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RF Power Amplifiers: An Overview

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The basic amplifier classes include Class A, Class AB, Class B, Class C, and Class D, each with varying levels of efficiency and linearity.

  • Advanced amplifier topologies like Class F, Class G, and Class I offer unique advantages in terms of efficiency, power consumption, and waveform shaping.

  • Recent technological advancements and complex RF power amplifier topologies exist, such as envelope tracking, digital predistortion, Doherty amplifiers, GaN technology, and efficiency enhancement techniques

Power Amplifier board with RF testing equipment

Power Amplifier board with RF testing equipment

Within the signal chain of radio frequency network, RF power amplifiers take their place as the final element directly driving the antenna, following components such as low-noise amplifiers (LNAs), mixers, and other signal processing stages. Power amplifiers are responsible for the crucial task of amplifying the signal to its required power level for transmission. Serving as the ultimate stage in the RF architecture, power amplifiers ensure that the signal is boosted to a level suitable for efficient and reliable signals over long distances. 

 RF power amplifiers are virtually all wireless devices, from wireless communication systems satellite communication, broadcasting, mobile phones and IoT devices. Read on as we discuss the fundamentals of RF power amplifiers, explore the basic amplifier classes, and delve into more advanced types and recent technological advancements.

RF Power Amplifiers’ Role in Wireless Communication

An RF power amplifier is an electronic device designed to increase the power level of radio frequency signals. It takes a weak input signal and boosts it to a higher power level suitable for transmission over long distances or to drive high-power loads, most often antenna. RF power amplifiers are characterized by their ability to handle high power levels efficiently while maintaining linearity, signal quality, and low distortion. 

They are implemented using active devices, specifically transistors, that operate in the linear region to prevent signal distortion. Essential factors contributing to the performance and operation of power amplifiers include gain, power output, bandwidth, power efficiency, linearity (which ensures minimal signal compression at the rated output), I/O impedance matching, and heat dissipation.

Amplifier Class

Amplifier Notes

Conduction Angle

Class A

Single transistor conducts full cycle

θ = 2π

Class B

Two transistors each conduct half cycle

θ = π

Class AB

Two transistors each conduct slightly over half cycle

π < θ < 2π

Class C

Single transistor conduct less slightly less than half cycle

θ < π

Class D, F, G and I

Utilizes non-linear switching (transistor either ON or OFF)

θ = 0

RF Power Amplifier Classes

Basic architectures for RF power amplifiers can be broadly classified into several amplifier modes, known as “classes”. Amplifiers’ classes are determined by how much of the cycle of the radio signal a given transistor conducts current. The most common amplifier classes include Class A, Class AB, Class B, Class C, and Class D, in increasing efficiency but decreasing linearity. 

  • Class A amplifiers are known for their high linearity but are less efficient since they operate in the active region at all times, resulting in significant power dissipation. They generally consist of a single transistor with a conduction angle of 360 degrees.
  • Class B amplifiers are more efficient than Class A amplifiers but have lower linearity. They use a biasing scheme that allows two transistors each to operate in the linear region during their respective half-cycle. In other words, the top transistor conducts during positive half-cycles, and bottom transistor in the negative half-cycles, with each transistor featuring a conduction angle of 180 degrees.
  • Class AB amplifiers provide a compromise between linearity and efficiency. They use a biasing scheme where  two transistors each operate in the linear region during their respective half-cycle with additional compensation for the transistors’ crossover distortion near 0V (approximately one diode voltage drop). This results in a conduction angle between 180 degrees to approximately 200 degrees.
  • Class C amplifiers are highly efficient but have limited linearity. They operate the active device in a highly saturated state, conducting only during a small portion of the input signal cycle and have a conduction angle generally around 90 degrees. Although the efficiency of the amplifier is very high, it also features heavy distortion near the output signal. Class C amplifiers generally consist of a single transistor with a parallel resonance circuit tuned for the amplifier’s purpose. 
  • Class D amplifiers, also known as switching amplifiers, achieve high efficiency by using pulse width modulation (PWM) techniques. They switch the active device on and off rapidly, resulting in a digital representation of the input signal. Generally, class D amplifiers are not used in RF applications as the switching speed of the transistors in their saturation mode may result in large I-V products which reduces efficiency. 

Other Amplifier Topologies

In addition to the basic amplifier classes, Class F, Class G, and Class I amplifiers also exist, each with unique characteristics and advantages in terms of efficiency, power consumption, and waveform shaping.

  • Class F amplifier achieves improved efficiency and output by incorporating harmonic resonators in the output network, shaping the output waveform into a square wave. As infinite harmonic tuning is approached, Class F amplifiers can achieve high efficiencies exceeding 90%.
  • Class G amplifiers provide enhancements to the conventional Class AB amplifier design. They utilize multiple power supply rails with varying voltages and automatically switch between these rails based on the input signal. This continuous switching operation reduces average power consumption, minimizing power loss caused by heat dissipation.
  • The Class I amplifier employs a configuration with two sets of complementary output switching devices in a parallel push-pull arrangement. Both sets of switching devices sample the same input waveform. Consequently, one device handles the positive half of the waveform while the other handles the negative half. This switching mechanism resembles that of a Class B amplifier.

Advanced RF Power Amplifier Types and Technology Trends

In recent years, significant advancements have been made in RF power amplifier technology to meet the increasing demand for larger communication ranges, improved efficiency, and more compact designs. Some of the advanced types of RF power amplifiers and recent technological advancements are as follows:

  • Envelope Tracking (ET) Amplifiers: ET amplifiers utilize envelope tracking techniques to improve efficiency by dynamically adjusting the supply voltage to the amplifier based on the envelope of the input signal. This allows the amplifier to operate at high efficiency even for signals with high peak-to-average power ratios.

  • Digital Predistortion (DPD): DPD is a technique used to compensate for nonlinearities in RF power amplifiers. By pre-distorting the input signal based on a model of the amplifier's nonlinearity, DPD can significantly improve linearity and reduce distortion.

  • Doherty Amplifiers: Doherty amplifiers are highly efficient amplifiers that use a combination of two amplifiers, a carrier amplifier, and a peaking amplifier. The carrier amplifier operates at a high output power level, while the peaking amplifier supplements the carrier amplifier's output power during high-power signal peaks. They are primarily used in cases where there needs high peak to average power cases.

  • GaN (Gallium Nitride) Technology: Gallium Nitride-based power amplifiers have gained popularity due to their high power density, efficiency, and wide bandwidth capabilities. GaN technology enables the development of smaller, more efficient, and higher power amplifiers compared to traditional technologies like Silicon LDMOS.

  • Efficiency Enhancement Techniques: Various techniques, such as load modulation, harmonic tuning, and dynamic biasing, are employed to improve the efficiency of RF power amplifiers. These techniques optimize the amplifier's performance at different power levels and operating conditions.

Having a strong understanding of the basic amplifier classes helps in selecting the right amplifier for specific applications. Ready to explore the world of RF power amplifiers and their advancements further? Experience the cutting-edge capabilities of RF design with Cadence AWR software. With its comprehensive tools and simulation capabilities, you can dive deeper into amplifier classes, topologies, and the latest technological innovations. Take your RF design expertise to new heights with Cadence AWR

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