# Alternative Categorization of Radio Frequency Power Amplifier for Generalized Design Insights
## 3 Alternative Categorization
### Frequency
#### 3.2.2 Continuous Approach  
The other method of widening the bandwidth of an RFPA is more analytical but does not require any additional control circuitry, thereby reducing the overall complexity of the system. The continuous class, initially introduced by [PERSON] et al. [28], laid the theoretical foundation for the design of broadband RFPAs. Among those, the continuous F [29; 30], continuous AB [31], and continuous X [32] amplifiers have gained recognition for their ability to operate efficiently over a wide frequency range.  
The mixed-mode design concept, which was initially introduced by [PERSON] et al. [33], was primarily utilized to achieve highly efficient designs. However, as the notion of broadband performance gained prominence, it evolved into the concept of Continuous Class E (CCE) [34; 35].  
By analyzing the waveforms in the Fourier domain and evaluating harmonic impedances, designers can extend the accessibility of load terminations, thereby enhancing efficiency across a broad spectrum of frequencies. The load trajectory for the fundamental and the second harmonic for class J/B continuum is illustrated on the Smith chart in Figure 6. The corresponding time domain waveform was previously presented in Figure 5. This method involves calculating the trajectory of the load network using Equation (13).  
Based on this trajectory, a single load network is then synthesized to satisfy the calculated matching condition. This load network could also be generated by employing hit and trial methods using standard electronic design tools, but it lacks the guarantee of convergence. To ensure a more reliable implementation, two standard techniques have been widely applied: the analytical technique [36] and the real frequency technique (RFT) [37].  
The analytical method involves determining the load network's parameters using filter theory [38]. As it is based on established circuit models and theory, the analytical approach provides a systematic way of designing the load network and ensures good performance and convergence [39].  
_RFT_ uses network parameters directly in the frequency domain to synthesize the load network [40; 41]. By utilizing actual frequency domain measurements, this approach offers a more accurate representation of the load network's behavior and impedance characteristics. It is considered a powerful and practical method for synthesizing the load network either through analysis [42] or by using source pull or load pull/source pull measurements [43] and achieving the desired wideband amplifier performance.  
Despite the various design techniques available, striking a balance between high efficiency and large bandwidth in RFPAs remains a significant challenge. This difficulty arises due to the inherent trade-offs between these two objectives in amplifier design. Achieving both high efficiency and large bandwidth requires innovative design strategies that carefully balance these conflicting objectives. This might involve hybrid approaches that combine elements of both step and continuous methodologies.