Doherty Power Amplifier Design
- the Doherty power amplifier offers many advantages in terms of efficiency improvements, but the design of them is notorious exacting if the optimum performance is to be achieved..
The concept behind the Doherty amplifier is relative straightforward. However Doherty amplifier design is notoriously tricky if the optimum performance is to be achieved.
The amplifier design presents some interesting issues when realising some of the key techniques for implementing the Doherty amplifier design.
Doherty amplifier operation
Before looking at the Doherty amplifier design, it is necessary to look at the basic operation of the amplifier.
The Doherty amplifier design is uses a main or carrier amplifier that is typically biased for Class AB operation. A second active device, typically named the auxiliary or peaking amplifier that is normally biased for Class C operation.
Basic Doherty amplifier design concept
The signal enters the overall Doherty power amplifier and is presented to a splitter. This creates two signals that are phase shifted by 90° with respect to each other. The reason for this is that inductive splitters are used to reduce the power loss and these create a 90° phase shift between the two signals.
One output is presented to the carrier amplifier. This is designed to accommodate the lower power levels encountered around the average power level. This is designed to provide optimum efficiency for these power levels.
Basic Doherty amplifier operation
The signal is also presented to the peaking amplifier. This is biased so that it only started operating when large peaks are present that the carrier amplifier would not be able to accommodate on its own. Being a higher power amplifier, this would not provide high levels of efficiency for the lower power levels and therefore it only operates when higher power levels are present. In this way optimum efficiency is obtained over the power range.
One the signal has passed through the amplifier circuits themselves, the outputs are combined using a reverse of the splitter circuit. As this also has a 90° phase shift, this is used to counteract the phase shift at the input. As a result, signals from the two amplifier sections remain in phase.
Doherty amplifier design issues
Doherty amplifier design present some interesting issues. The main design challenges include:
- Phase maintenance: In theory the phase of the signals through the different paths should be that same at the combination point. The splitter introduces a 90° phase shift on one leg, and this can be cancelled at the combination stage as a 90° shift also occurs in the combiner and this can be added in the other leg. However amplifiers will introduce a phase shift and this may not be equally match as one is designed to handle the lower power levels and the other for the peaks. This means that their characteristics will be different (in the asymmetric case).
- Impedance matching: Ensuring that the impedance of both amplifiers is sufficiently maintained over the operating range can present issues in some designs.
- Bandwidth: Typically Doherty amplifier designs are limited in terms of their bandwidth. The splitters and combiners are limited in their bandwidth, and outside this their phase shift varies significantly, impairing the performance of the overall amplifier design.
- Linearity maintenance: It is found that kinks or disturbances in the linearity of the amplifier can occur as the peaking amplifier starts to operate. This adds distortion to the waveform being amplified. Care is also required to ensure linearity over the whole of the operating range.
By Ian Poole
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