## 3 Dual-Band Doherty Power Amplifier
### Dual-Band Design Improvements  
The linearity of the DPA can be improved by employing the ET technique at the gate of each transistor [38], where the effect of the third order modulation can be cancelled. In addition, varactor diodes have also been used adaptively for improving the bandwidth [39; 40]; these techniques have very complex circuity. Moreover, dual-band reconfigurable Doherty amplifier was introduced by [21] where a PIN diode was used adaptively in to remove the reactance components of the load each frequency band. There was an enhancement about 5% of power added efficiency (PAE) compared with a traditional DPA, at the same time, the diode was used to switch between the two bands, where the state of the diode determines the selected band. Moreover, a large back-off compared to previous works been achieved in [23; 41; 42], which describe a dual-band multiway DPA. A tri-way DPA was designed to cover the two bands 0.9 GHz and 2.31 GHz using an unequal power divider as shown in Figure 12, where three Wilkinson power dividers are used to produce a quad-way power divider, the first power divider was split the input power unequally, whereas the next two power splitter split the power equally in order to avoid adjusting the auxiliary gate bias by showing the importance of power splitter to manipulate the gain of the auxiliary amplifier that varies depending on the frequency [43]. The saturated designed power was 46 dBm, whereas 61% and 44% efficiency at 9 dB back-off power were achieved at both bands respectively. It can be noticed that the amount of output current of the auxiliary amplifier is less than for the main amplifier due to the lower gain of the auxiliary amplifier which is deeply biased; in this case, less efficient load-modulation will occur at a high-power level. For this reason, an appropriate selection of the power ratio should be chosen to maximize the load-modulation effects. Depending on the probability density function of a signal with a non-constant envelope, i.e., it has a substantial value of PAPR, the output signal spends most of the time near to the average output power. Hence, the power amplifier should be designed to produce its highest efficiency near the average power. Adaptive input power control for a dual-band DPA was used in [44], which relied on the adaptive power splitter invented by [PERSON] et al. [45]. The adaptive input can be used to deliver more power to the main amplifier than the auxiliary amplifier when the DPA is working in the low power region and vice versa. The main idea of [45] is to make the auxiliary amplifier turn on later so that, at the OBO, early load modulation can be avoided and after that, more power will be delivered to the auxiliary amplifier to compensate the low gain due to the biasing condition and improving the load modulation. Extending the bandwidth of a dual-band of DPA using a novel output combiner was realized in [46] where a resonant shunt LC tank was added at the combining node to maintain the main amplifier back-off impedance and reactance compensation by reducing the reactance variation with frequency, however, this technique is limited by the LC band. In the work presented by [46], the 20 W peak power  
Figure 12: Tri-way Dual-band DPA with frequency dependent power division [43]. DPA used two GaN HEMT devices with different capabilities, where a 10 W device was used for the main amplifier and operated as class F, whereas a 25 W device was used for the auxiliary amplifier and biased as class C. This amplifier showed a PAE of 42% when the dual-band input contained long term evolution (LTE) and WCDM signals concurrently with 15 MHz bandwidth for each. In [47; 48], the main and auxiliary power amplifiers operating classes was changed to class ABJ, where harmonics control was employed. The main thing to note is the linearity measurements in terms of adjacent channel power ratio (ACPR) were less than 25 dBc for both bands.