To create a broadband, four-way DPA using the iDPA architecture, we use two iDPA amplifiers, each housed in a dual-path package. The output matching networks (OM in Figure 3) are designed with an electrical length equivalent to a 180° phase shift, and a wideband impedance inverter is used, showing an improved fractional bandwidth from 28 to 50% .
Through simulation, we found that there are two operating modes for this topology, one we call ‘linear’ and the other ‘maximum efficiency’. Although the second sounds attractive, it has a non-linear output that is difficult to overcome using digital pre-distortion, especially for the highly linear modulation schemes used in multi-carrier GSM. Linear operation happens when both Peak2 and Peak3 transistors are biased at the same gate voltage, giving two efficiency peaks when operating in back-off mode.
A second set of simulations, using electro-thermal non-linear models, gives the efficiency response versus output power curves for LTE bands from 1.805 to 2.17 GHz, as shown by the narrow lines in Figure 4. The figure also shows that a 100 W power amplifier using the four-way iDPA topology can achieve substantial improvements in efficiency operating at various levels of back-off, when compared to a standard symmetric iDPA operating at its central frequency (represented in bold blue).