# S-Band Class-C-F Power Amplifier with 2 nd Harmonic Control at the Input
## 1 Introduction  
The topic of high efficiency power amplifiers (PAs) will persist as long as the communication industry flourishes. As the most power-consuming devices in a transmitter, PAs generate the most heat. Therefore, improvements in efficiency of PAs not only promote overall system efficiency but also relieve issues of the cooling system.  
A traditional way to achieve improvements in efficiency is biasing a device deeper from class-A. It reduces conductive angle, while sacrificing the gain (up to 6 dB for class-B biasing point and even more for class-C). However, taking the advantage of properly-phased harmonics, in shaping the voltage and current waveforms at the active device output, leads to the minimization of power waste. In other words, the efficiency will be improved. In fact, waveform-engineering has demonstrated its effectiveness, while simultaneously increasing the output power delivered at fundamental frequency [1; 2; 3; 4]. Various classes of PAs have been developed according to this approach when designing the relevant amplifiers [5; 6; 7].  
A classic class-F PA, for instance, utilizes the 3 rd harmonic to shape the drain voltage waveform into a rectangular wave [2; 5; 8; 9], thus improving the output power, the drain efficiency (DE) and the gain up to 15% (theoretically) higher than the one achievable with a sinusoidal output voltage (i.e., tuned load case). Similarly, the proper generation and control of 2 nd or 2 nd and 3 rd harmonic impedances would further improve the PA performances with respect to a tuned load configuration [2]. Besides that, other types of continuous mode PAs make use of harmonics together with the complex fundamental load to increase the bandwidth features, meanwhile maintaining the DE, gain, and output power at higher levels [10; 11; 12].  
Harmonic manipulation has been proven to increase the effectiveness in performance improvement for single stage design solutions by properly exploiting the intrinsic non-linearity of the active device itself. However, most of the designs were biased at a class-AB biasing point. When a transistor is pushed into a class-C biasing range, instead of showing improvement in efficiency, it decreases due to the wrong phase displacement of the harmonic at the drain node, becoming unsuitable for the realization of a class-F PA. However, since the active device typically acts as a voltage controlled current source, there is the possibility to manipulate its voltage harmonic at the input, in order to properly modify the amplitude and phase relationship between the current harmonics and the fundamental one at the drain node [2].  
This paper presents the design and experimental verification of a class-F PA biased in class-C, thus defining a new class-C-F PA. The realization and measurement results show the effectiveness of the above quoted harmonic manipulation at the input port of the device. In fact, by adjusting the loading impedance of the 2 nd harmonic at the input, it can be properly modified for the output current harmonic contents, thus realizing a class-C-F PA by properly phasing the relevant 3 rd harmonic at the output. The designed class-C-F PA achieves a DE higher than 60%, while supplying an output power of 37 dBm over a frequency range from 2.2 GHz to 2.5 GHz in simulation. In the measurement, it achieved a DE of 59.7% and output power of 36.9 dBm at center frequency, well in agreement with simulations.