base-station market for
fourth-generation applications and beyond.

May 2016

ET [6] dynamically adjusts the voltage bias of
the transistor according to the input signal, thereby
boosting power efficiency at lower signal levels. The
act of changing the voltage bias can force the PA to
operate at the highest possible instantaneous efficiency (near saturation) over the whole dynamic of
the driving signal [7], [8]. The consequence is a very
nonlinear input-output relationship, which causes
spectral regrowth at the PA output and affects the inband signal quality. Additional parameters such as
timing alignment between the drain voltage and RF
source, envelope amplifier bandwidth, and envelope
shaping [9], [10] are some of the factors that need to
be considered when designing an ET PA, which can
increase the design complexity.
DPD [11], [12] extends the operating PA region by
allowing system engineers to operate the PA in the
nonlinear region, and it permits a low-power, highefficiency PA to be used rather than a high-power,
less efficient PA. The combination of Doherty and
DPD, or of ET and DPD, enables very highly efficient transmitters.
Table 1 provides a technological overview of the
Doherty and ET architectures. The Doherty architecture
improves back-off efficiency, but ET has the advantage
of increasing efficiency even at very low output powers.
This results in a very nonlinear response and requires
a very complex DPD in order to allow linearization of
the system. The design complexity is also increased for
ET because the designer has to expect a variable drain
voltage for the transistor. In addition, the bandwidth of
ET is limited due to constraints on the envelope amplifier design; however, in practice, the actual envelope is
not necessarily required: average, speed-reduced, and
filtered versions can be used [13]. Current research is
focusing on increasing both efficiency and bandwidth
for these architectures, which are receiving considerable attention as the next generation of multiband transmitter designs.

background-©graphic stock, racecars-image licensed by ingram publishing

operating in the back-off region of the power amplifier (PA) for these crest-reduced signals using complex
modulation schemes. Traditionally, to avoid severe
nonlinear distortion and signal clipping, these amplifiers have to be operated at a back-off level equal to the
PAPR of the signals. This negatively impacts efficiency
as the back-off level increases.

Table of Contents for the Digital Edition of IEEE Microwave Magazine - May 2016