When considering the performance of RF Power Amplifiers (PA’s) or indeed other non-linear Devices, it is the terminal RF I-V waveforms that are the unifying theoretical link between transistor technology, circuit design, and system performance.
A logical expectation would be to find WaveForm Engineering driving both the measurement and mathematical analysis requirements of the RF PA design process at all stages—transistor optimization, circuit design, and system integration. However, due to limitations in measurement architectures the RF PA design approach has had to be based on other measurable properties, e.g., dc I-V characteristics, bias dependent S-parameters, load-pull contours, AM-AM and AM-PM behavior, scalar power, and output spectrum, to name but a few. This often led to a breakdown in the design chain where different departments employed their own, differing measurement techniques, this works until an issue arrises then the source of the problem can often be difficult to find.
The past 15 years have, however, seen the maturing of a number of RF characterization systems capable of measuring RF voltage and current waveforms. Coupling such systems with impedance control or emulation hardware also enables engineering of these RF waveforms during measurements. The result is a system that provides the PA designer engineer with a practical RF I-V waveform measurement and engineering solution. There is now a real opportunity to complement the present RF PA design approaches with waveform theory and experimental waveform information, finally allowing for robust linking of measured performance mathematically back to theoretical expectations.
In addition, as PA designers are forced to meet increasingly demanding system requirements and thus consider more complex high-efficiency modes of operation, the black-box approach is becoming impractical. This is because it requires the systematic variation of many more parameters such as harmonic source and load impedances - clearly randomly optimizing so many variables becomes prohibitive. This experimentally based design process could be made more efficient if coupled with waveform measurements. This is a good example of where RF waveform measurement systems can contribute significantly to both RF power transistor characterization/development and RF PA design/optimization. The practical consequence of integrating RF waveform measurements with solutions that provide experimental control (engineering) of the terminal impedances is that the engineering of the RF current and voltage waveforms can be experimentally monitored. Systematic experimental PA design investigations can now become WaveForm Engineering driven; the design goal finally becoming the realization of the theoretically derived optimum waveforms. It is important to note that, even if the stimulus signal is confined to a single-tone (continuous wave) signal, the RF waveform generated by the nonlinear DUT is spectrally rich—it will also contain harmonic frequency components.
In summary, from a nonlinear PA design perspective, the integration of RF WaveForm Engineering capability, whether passive or active, with RF waveform measurement capability is essential. With such systems, the practical design of PAs achieved by directly employing the theoretically based WaveForm Engineering approach is now experimentally possible.