A Transformer-Less Load-Modulated (TLLM) Architecture for Efficient Wideband Power Amplifiers

An architecture and design procedure for transformer-less load-modulated power amplifiers (PAs) having high efficiency at power back-off is presented. This architecture utilizes a comparable load modulation concept as in the Doherty PA; however, contrary to the Doherty PA, it neither requires an output impedance transformer, nor offset lines, which are the main limiting factors in designing wideband Doherty amplifiers. In this architecture, the proper load modulation at the transistor drain levels is directly dependant on the design of the main and peaking amplifiers´ matching networks. It is shown that the design of the output impedance of the peaking amplifier as a frequency-varying susceptance rather than as a quasi-open circuit impedance (as in Doherty amplifiers) helps in achieving better wideband performance. Based on the analytical results, a step-by-step procedure is proposed for designing wideband load-modulated PAs having high efficiency over a wide power back-off region. Based on the proposed architecture and its design procedure, a 10-W PA is designed and prototyped in the 1.96-2.46-GHz band exhibiting a power-added efficiency of more than 40% at a 6-dB power back-off. In comparison with a symmetrical Doherty amplifier design using the same device, the proposed architecture exhibits a 250% enhancement in its fractional bandwidth.