## 2 Design of the Proposed DPA  
### Design of the Dynamic Power Dividing Network  
Figure 1a shows a schematic diagram of the dynamic power dividing network proposed in this paper. It is implemented at the interstage of the 2-stage DPA and distributes the input power to the carrier and peaking amplifiers. While the conventional DPAs have used power dividers, such as a Wilkinson power divider or a quadrature hybrid, to achieve high isolation between the carrier and peaking amplifiers, the proposed dynamic power dividing network was designed to deliberately weaken the isolation and to use the nonlinear input impedance of the peaking amplifier for better performance. \(Z_{\text{C}\_in}\) and \(Z_{\text{P}\_in}\) of Figure 1 represent the input impedances of the carrier and peaking amplifiers, respectively. The input impedance of the peaking amplifier, \(Z_{\text{P}\_in}\), goes through a large transition while input power increases due to the nonlinear input capacitance, while \(Z_{\text{C}\_in}\) of the carrier amplifier has relatively small variation over the input power [3]. Therefore, the transition  
of \(Z_{P\_in}\) can be utilized to provide different power dividing ratios at the DPA low and peak power levels by controlling design parameters of the interstage transformer, phase offset, and the subsidiary impedance matching network. Figure 2 shows various dynamic power dividing ratios and simulated performances that were obtained from the dynamic power dividing networks with three combinations using differently optimized components compared to the conventional case using a hybrid power divider. The power dividing ratio (\(P_{in\_C}/P_{in\_p}\)) in Figure 1(a) represents the ratio of the power delivered into the carrier amplifier to the power delivered into the peaking amplifier while increasing the input power from the drive stage. Cases 1 and 2 have the peaking amplifier biased to Class-B and have the dynamic power division applied. The difference of the power ratio between the low and peak power levels is designed to be more than 3 dB for cases 1 and 2 as shown in Figure 1(a). At peak power, case 1 has equal power division to the carrier and peaking amplifiers, and case 2 has 1 dB more power driven to the peaking amplifier at the peak power. Case 3 has the dynamic power division applied but has the peaking amplifier biased to Class-AB, and therefore, the variation of the power dividing ratio is low. Case 4 is the conventional case where the peaking amplifier is biased to Class-AB, and no dynamic power dividing is applied. It can be seen in Figure 1(a) that the relatively large transition of \(Z_{P\_in}\) can considerably boost the gain at the low power levels for cases 1 and 2 compared to cases 3 and 4, along with improvement of PAE resulting from the Class-B bias as shown in Figure 1(b). Meanwhile, the AM-AM characteristics can be improved when appropriately more input power is driven to the peaking amplifier at the peak power level. Case 1, which has equal power delivered to the carrier and peaking amplifiers at the peak power level, shows the poorest AM-AM characteristics, as shown in Figure 1(c). Case 2 has more input power to the peaking amplifier by 1 dB at the peak power level, but has slightly unfavorable AM-AM characteristics compared to the conventional case 4 due to the deeper class of the peaking amplifier. Case 3 shows good AM-AM characteristics, but it has lower PAE compared to cases 1 and 2 since it has more input power to the Class-AB peaking amplifier at the peak power level. The relationship of efficiency and dynamic range of power division is presented in Table 1.  
In this paper, case 2 was selected and designed for the DPA configuration. Case 2 has advantages in enhanced power gain using the large variation of the power division ratio by more than 3 dB according to the input power level and in high efficiency with the Class-B peaking amplifier.