## 2 Results
### Large-Signal Investigation for PA Design
#### 2.2.2 Comparison with Other Configurations  
In order for an appropriate comparison of the LS operation, the bias conditions of each transistor of the independently biased configurations, including independently biased cascode and independently biased 3-stack, are individually optimized to achieve their best performance. The conventional configurations including single-stage, conventional cascode, and conventional 3-stack cannot do this due to the lack of the additional bias terminals. The LS investigation is conducted at an operation frequency of 3.5 GHz which is useful for using in next-generation of wireless communication systems. The GaN HEMT LS model is provided by WIN Semiconductor Corp. Figure 13 shows the comparison in output power and power gain among the configurations. Obviously, single-stage and cascode configurations have significantly low power gain and output power compared with that of the 3-stack configurations. The proposed 3-stack configuration has similar output power and power gain as that of the conventional 3-stack one. Both of them have the highest output power and power gain compared with the other configurations. This is because the 3-stack configurations can deliver higher output voltage swing. Figure 11: LS investigation with \(V_{\text{g2}}\) variation. Figure 12: LS investigation with \(V_{\text{g3}}\) variation. It is noted that the efficiency of each configuration is obtained by using a load/source pull technique based on the LS GaN HEMT model to find out their optimum load/source impedances as mentioned previously. These optimum impedances for each configuration are given in Table 2.  
Additionally, the bias conditions of each configuration are given in Table 3. In contrast to the SS investigation in which operation condition is set at the class-A, in LS investigation the operation conditions of the transistor are set at a class-AB for the non-linear operation. Along with efficiency, another key performance when designing power amplifier is the linearity. Although both the 3-stack configurations have similar output power and power gain, the efficiency of the conventional 3-stack one is poorer than that of the proposed 3-stack one, as indicated in Figure 14. As shown in the figure, the proposed 3-stack has superior efficiency among the configurations.In this paper, IMD3 is tested at a center frequency of 3.5 GHz with a frequency spacing of 4 MHz. In Figure 15 the IMD3 performances of the configurations corresponding to their efficiency are shown. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Configurations** & **Source Impedance (f)** & **Load Impedance (f)** \\ \hline Single-stage & 22.3 + j31.2 & 233 + j273 \\ Conventional cascode & 56 + j293 & 223.2 + j250 \\ Independently biased cascode & 56 + j293 & 223.2 + j250 \\ Conventional 3-stack & 55.9 + j101 & 179 + j273 \\ Independently biased 3-stack & 55.9 + j101 & 179 + j273 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Optimum source and load impedances of the compared configurations. Figure 13: Output power and power gain characteristic. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Configurations** & \(V_{\text{g1}}\) & \(V_{\text{g2}}\) & \(V_{\text{g3}}\) & \(V_{\text{d1}}\) & \(V_{\text{d2}}\) & \(V_{\text{d3}}\) \\ \hline Single-stage & \(-2.6\) V & X & X & 44 V & X & X \\ Conventional cascode & \(-2.6\) V & \(-2.6\) V & X & 44 V & X & X \\ Independently biased cascode & \(-2.6\) V & \(-2.6\) V & X & 6 V & 44 V & X \\ Conventional 3-stack & \(-2.6\) V & \(-2.6\) V & \(-2.6\) V & X & X & 44 V \\ Independently biased 3-stack & \(-2.6\) V & \(-2.6\) V & \(-2.6\) V & 5 V & 4 V & 44 V \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Bias condition of configurations for large-signal (LS) investigation. In IMD3 test, the limit level should be at \(-35\) dBc. The figure shows that IMD3 of the single-stage cannot reach this level because it does not use any linearity improvement method. On the other hand, the other configurations can reach this IMD3 level. Table 4 summarizes this comparison.