Designing a more efficient broadband 100 W Doherty power amplifier for cellular base-stations: Page 3 of 5

October 10, 2016 // By X Moronval, J Gajadharsing, Ampleon
The proliferating frequency bands and modulation schemes used in modern cellular networks are making it increasingly important that the RF power amplifiers used in base-stations offer the right combination of output power, multiband support and efficiency – at both peak and average power outputs.

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% [5].

Figure 3: Proposed architecture for the 4-way iDPA (Source: Ampleon).

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).

Figure 4:  Simulated efficiencies of the four-way iDPA operating at 1.805 – 2.17 GHz (narrow colours) compared with a symmetric iDPA topology (bold blue) operating at 1.99 GHz (Source: Ampleon).

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