# S-Band Class-C-F Power Amplifier with 2 nd Harmonic Control at the Input
## 3 Design and Measurements of Class-C-F PA  
In order to validate the concepts presented in previous section, a single stage PA centered at 2.4 GHz has been designed and measured, by using the 6 W GaN transistor CGHV1F006s from CREE. The pinch off voltage of the device is \(-3\) V, and its knee voltage is nearly 6.5 V with a drain maximum current of roughly 0.6 A. For the design of the amplifier, the drain voltage was limited to 30 V for practical reasons (i.e., test equipment limitation), while the gate bias voltage was set to \(-3.5\) V. The schematic of the designed amplifier is shown in Figure 6, meanwhile the photo of the realized hardware is shown in Figure 7.  
The key point of the class-C-F PA design is to accomplish the gate waveform similar to the ideal one shown in Figure 5 through the proper control of the harmonic impedances at the gate node. In fact, the input matching network (IMN) was designed not only to fulfill the conjugate matching condition at fundamental frequency but also for properly controlling gate voltage by adjusting the 2 nd harmonic loading condition. In particular, instead of shorting the latter at the input, as typically done in a tuned load PA design, corresponding to Point B highlighted in Figure 8, it has been modified up to point A, in order to properly shape the intrinsic gate voltage, as shown in Figure 9.  
The phase and amplitude relationship between the 2 nd gate voltage harmonic and fundamental one has been checked in the design process, in order to maintain a top flattened waveform shaping, as shown in Figure 9. The amplitude of such a ratio (i.e., parameter \(h_{2}\) as defined in (3)) is reported in Figure 10.  
Figure 5: Theoretical gate waveform for class-C-F PA.  
Figure 6: Schematic of the designed class-C-F PA. VGG = Gate Bias; VDD = Drain Bias; RF = Radio Frequency. Figure 8: Input (extrinsic) and output (intrinsic) impedance for class-C-F PA.  
Figure 10: The simulated harmonic amplitude ratio between 2 nd harmonic and fundamental frequency at gate node. Figure 7: Photo of the realized class-C-F PA.  
Figure 9: Simulated waveform with respect to the impedance in Figure 9.0  
Not only is the amplitude crucial but also the phase, since properly phasing the 2 nd harmonic can guarantee the flatness behavior of the gate voltage when it achieves its maximum value (see Figure 9). It is to highlight that both amplitude and phase should be carefully monitored. In fact, even if the amplitude of the 2 nd harmonic was higher for the loading condition B, after the PA saturation, its phase relationship with the fundamental frequency component would be unsuitable for the required waveform shaping, becoming detrimental to the PA performances, as it will be shown later. Thus, in practical design, both amplitude and phase have to be controlled, eventually compromising the amplitude a little, as presented in Figure 10, where maximum value is quite close to the ideal value of \(\sqrt{2}/4\). The proper input harmonic loading control, together with the strong truncation effect caused by the class-C biasing condition, results in a rectangular-like (or trapezoidal-like) output current waveform synthesized at the intrinsic drain node, as shown in Figure 9.1 For the design of a class-F PA, the condition of low impedance for the 2 nd harmonic and high impedance for the 3 rd harmonic of the output voltage was assured through the design of the OMN, also shown in Figure 9.2 The simulated output loads at the device's intrinsic current source plane are shown in Figure 9.3  
The load lines resulting from different input/output harmonic terminations are reported in Figure 11, from which it can be highlighted how the proposed approach demonstrates the possibility to design (and realize) a class-F PA biased in class-C, without risking to over-drive the active device. The simulated DE, output power, and gain behavior are also compared with the measurement results, as reported in Figure 12a-c, respectively, showing a good agreement.