(Editor's Note: there is a linked list of this author's previous guest columns at the end, below "About the author".)
In a world of everything Doherty, there can be several variations on the standard Doherty amplifier architecture, in order to optimize the operation for different signal conditions. All of the variations can be subdivided into three basic formats, those being symmetrical, asymmetrical, and N-way Doherty. These basic versions have different performance parameters that can make one version better than the others, depending on several factors in the application.
•The symmetrical Doherty is the oldest and most-common version of Doherty amplifier architectures, and it has several benefits and drawbacks. Benefits include an inherent common device for both the peaking and carrier sides of the amplifier, and a relatively simple, 3-dB power splitter on the input side of the amplifier.
The detriment to this design is that it is theoretically only optimized for a 6-dB back-off ratio from the P3 dB saturation capability of the combined pair. If the test signal has a higher peak-to-average (PAR) value than 6 dB, then the symmetrical Doherty will have to be backed off further, resulting is a less-than-optimum efficiency operating point.
•The asymmetrical Doherty is different from the symmetrical version in the fact that it is based on transistors with different peak-power capabilities. Given that the peaking amplifier side is typically biased under a light Class C mode of operation, then for any given amount of silicon, a Class AB bias will produce a greater amount of saturated power than a Class C due to the Class AB's longer conduction angle.
Depending on the design requirements, the Class C amplifier may need to be somewhere on the order of 1 to 3 times the die periphery compared to that of the Class AB carrier side. The main benefit of an asymmetrical Doherty is that its optimum efficiency operating point can be in the range of 6-to-9 dB back-off from the P3 dB saturation level.
•The final category of Doherty is the N-way Doherty. "N-way" implies that the peaking amplifier consist of not one but, rather, two or more different Class C amplifiers. With the N-way, the various peaking amplifiers can be added in successions at various levels of input signal. Under very low drive levels, only the carrier amplifier will be turned on.
At higher drive levels, all of the peaking amplifiers will be engaged and will be contributing to the overall output-combined signal level. The main benefit of the N-way Doherty design is similar to the asymmetric design, as it offers a high-efficiency response from Psat down to high levels of back-off.
While this provides a first-order assessment of the relative strengths and weaknesses of various types of Doherty, the decision-making process to select one particular Doherty configuration should also take into account parameters such as RF Gain, RF bandwidth, linearity, and linearization response with Digital Pre-distortion. ♦
About the author
Leonard Pelletier is lead RF applications engineer at Freescale Semiconductor and is a regular Guest Contributor on RF topics for RF DesignLine. He can be reached for questions and suggestions for future topics by either commenting in the box below or by contacting him directly at firstname.lastname@example.org. You can follow him on Twitter at www.twitter.com/RFLeonard.
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