**5. Conclusions**

The paper presents the method to improve the amplifier transmittance flatness during pulse as well as other time-varying parameters of amplified signals without deterioration of other significant amplifier parameters such as output power, PAE and gain. The method comprises the Envelope simulations and sophisticated measurement. We simulated the transmittance phase and amplitude in time, e.g., during the pulse. The results of such simulations were consistent with the measurement results. The formulas for the amplifier transmittance were derived. These formulas enable the power amplifiers with minimum transmittance phase changes to be easier designed. For the transparency of this work, presented results relate to simple pulses, but the calculation can be done for a more complex radio signal, like 5G. We are currently working on the application of the presented method to this kind of signal. By applying new design approach, it was possible to improve phase changes on test pulses from 0.5◦ to 0.2◦ and decrease amplitude variation from 0.8 dB to 0.2 dB during the pulse width of 40μs and 40% duty cycle with the 17 W of output power and PAE more than 62%. Though we used GaN-on-Si HEMT, the results are very promising, and we are currently testing and modeling amplifiers with GaN-on-SiC HEMTs.

**Author Contributions:** D.K. designed, and performed characterization, analysis and optimization of the microwave amplifiers as well as measurements of PAs. W.W. performed the design and optimization of GAN HEMT PAs as well as performed modeling of GaN HEMTs and. D.G. performed large-signal electro-thermal modeling of HEMTs as well as performed analysis verification. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the National Centre for Research and Development WidePOWER Project, Contract No. 1/346922/4/NCBR/2017.

**Conflicts of Interest:** The authors declare no conflict of interest.
