A Comprehensive Analysis of Bandwidth and Size for Four-Types of Series-Combined Transformer-Based Doherty Output Networks: Theory and Examples

This paper presents theory-based design guidelines for four types of series-combined transformer (SCT)-based Doherty output matching networks to maximize bandwidth (BW). Intuitive, novel approaches to analyzing the series Doherty combiners are proposed. Compared to the previous analysis via only ideal or otherwise insufficient models, this article systematically derives closed-form design equations based on non-ideal transformer-based practical circuit models. With two different combinations of SCT models, and four capacitor configurations, Doherty load modulation is clearly verified for all four types. Furthermore, the design parameters are simplified, and the equations are validated, for both symmetrical and asymmetrical SCT. To anticipate maximum achievable BW and dominant design factors, four design examples of each type are compared using circuit simulation and numerical analysis. The clear trend observed for all four types is that higher magnetic coupling coefficient results in larger fractional BW. It is also demonstrated that the distinct “sweet spots” for the four types, when maximizing BW depend mainly on only 2 or 3 variables, including turn ratios and/or self-inductances. The SCT sizes are also compared by the sum of two transformer dimensions. To validate the circuit model, a design example and design procedure for best practice are provided: a layout-based full-EM structure demonstrates close-to-theoretical BW performance. For further verification, the four types of SCT-based Doherty power amplifiers are simulated and compared.