Skip to main content

RF Power Amplifier Circuits

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The primary function of RF power amplifier circuits is to amplify modulated frequency band signals to the required power level while ensuring minimal interference with adjacent channels. 

  • Various topologies like Cascode, Totem Pole, and Doherty Amplifiers are employed in RF power amplifier circuits, each offering unique advantages such as high gain, improved efficiency, and better signal isolation. 

  • Non-CMOS materials like gallium arsenide and gallium nitride can also be used in RF amplifiers for their high-frequency performance.

RF power amplifier diagram

RF power amplifier diagram

RF power amplifier circuits amplify the modulated frequency band signal to the necessary power level. It is vital that this process doesn't excessively interfere with adjacent channel communications. At the same time, there's a need to maintain the integrity of the amplified high-power signal, ensuring it remains free from distortion. For this reason, there are a variety of architectures and topologies that are used in RF power amplifier circuits. 

RF Power Amplifier Circuit Topologies

Topology

Description

Advantages

Applications

Cascode Topology

Combines a common-emitter stage with a common-base stage in series.

High gain, improved bandwidth, better isolation, minimized Miller effect, improved stability.

UHF and microwave amplifiers where high gain and bandwidth are crucial.

Totem Pole (Push-Pull)

Uses two transistors to amplify positive and negative half-cycles of the waveform, respectively.

Reduces even-order harmonics, improves efficiency, and offers better thermal stability.

High efficiency, low distortion applications like audio amplifiers and RF transmitters.

Stacked Transistors (StackFET)

Multiple transistors stacked in series to share the supply voltage, increasing voltage handling capabilities.

Allows higher voltage operation and higher power output without exceeding voltage ratings of individual transistors.

High-power applications like satellite communications and radar systems.

Doherty Amplifier

Utilizes a main (carrier) amplifier and an auxiliary (peaking) amplifier.

Enhances efficiency at peak and back-off powers, efficient over a wide range of output powers.

Cellular base stations and broadcast transmitters, especially in 4G and 5G networks.

Balanced Amplifier

Employs two amplifying elements receiving the same input but with a phase difference (usually 180 degrees).

Improved isolation, reduced distortion, cancellation of even-order harmonics.

High linearity and moderate power level applications such as RF front-end circuits.

Transformer-Coupled Amplifier

Utilizes transformers for impedance matching at input/output or between stages.

Efficient power transfers. Can provide voltage/current step-up or step-down.

RF applications where impedance matching is critical for maximizing power transfer.


 

Wilkinson Power Combiner

A power combiner that uses quarter-wave transformers to split or combine RF power while maintaining isolation between ports.

Low loss, good isolation between output ports, ability to handle high power.

RF systems requiring power combining or splitting with minimal loss and high isolation, such as antenna arrays.


 

Methods of RF Power Amplifier Circuit Classification

RF power amplifier circuits can be categorized based on several criteria. 

  • In the case of frequency bands, they can be split into narrowband and broadband types. 

    • Narrowband amplifiers typically utilize frequency-selective networks, like LC resonant circuits, for their load circuits. 

    • Broadband amplifiers avoid frequency selection networks and use transmission lines with a broad frequency response as their load. Achieving impedance transformations across a wide bandwidth presents challenges, leading to the common practice of designing wideband amplifiers with a 50 Ω output load.

  • In terms of network properties, amplifiers can be classified into non-resonant and resonant types.

    • Non-resonant amplifiers feature matching networks that are not resonant systems, employing components like high-frequency transformers or transmission line transformers. These systems generally have purely resistive load properties.

  • Based on conduction angle – referring to the portion of the input signal cycle during which the amplifier is conducting. This classification is crucial, as it significantly influences the amplifier's efficiency, linearity, and overall performance. Different conduction angles cater to varying requirements in RF power amplifier design, balancing between efficiency and signal fidelity.

Non-CMOS RF Amplifier Circuits

Gallium arsenide (GaAs) and gallium nitride (GaN) are non-CMOS materials widely used in RF power amplifier circuits due to their superior properties in certain applications. 

GaAs, known for its high electron mobility, excels in high-frequency operations, making it a popular choice for microwave frequency bands. However, GaAs amplifiers tend to have power limitations, primarily due to the material's lower thermal conductivity compared to silicon, leading to challenges in heat dissipation at higher power levels. 

Gallium Nitride, on the other hand, has emerged as a robust alternative, especially in high-power applications. GaN is distinguished by its high breakdown voltage and thermal stability, allowing for higher power density and efficiency, particularly at microwave and millimeter-wave frequencies. Despite these advantages, both GaAs and GaN amplifiers face efficiency challenges, particularly in linearity and power-added efficiency (PAE), which are critical in applications like telecommunications and broadcasting.

LDMOS-Based Amplifiers in RF Power Amplifier Circuits

LDMOS amplifiers are renowned for their high power efficiency and ability to handle large power levels, making them ideal for applications like broadcast transmitters, cellular base stations, and RF heating. One of the key advantages of LDMOS over CMOS-based devices is their superior power handling capability, attributed to the LDMOS's higher breakdown voltage and better thermal performance. This allows LDMOS amplifiers to operate at higher voltages and with greater power outputs, crucial in high-power RF applications. 

Additionally, LDMOS devices exhibit better ruggedness and reliability under extreme operating conditions, further bolstering their suitability for demanding RF applications. While CMOS is often favored for low-power, integrated circuits due to its lower cost and compatibility with digital circuitry, LDMOS stands out in scenarios where high power, efficiency, and reliability are paramount.

Cadence’s AWR Software for RF Power Amplifier Circuits 

In the world of RF power amplifiers, where precision, efficiency, and power are paramount, practical application, simulation, and optimization are essential to bring these concepts to life and ensure they meet real-world demands. This is where Cadence's AWR software comes into play. 

Cadence’s AWR software is an indispensable tool for RF designers, offering advanced simulation capabilities that enable the practical implementation and optimization of RF amplifier circuits. By leveraging AWR software, designers can simulate and analyze the behavior of various amplifier topologies.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.