Design the RF circuit of the Doherty amplifier from scratch

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

Design the RF circuit of the Doherty amplifier from scratch

Posted Date: 2024-01-31

Doherty amplifiers are used in many RF power applications. Although a reference design may be frequently used, its operation and RF circuit design challenges need to be understood so that a custom design can meet its requirements.

The Doherty amplifier design requires that both amplifiers for efficient operation, as well as the separation, matching combination and phase, be optimized to achieve the desired results and improve efficiency.

Designing the RF circuitry for a Doherty amplifier from scratch is a complex process that requires a deep knowledge and understanding of the technology and performance of each component of the amplifier.

Doherty amplifier operation

Before understanding the RF circuit design of Doherty amplifier, it is necessary to understand the basic working principle of RF amplifier.

Basic concepts of Doherty amplifiers

The RF circuit design of the Doherty amplifier uses a main amplifier or carrier amplifier that is typically biased for Class AB operation. The second active device, often called the auxiliary amplifier or peaking amplifier, is usually biased for Class C operation.

The signal enters the entire Doherty power amplifier and is presented to the splitter. This produces two signals that are 90° phase-shifted from each other. The reason for this is the use of an inductive splitter to reduce power losses and create a 90° phase shift between the two signals.

One output is presented to the carrier amplifier. This is designed to accommodate lower power levels encountered around average power levels. This is designed to provide optimal efficiency for these power levels.

Doherty amplifier work area

The signal is also presented to a peaking amplifier. This is biased, so it only starts to work when there are large peaks that the carrier amplifier cannot adapt to on its own. As a higher power amplifier, this does not provide a high level of efficiency for lower power levels, so it only operates when higher power levels are present. In this way, optimal efficiency is achieved within the power range.

A signal has passed through the RF amplifier circuit itself and the output has been combined in reverse via the splitter circuit. Since it also has a phase shift of 90°, it is used to cancel the phase shift at the input. Therefore, the signals from the two amplifier sections remain in phase.

Doherty amplifier operation: details

Basic Doherty amplifier theory requires that the signals between the two halves be matched in phase so that the combination occurs in such a way that the two signals are added together to provide the desired output.

Power distribution at the input of the Doherty amplifier is relatively simple. Power distribution is done using quadrature splitters: typical topologies include Lange or branch line technology.

The input works like a balanced amplifier. It has the same characteristic that if the reflection coefficients are equal in magnitude and phase, the reflection coefficient of a mismatched amplifier is reduced. The reflected wave is dissipated in the load terminating the isolated port of the coupler.

Doherty amplifier circuit block

The combination of the two amplifier signals at the output creates further problems. The two signals are 90° out of phase and a quarter wave line is used in the output circuit of the peaking amplifier to make them in phase with each other again.

Impedance also needs to be accurately matched to ensure efficiency is maintained. The impedance of both RF amplifiers is Z0/2. This is boosted to Z0 by the quarter wave transformer.

This seems simple, but given that peaking amplifiers only work at the peak, the amplifier works in a non-linear manner. During operation, the response of one amplifier can actively pull the load pull of the other amplifier because they are not isolated. This means that nonlinear analysis is required to complete the design.

It is possible to operate both RF amplifier sections in the same class but use an adaptive biasing scheme to turn on the peaking amplifier when required - usually the carrier amplifier operates in Class A or AB and the peaking amplifier operates in Class C. Another approach is to use unequal sized devices, or you can use unequal power dividers at the inputs.

Doherty amplifier design issues

During the RF circuit design process, the developer's goal is to provide the best performance as efficiently as possible under the expected conditions. However, these goals cannot be achieved simultaneously and require significant trade-offs.

To achieve the best overall performance, it is necessary to find a parameter set and operating point that provide a good compromise between the design's sensitivity to frequency, phase, and level changes. This requires a deep understanding of the characteristics of amplifiers, splitters, and combiners.

Typically, RF circuit design techniques involve using a manufacturer's reference design, which can then be fine-tuned. It is often difficult to fully optimize the design for the specific application being explored, as only minor design changes are often made to the reference design.

As can be seen, the RF circuit design of the Doherty amplifier brings many interesting and challenging aspects if the entire amplifier is to work properly:

Phase Maintenance: Theoretically, the phase of signals passing through different paths should be the same at the combining point. The RF splitter introduces a 90° phase shift in one leg, which can be eliminated at the combining stage, as a 90° shift also occurs in the combiner and can be added in the other leg. However, the amplifiers introduce phase shifts, which may not match because one is designed to handle lower power levels and the other to handle peaks. This means that their characteristics will be different (in the case of asymmetry).

Impedance Matching: Ensuring that the impedance of both RF amplifiers is adequately maintained over the operating range may cause problems in some designs. Optimizing different electronic components to achieve this goal can be difficult.

Linearity Maintenance: Studies have found that when a peaking amplifier begins to operate, there may be kinks or disturbances in the linearity of the amplifier. This will increase the distortion of the placed shape. In addition, care needs to be taken to ensure linearity over the entire operating range.

Bandwidth: Typically, Doherty amplifier designs are limited in bandwidth. In addition to the limited bandwidth of some electronic components, including splitters and combiners, their phase shifts can vary widely, affecting the performance of the entire amplifier design.

In most cases, using a reference design of a Doherty amplifier, this usually only requires small adjustments to the values ​​of certain electronic components. Typically, these reference designs are provided for some of the most common end uses, so little work on the design is required.

Often, a reference RF design, including PCB layout, can be incorporated into the overall printed circuit board layout with minor changes to certain electronic component values. Still, care needs to be taken to ensure that the final RF circuit design performs as required. Even small changes to PCB layout can have a significant impact on performance.

Despite its RF circuit design challenges, Doherty amplifiers have established themselves firmly in applications such as the final power output stage in cellular base stations and other wireless communications and radiocommunication applications.

Despite the difficulties involved in the design process, when the RF design is optimized, Doherty amplifiers are able to provide significant performance improvements in efficiency and other areas. These are very useful when developing new cellular base stations, wireless communication systems, and various types of radio communication systems.
Review Editor: Huang Fei

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