Doherty amplifier variants
The following are variations on the basic concept, which might be more appropriate for some applications.
Commonly used variants, plus their hybrids, include:
Doherty amplifier cascading variant
Cascading two or more stages can free up bias selection of the final stage, but renders the design liable to greater amplitude/phase dispersion and variation.
Cascading two or more stages, inside the split/combine section, can free up bias selection of the final stage. This can increase both output power and efficiency.
The downside of this solution is the increased number of devices, and their associated gain/phase variations. This effect might even be sufficient to reverse the improvements achieved through common biasing.
Doherty amplifier adjustable splitter variant
With an adjustable splitter, higher performance can be achieved across the bandwidth. But, this bandwidth improvement cannot be achieved instantly or simultaneously, plus adaptation might be needed.
Combining the simple cascade with an adjustable input splitter (intended to provide different splitting for different frequencies) can improve matters, over the "Cascading" case.
Depending on the splitter design however, the improvements may only be applicable to a narrowband. Furthermore, adaptation of the split ratio for different frequencies might require additional control and monitoring circuitry.
Doherty amplifier adjustable splitter variant
A (frequency) dispersive split design is one which intentionally provides intentionally different amplitude and phase splitter values at different frequencies. In a simple design, this might utilize a high pass structure in one path, and a low pass structure in the other. For example, this might provide the typical 90 degree phase difference in the middle of the band, but 80 degrees and 100 degrees at the low- and high-band edges respectively.
Doherty amplifier digital doherty variant
This architecture is characterized by an input split which stretches back into the digital domain, prior to digital-to-analog conversion (DAC). The ability to apply digital signal processing (DSP) techniques to the signal applied to both paths provides for a potentially unsurpassable performance elicitation from a given piece of RF hardware. Compared to the Zeroth implementation, it can offer 60% greater output power, 20% more efficiency, 50% more bandwidth without degrading predictable linearizability.
