# Design of K-Band Power Amplifier with 180-Degree Phase-Shift Function Using Low-Power CMOS Process
## 2 Design of the CMOS Power Amplifier with LP Process
### 180\({}^{\circ}\) Phase Shift Function in the First Driver Stage  
As described in the previous section, the power amplifier was designed as a four-stage structure to secure sufficient power gain in the LP CMOS process. However, when the power amplifier is composed of four stages, the overall IC area inevitably increases. In particular, compared with the power amplifier based on the GP CMOS process, the LP CMOS-based power amplifier requires more amplification stage, considering the low \(g_{m}\) of the LP CMOS process. The increase in the area makes the configuration of the beamforming system difficult. Therefore, in this study, the 180\({}^{\circ}\) phase-shift function, which occupies a large area in the beamforming transceiver, was embedded in the power amplifier to reduce the area of the entire transceiver. The phase-shift function was embedded in the first driver stage with the lowest power handling. Because each stage has a power gain, the power that the first stage deals with is the lowest, and the power increases as it goes to the power stage. Therefore, if the phase-shift function is implemented in the second or third stage rather than the first stage, the size of the transistor for implementing the phase shift function increases, thereby increasing the additional power consumption, causing the power amplifier to lose  
Figure 2: Schematic of the designed CMOS power amplifier. efficiency. In addition, in extreme cases, if the phase-shift function is implemented in the power stage, the power-matching technique should be implemented simultaneously, which hinders securing the output power of the power amplifier. Accordingly, by embedding the phase-shift function in the first driver stage, it was possible to minimize the power amplifier's output power degradation due to the embedding of the phase-shift function. Figure 3 shows the typical schematic of the amplification stage for embedding a 180\({}^{\circ}\) phase-shift function [9]. As shown in Figure 3, in general, to realize the 180\({}^{\circ}\) phase-shift function, two identical amplifiers, which are located in parallel to each other, may be used. In Figure 3, \(V_{\text{CG},P}\) and \(V_{\text{CG},N}\) were the gate bias of the common-gate (CG) transistors while \(V_{\text{CS}}\) was the CS transistor. In the typical structure, one of the two amplifiers is selectively operated by \(V_{\text{CG},P}\) and \(V_{\text{CG},N}\), thereby implementing the 180\({}^{\circ}\) phase-shift function. However, in this case, due to the two amplifiers, the required chip area could increase. Figure 4 shows the schematic of the proposed first driver stage for embedding a 180\({}^{\circ}\) phase-shift function. In this work, considering the overall chip area of the power amplifier, instead of using two identical amplifiers, the phase-shift function was implemented by modifying a single-cascode structure. As shown in Figure 4, from a general cascode structure, a CG transistor was designed by dividing it into two. \(\text{M}_{\text{CG},\text{P1}}\), \(\text{M}_{\text{CG},\text{P2}}\), \(\text{M}_{\text{CG},\text{N1}}\), and \(\text{M}_{\text{CG},\text{N2}}\) were the divided CG transistors. The input signal path was converted to 180\({}^{\circ}\), according to the \(V_{\text{CG},N}\) and the \(V_{\text{CG},P}\). The \(V_{\text{CG},P}\) (or \(V_{\text{CG},N}\)) was gate bias voltage of \(\text{M}_{\text{CG},\text{P1}}\) and \(\text{M}_{\text{CG},\text{P2}}\) (or \(\text{M}_{\text{CG},\text{N1}}\) and \(\text{M}_{\text{CG},\text{N2}}\)) As a result, the CS transistor of \(\text{M}_{\text{CS}}\) operated at all times in the 0\({}^{\circ}\) and 180\({}^{\circ}\) phase-shift modes. On the other hand, as can be seen in Figure 4, \(\text{M}_{\text{CG},\text{P1}}\) and \(\text{M}_{\text{CG},\text{N1}}\) (or \(\text{M}_{\text{CG},\text{P2}}\) and \(\text{M}_{\text{CG},\text{N2}}\)), which are divided-cascode transistors, operate selectively, according to each mode, and consequently perform a phase shift function.