# Review of Ka-Band Power Amplifier
## 3 Topologies
### Doherty Power Amplifier  
With the development of satellite communication technology, in order to obtain a higher transmission rate, the signal contains multi-channel subcarriers with variable phase and amplitude, but also makes the signal peak-to-average-power ratio (PAPR) larger. Signals with high PAPR require power amplifiers with higher efficiency and linearity. The Doherty power amplifier (DPA) has been widely used because of its relatively simple implementation and obvious efficiency improvement [93; 94]. A conventional DPA typically includes a power splitter, carrier amplifier, peaking amplifier and load modulator. Among them, the carrier amplifier is also the main amplifier, which is in the class AB bias, and the peak amplifier is also the auxiliary amplifier, which is in the class C bias. The Doherty PA circuit schematic diagram is shown in Figure 22.  
The input signal is divided into two signals of equal amplitude and phase through the power splitter, which enter the carrier amplifier and peaking amplifier, respectively. After amplifier amplification, the output is synthesized by the load modulation network. Load modulation networks are generally composed of a \(\lambda/4\) transmission line to match the output impedance of the carrier amplifier and peak amplifier to 50 \(\Omega\) through impedance transformation. Generally, carrier amplifiers and peak amplifiers use the same device, but due to the low input bias of the peak amplifier, the output current of the peak amplifier cannot be the same as that of the carrier amplifier. Therefore, the optimal load modulation cannot be realized, resulting in low output power and efficiency. To solve the problem of low output power and efficiency of DPA, the common solutions use asymmetric DPA topology, using new load modulation scheme to realize the full modulation, etc. Asymmetrical DPA topologies include asymmetrical input splitter [95] and asymmetrical stack gate bias [96]. In 2013, a 32 GHz transformer-based Doherty PA with Wilkinson  
Figure 22: Doherty PA circuit schematic diagram. splitter was presented. The Wilkinson splitter distributes the input signal unequally to carrier amplifier and peaking amplifier, and matches the output impedance to 50 \(\Omega\) to improve the output power. The fabricated prototype exhibits measured P1 dB of 25.1 dBm, peak PAE of 38% and record PAE of 27% at 6 dB back-off power. In [96], [PERSON] proposed a stack-FET DPA. The gate bias of the common-gate transistors in the stacked FET can control the current of peaking amplifier (see Figure 23). With higher gate bias, the peaking amplifier's higher current at saturation enhances efficiency. The experimental results demonstrate that the peak PAE is 37%, PAE at 6- dB back-off power is 27% and P1 dB is 28.2 dBm at 28 GHz. In 2018, [PERSON] presented a new load-pull-based design technique in order to choose an appropriate device size and the impedance of the \(\lambda/4\) inverter to achieve the highest PAE [97]. With this new method, the experimental results show PAE of 32% at 6 dB back-off and 28.5% at 8 dB back-off. In addition to the above methods, the parallel series parallel power combination technology [98] commonly used in bulk CMOS technology is used to improve the output power, and the high turn ratio current source transformer [99] is used to improve the fallback efficiency (see Figure 24). In the GaAs process, in order to reduce the chip area, the methods mentioned are as follows: Lange coupler [100; 101]; input broadside coupler [102]; removal of the offset line when the power ratio between the main and auxiliary PA is appropriate [103]; replacing the transmission lines and offset lines with LC elements [104], and so on. Figure 24: Transformer-based DPA in 90 nm CMOS [99]. Figure 23: Stacked-FET MMIC DPA in 120 nm GaAs pHEMT [96]. The bulk CMOS technology is relatively small in area compared to the GaAs process, but the efficiency and Psat of bulk CMOS are limited due to lossy and complex Doherty output networks.