Hitting the Sweet Spot: A Single-Ended Power Amplifier Exploiting Class AB Sweet Spots and Optimized Third Harmonic Termination

The single-ended RF power amplifier (PA) is a standard design approach in the RF engineer's tool kit, but high efficiency and good linearity may only be achieved if the harmonic terminations and biasing conditions are carefully examined. This is demonstrated in a single-ended PA using class-AB "sweet spots" and an optimized third harmonic termination, which won first place at the 2016 IEEE Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium's high-efficiency PA student design competition, sponsored by Technical Committee MTT-5. A linear, single-ended PA operating at 3 GHz was designed by exploiting the sweet spots present in class-AB amplifier responses and optimizing the third-harmonic termination. The improvements to PAE and IMD3 from the third-harmonic tuning are clear from the simulation results, and the class-AB sweet spots were shown in both simulation and measurement. The actual measured performance was 44% two-tone PAE at -30 dBc IMD3, giving a FOM of 58.8; this is a good result considering the variable-envelope signal used, which supports the proposed methodology. While the PA was unable to reach the outstanding FOM performance shown in previous years, the single-ended design is simpler and more cost effective than a linear DPA. The good agreement seen between the simulation results and measured data also demonstrates the value of the design method when applied through the use of suitable CAD tools.