RF Power Amplifier

RF power amplifier(PA) is the last stage which handles maximum amount of power in a transmit chain. In order to deliver large amount of power to load or antenna, it has to handle large signal swings which introduces distortion in the power output due to non-linear characteristics of the transistors. The amount of the distortion a PA can tolerate depends on the modulating signal (wireless standard or modulation technique) to be amplified. Since a PA handles relatively portion of total power, the power efficiency of power amplifier has considerable influence on the overall efficiency of a transceiver/radio.

Several types of power amplifier architectures are proposed in literature, and the choice of architecture is greatly influenced by modulation type, communication standard, cmos (any other) technology, cost along with designer’s choice. Ofcourse, apart from these, there are lot other parameters to influence the architecture selection.

Performance Metrics

Following are important metrics often used to guage the performance of a power amplifier. Some metrics are defined in a given standard, others arise from the designer’s perpective.
Metrics defined in standard

Output Power
Spectral Mask
ACPR (Adjacent Channel Power Ratio)
Signal Modulation
Error Vector Magnitude

Metrics not defined in standards, but still important from the design perspective are

PAE (Power Added Efficiency)
Drain Efficiency
Power Gain
1dB Compression Point (P_1dB)

[ Read more . . . ]

Power Amplifier Classes

Power amplifiers are broadly classified into two categories, linear PAs and switch-mode PAs. In linear power amplifiers the active device works as a transconductor, whereas in switching mode power amplifier it act as a switch.

Linear PAs : Class-A, Class-B, Class-AB, and Class-C
Switch-mode PAs : Class-D, Class-E, Class-F, Class-G and Class-H

Amplifier classes of operation [ Read more . . . ]

Linearity Enhancement Techniques

FeedForward Linearization
– Based on inherently wideband technology
Digital Predistortion
– Limited Bandwidth (DSP implementation)
RF-Based Predistortion
– Limited accuracy of function model
– Implemented at RF with low complexity
– Adaptation is required
Cartesian Feedback
– Stability considerations limit bandwidth and accuracy
LINC (LInear amplification using Nonlinear Components)
– Sensitive to component drift and has a high level of complexity
Dynamic Biasing
– Limited ACI suppression

Efficiency Enhancement Techniques

Adaptive bias
Doherty Power Amplifier
Envelope Elimination and Restoration (EER)