## 3 Dual-Band Doherty Power Amplifier
### \(\Pi\)-And T-Networks for Dual-Band DPA  
The classic DPA has two simple \(\uplambda\)/4 transformers: the impedance inverter and global matching, as shown in Figure 1. These two transmission lines are different in their purpose of work. A simple \(\uplambda\)/4 transformer (global matching) can be replaced with a two-section transmission line form which is easily adjusted to give the exact impedance transformation at two frequencies, as shown in Figure 5 (more details of dual-band transformer can be found in [28]). The inherently larger bandwidth of the two sections can be maximized when no matched operation is required between the two operating bands. However, the two-section form cannot be assumed as an impedance inverter, due to the different of work purpose; instead, the impedance inverting section is realized for split band operation as a \(\Pi\)-network by adding two stub sections to a single near-\(\uplambda\)/4 section [25]. In a comprehensive passive design of a dual-band DPA (1.8 GHz and 2.4 GHz) presented by [25], the design of each amplifier depended on the load-pull/source-pull simulation to find optimum load and source impedances. For dual-band harmonic control, two shunt stub lines in addition to (TL\({}_{1}\) and TL\({}_{2}\)) are used as shown in Figure 6. The first is a short circuit terminated with a length of \(\uplambda\)/4 at f2, where it used to cancel the effect of the second harmonic at the upper band at point A, whereas the second with a length of \(\uplambda\)/8 at f1 is open to suppress the second harmonic of the lower band at point B. The designed amplifier of [25] was showing good performance. Additionally, a T-network was used in a dual-band DPA working at 900 MHz and 2000 MHz in [7] as an alternative component for the \(\uplambda\)/4 inverter, as shown in Figure 7. In the output matching network, a T-network with a shorted shunt line in addition to an open stu  
Figure 5: Dual-band global matching network. Figure 6: Dual-band Doherty power amplifier (DPA) schematic designed by [25]. stub was used to adjust the phase of output, whereas the T-network with a shorted shunt was used as a dual-band impedance transformer and as a dc feeder at the same time. The suggested solutions of using \(\Pi\)- or T-topology in [7; 25] were only used for impedance inverters, nevertheless, a modified \(\Pi\)-network is employed as an output combiner [18], which eliminates the need for using an offset line by absorbing the output parasitic of both the main and auxiliary amplifiers; three networks are used in the dual-band design as illustrated in Figure 8, the first is a short-circuited \(\Pi\)-network at the main path, which works as dual-band impedance inverter, and double \(\Pi\)-networks at the auxiliary path work as a \(-180^{\circ}/0^{\circ}\) impedance transformer. The linearizability of the design in [18] is enhanced by providing small low-frequency impedances for both amplifiers; The shunt shorted-stub of the \(\Pi\)-network helps to reduce the effect of output parasitic capacitor of both amplifiers, so that the bandwidth limitations will be reduced; the shorted stubs can also be used as a dc feeder for both transistors. Besides, Dual-band DPA attempt targeting 2.14 GHz and 3.5 GHz was done by [29]. It explained alternative components that can work as dual-band input power splitter, impedance inverter network, and phase compensation network. In general, a cascade of two transmission line sections is used for the impedance transformer network; however, as noted, this configuration cannot be used as an impedance inverter network because the relationship between the voltage of one side does not depend on the current of the other side, so that, \(\Pi\)-network or T-network is used for input power splitter and impedance inverter network.