## 2 Efficiency-Enhanced Method  
To address the issue of low power back-off efficiency caused by the high peak-to-average power ratio (PAPR) of WLAN signals, this work proposes a fully integrated PA with two operation modes to improve back-off efficiency. The PA consists of two stages, and the power cells of the second stage are reconfigurable and divided into five units. As illustrated in Figure 1, the proposed PA has two operation modes: high-power mode and low-power mode. In high-power mode, all power cells in the second stage of the PA are activated for high output power. Additionally, gm linearization, PMOS compensation, and the on-chip high Q output matching network are employed to ensure high linearity and output power. In the low-power mode, some of power cells in the second stage of the PA are turned off to improve efficiency and reduce DC power consumption, because the second stage of the PA mainly affects the total efficiency. The power-added efficiency (PAE) can be improved by \(\times\) 1.5 in low-power mode with the same output power in the high-power mode. Moreover, with the on-chip power controller circuit dynamically configuring the PA's operation mode according to the output power level, the PA allows not only static control but also dynamic control. In the dynamic control switching mode, the average power-added efficiency (PAE) over the entire output power range is 1.5 times higher than that of the PA without mode switching. In the case of dynamic mode switching, the power controller detects the power of the circuit and generates a control signal to adjust the power cells and input capacitors. When operating at low-output power levels, the power detector detects that the power is low and generates a control signal to turn off four of the five power amplifier units, leaving only one unit turned on. As a result, the power-added efficiency (PAE) can be improved due to the reduced power cell size, and the DC current can also decrease significantly. As the output power continues to increase, the remaining four units are turned on by the control signal. In high power conditions, all power cells are turned on to provide high linearity and power. Through the method of this work, the back-off power efficiency can be greatly improved. Figure 2a shows the simplified single-ended schematic of the proposed reconfigurable power cells. The small signal equivalent circuit of the power cell is shown in Figure 2b. Assuming that all the transistors work in the saturation region and the output impedance is ignored, the small signal voltage gain can be derived as  
\[A_{v}=\frac{V_{out}}{V\!n}=-Z_{L}\frac{jwM}{(Rs+R_{L})+(jwLs+jX_{L})}.gm2\! \left\{gm1-\frac{jwC1g_{m1}}{jwC1+gm2\frac{C1}{Cg^{2}}}\right\}\]  
Figure 1: Architecture of the proposed CMOS PA.  
where \(Z_{L}=R_{L}+jX_{L}\), \(M=k\sqrt{L_{1}L_{2}}\). Thus, there will be phase and amplitude discontinuity during the dynamic mode switching between the high-power mode and low-power mode, causing AM-AM and AM-PM distortion. The large AM-AM and AM-PM distortion may cause severe linearity degradation. Reference [40] shows that the ACLR degrades rapidly with larger gain and phase discontinuities. To address the issue of gain and phase discontinuities caused by the dynamic power control mode, this work proposes the addition of variable capacitors at the input matching to ensure minimal discontinuities in total gain and phase. Therefore, in order to achieve high performance, careful design considerations must be taken into account. Specifically, the gain variation at the input matching must be less than that of the second stage PA, and the total variation of the gain should not exceed 1\(\sim\)1.5 dB. This approach can help minimize distortion and ensure high linearity. The remaining distortions could be compensated by digital pre-distortion (DPD). Power cells are carefully designed to maximize efficiency and linearity. The detailed circuit design will be presented in Chapter 3. Figure 2: (a) Simplified schematic of the proposed reconfigurable single-ended power cells.