## 4 Conclusions  
The techniques for designing dual-band Doherty amplifier have been reviewed and presented. The main limitation of each technique has been also addressed. The dual-band DPA was found as a promising technique that can be used in modern and future communications systems (e.g., satellite communications and mobile communications) where carrier aggregation and SDR can be applied. It was concluded that the offset line length mainly depends on the parasitic effect of the amplifier, in addition to the matching network characteristics. Using modern RF switches, which may include MEMS, can help to make the design of dual-band power amplifiers easier. The linearization process depends on characteristics of the PA devices, which vary depending on the operational frequency band, so that, in multi-band power amplifiers, the linearization process will be more complex and may be extremely so when the frequency bands are used concurrently. A solution for dual-band linearization is to use 2D-DPD, which will be able to achieve the required linearity at the cost of complexity. The DPA back-off depends mainly on the load modulation between the main and auxiliary amplifier, and at the same time, can be increased as the power ratio between the peaking power and the main power increases; however, selecting device sizes for the main and the auxiliary amplifiers is not always easy because it depends on the market availability of the devices. The use of MMIC technology gives flexibility in selecting the device peak power ratio; nevertheless, the output power of this technology is still low. For low frequency up to 2.2 GHz, LDMOSFET technology is often preferable; however, the GaN HEMT has smaller input and output capacitance due to its small periphery, and its transition frequency is high. Currently, the GaN HEMT has a good broadband performance, high efficiency, and large breakdown voltage, due to its features. It has given the designers a better ability to design an efficient PA.  
The market for mobile communications covering more than one frequency band is growing. In this frame, the dual-band or multi-band Doherty amplifier can support the operation of the PA to cover two or more frequency bands with good efficiency at the back-off power, for which, the cost and the size can be reduced at the same time to preserve energy efficient operation.  
Conceptualization, A.M.A. and M.A.Y.; Methodology, A.M.A., N.M. and M.A.Y.; Software, A.M.A., R.A.A.-A. and M.A.Y.; Validation, A.M.A., R.A.A.-A., N.M. and A.R.; Formal analysis, A.M.A., R.A.A.-A., J.N., N.M., N.O.P., Y.L.A.A.-Y. and A.R.; Investigation, A.M.A., M.A.Y., R.A.A.-A., J.N., N.M., N.O.P., Y.L.A.A.-Y. and A.R.; Resources, A.M.A., M.A.Y., R.A.A.-A., J.N. and Y.L.A.-Y; Data curation, A.M.A., M.A.Y. and R.A.-A.; Writing--original draft preparation, A.M.A., M.A.Y. and R.A.A.-A.; Writing--review and editing, A.M.A., M.A.Y., J.N. and R.A.A.-A.; Visualization, A.M.A., M.A.Y., R.A.A.-A. and J.N.  
This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424.  
Authors wish to express their thanks for the support provided by the innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET-722424.  
The authors declare no conflict of interest.