What are Doherty Amplifiers?

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- everything RF

May 25, 2023


A Doherty Amplifier is a type of RF power amplifier that has two amplifier stages - one for low-power signals and one for high-power signals. The innovative dual-stage design of Doherty amplifiers offers significant advantages in terms of power efficiency and linearity.

The Doherty amplifier was invented by William H. Doherty in the early 1930s and has since evolved to become a crucial component in modern communication systems. The fundamental principle behind the Doherty amplifier is the combination of two amplifier stages: a carrier amplifier and a peaking amplifier. It was designed to improve efficiency while maintaining good linearity characteristics.

The Doherty amplifier generally operates in a class-AB (provides gain at any power level) or class B mode, where the carrier amplifier operates as a linear amplifier, while the peaking amplifier at Class C kicks in only during high-output power demands. The carrier amplifier handles the lower power level signals, while the peaking amplifier efficiently amplifies the higher power level signals. This division of labor allows the Doherty amplifier to maintain high linearity at lower output power levels and high efficiency at higher output power levels.

Benefits of Doherty Amplifiers:

  • Improved Power Efficiency: Doherty amplifiers are renowned for their exceptional power efficiency. By utilizing two amplifiers that work in tandem, they can achieve higher efficiency levels compared to traditional amplifiers, resulting in reduced power consumption and improved overall system efficiency.
  • Enhanced Linearity: Linearity is crucial in wireless communication systems, as it ensures accurate signal reproduction without distortion. Doherty amplifiers offer excellent linearity characteristics, even at high output power levels. The combination of the carrier and peaking amplifiers allows the Doherty amplifier to maintain low distortion, minimizing signal degradation.
  • Wide Dynamic Range: Doherty amplifiers are capable of accommodating a wide range of input signal levels. They can effectively handle both low and high-power signals, making them suitable for applications where the power level can vary significantly, such as in wireless base stations.
  • Reduced Heat Dissipation: Due to their higher efficiency, Doherty amplifiers generate less heat compared to traditional amplifiers. This benefit translates into lower cooling requirements and reduced overall system complexity and cost.

Applications of Doherty Amplifiers

Doherty amplifiers find applications in various industries, including:

  • Wireless Communication Systems: Doherty amplifiers are extensively used in wireless base stations, enabling efficient transmission of signals in mobile networks. They help improve coverage and capacity while reducing power consumption and operating costs.
  • Broadcasting: Doherty amplifiers are employed in broadcasting systems to enhance the efficiency and performance of transmitters used in television and radio broadcasting. This ensures reliable signal transmission while minimizing energy consumption.
  • Radar Systems: Doherty amplifiers are utilized in radar systems, where high power levels and linearity are essential for accurate detection and tracking of targets. They help enhance the overall performance and efficiency of radar systems.

What are Asymmetrical Doherty Amplifiers?

Asymmetrical Doherty amplifiers are a variation of the traditional Doherty amplifier architecture. While the conventional Doherty amplifier employs two identical amplification paths, the asymmetrical Doherty amplifier introduces a difference in the characteristics of the carrier and peaking amplifier stages.

In an asymmetrical Doherty amplifier, the carrier amplifier is designed to have higher power capability and operates closer to saturation, while the peaking amplifier operates at a lower power level with better linearity characteristics. This design choice allows for improved efficiency and linearity across a wider range of output power levels.

The operating principle of an asymmetrical Doherty amplifier is similar to that of a conventional Doherty amplifier. The carrier amplifier handles the lower power level signals, and the peaking amplifier provides additional amplification during high-output power demands. However, by making the carrier amplifier more powerful and operating it in saturation, the asymmetrical design achieves better efficiency and linearity compared to the conventional symmetric Doherty amplifier.

The benefits of asymmetrical Doherty amplifiers include:

  • Enhanced Efficiency: The asymmetrical design allows for higher efficiency, particularly at lower output power levels. By operating the carrier amplifier closer to saturation, the overall efficiency of the amplifier is improved, resulting in reduced power consumption.
  • Improved Linearity: Asymmetrical Doherty amplifiers offer improved linearity performance across a wider range of output power levels. The lower-power peaking amplifier, combined with the higher-power carrier amplifier, helps maintain good linearity characteristics, minimizing signal distortion.
  • Extended Power Range: Asymmetrical Doherty amplifiers can accommodate a broader range of power levels compared to symmetric Doherty amplifiers. This versatility makes them suitable for applications with varying power requirements, allowing for efficient amplification of signals across different operating conditions.
  • Greater Flexibility: The asymmetrical architecture provides greater flexibility in optimizing the performance of the amplifiers. Engineers can tune the characteristics of the carrier and peaking amplifiers independently to achieve the desired trade-off between efficiency, linearity, and power output.

Asymmetrical Doherty amplifiers find applications in various industries where high-efficiency power amplification is crucial. They are commonly used in wireless communication systems, such as cellular base stations and wireless infrastructure, where efficient power amplification is essential for signal transmission while minimizing power consumption and operating costs.

In conclusion, Doherty amplifiers have emerged as a significant advancement in the field of power amplification, combining high efficiency, excellent linearity, and wide dynamic range. With their ability to reduce power consumption and enhance signal quality, they have become a key component in various industries, including wireless communication systems, broadcasting, and radar systems. As technology continues to evolve, Doherty amplifiers are expected to play an increasingly vital role in meeting the demands of next-generation wireless networks and communication systems.