# Highly Efficient GaN Doherty Power Amplifier for N78 Sub-6 GHz Band 5G Applications
## 4 Implementation and Simulation Results of the N78 Band Sub-6 GHz DPA
### The Performance of the Harmonic Suppression Network  
Figure 22 displays a photograph of the post-harmonic suppression network that was fabricated and combined with a 35.35-ohm impedance transformers microstrip line at the input port and a 50-ohm microstrip line at the network end to allow sufficient soldering space for the SMA coaxial connectors. The response of the proposed filter was measured using an R&S(r) ZVB vector network analyzer from 0 to 16 GHz, and the results are shown in Figure 23. To make the measurement easy, the source/load impedances of the filter were set at 50 \(\Omega\), and the filter exhibited a seventh-order general Binomial low-pass response that was visible. The measured response (blue solid line) of the filter had a low-pass characteristic with a 3 dB cutoff frequency of 4.9 GHz. The insertion loss reached 10 dB at 5.5 GHz and 26.3 dB at 7 GHz. These measurements were in good agreement with the simulated results. Figure 24 illustrates the effect of our proposed design (DPA with HS) on the attenuation output power of harmonics from the second to the fourth order in comparison to the harmonic content of a conventional DPA (DPA without HS) in terms of input power. The results demonstrate that the suggested topology provides excellent harmonic suppression with high efficiency versus input power. Although the Doherty amplifier is a balanced amplifier, our design considered the fundamental frequency. We incorporated a suggested harmonic suppression network at the output to prevent the occurrence of harmonics, which is clearly demonstrated in the harmonic balance simulation results shown in Figure 25. The impact of the post-harmonic suppression network is illustrated by comparing the harmonic spectra at the output of the RF amplifier. Figure 25 displays the spectra of four harmonics for power, voltage, and current obtained from the harmonic balance (HB) simulation. Each spectrum figure includes DC (0 Hz), fundamental (3.5 GHz), second (7 GHz), third (10.5 GHz), and fourth (14 GHz) harmonic components. Notably, the values of the second to fourth harmonics are all below \(-\)50 dBm, providing evidence of the effective suppression achieved by the proposed harmonic suppression network. This successful suppression contributes to increased amplifier efficiency and improved linearity of the output signal. These findings are further supported by the simulated time-domain output voltage and current waveforms for the proposed DPA, as illustrated in Figure 26.  
Figure 22: Photograph of the fabricated post-harmonic suppression network. Figure 23: Results of the electromagnetic simulation (dashed) and measured (solid) S-parameters for the proposed harmonic suppression network. (**a**) S11, (**b**) S12, (**c**) S21, and (**d**) S22. Figure 24: Comparison of power reduction at harmonic frequencies between the proposed design (DPA with HS) and the conventional design (DPA without HS). The proposed DPA components were assembled with the addition of two transmission lines for 50-ohm impedance at the input and output ports. Optimizing these transmission lines, converted from electrical lengths to physical dimensions, yielded the highest results via EM simulation (Figure 27). This simulation confirms the DPA's proper operation within the 3.3-3.8 GHz frequency range, as seen in Figure 27. The parameters of gain, output power, DE, and PAE at the center frequency of 3.5 GHz are presented. Notably, maximum PAE and DE values reach around 75.2% and 80%, respectively, while the amplifier achieves a large-signal gain exceeding 12 dB and an output power of approximately 42 dBm. As previously discussed, the proposed DPA structure significantly enhances PAE, DE, and gain. These improvements were investigated by implementing an unequal Wilkinson power divider at the amplifier's input and introducing a harmonic suppression network at the output.