## 4 Experimental Results
### Single-Tone Measurements  
The power added efficiency (PAE), AM-PM and Gain/AM-AM as a function of output power are presented in Figure 8 at 26 GHz (the center of 3 GPP/NR FR2 band n258) with varying V\({}_{\text{g1}}\) bias settings. Note that, as explained in Section 3, the power calibration moved the reference planes to the PA input and output (see the red dotted line in Figure 5); thus, the RF pads are excluded from the results shown here. The achieved saturated output power Psat = 20 dBm at 26 GHz, which was achieved with each bias level. The peak PAE = 26 % at 26 GHz, which was measured at the lowest bias voltage. The lowest gate V\({}_{\text{g1}}\) biased the PA close to the class B operating class, and thus the higher input power level self-biased the PA. As a result, the AM-PM response with I\({}_{\text{dQ}}\) = 17.7 mA decreased at a higher input drive than the other AM-PM curves (increasing self biasing seemed to linearize the PA). The total AM-PM below 16\({}^{\circ}\) was relatively low. The most notable difference vs. the bias level can be seen in the PAE curves, especially in the back-off region. For instance, at Pout = 14 dBm PAE varied by 6.5 percent units (from 9.7 % at I\({}_{\text{dQ}}\) = 48.7 mA to 16.2 % at I\({}_{\text{dQ}}\) = 17.7 mA), while the difference in gain was less than 2 dB (see Figure 9c)) (from 12 dB at I\({}_{\text{dQ}}\) = 17.7 mA to 13.7 dB at I\({}_{\text{dQ}}\) = 48.7 mA). This gain difference was slightly higher compared to the S-parameter measurements due to the different measurement type (power sweep) and calibration (excluding RF pads). Figure 7: Comparison of the measured and simulated S-parameters of the stacked PA with maximum bias setting (Idq = 48.7 mA). The simulation results are also shown in Figure 8 (dashed lines) using the maximum V\({}_{\text{g1}}\) bias. It can be observed that the simulated gain matched quite closely with the measured large signal gain curve at lower input power levels. However, simulations predicted a higher compression point and saturated output power than the measurements. Similar to the higher 1 dB compression point, the simulations predicted less AM-PM. The main difference can be seen in the PAE curves; the simulated PAE was clearly lower in the simulations. These differences were mainly a result of the fact that the simulated PA predicted a larger current consumption with the same gate bias voltages. The main figures of merit are presented in Figure 9 as a function of frequency. The solid lines indicate the smallest bias current (I\({}_{\text{dQ}}\) = 17.7 mA) and dashed lines show the maximum bias current (I\({}_{\text{dQ}}\) = 48.7 mA). It can be seen from Figure 9a that the highest 1 dB compression point was 18.8 dBm and Psat was 20.5 dBm at 24 GHz. In terms of power saturation, the difference between solid and dashed (minimum and maximum bias currents) curves was small. The PAE curves presented in Figure 9b show a similar behavior. The peak PAE was 29% at 24 GHz and the PAE decreased towards higher frequencies. The PAE difference between maximum and minimum bias currents was small at 3 dB compression and power saturation but increased in the back-off area. As mentioned above, the PAE difference between minimum and maximum bias currents was 6.5 percent units at 6 dB back-off at 26 GHz. The measured gain was reasonably constant with respect to the frequency at the saturation (see Figure 9c) but showed a larger variation at lower drive. The maximum gain of 13.7 dB was measured at 26 GHz using I\({}_{\text{dQ}}\) = 48.7 mA bias current. It can be seen that the optimum power matching occurred at a point 2 GHz lower than the optimum gain, which was likely due to the limited modeling of the large signal simulations.