Novel digital microwave PA becomes competitive in terms of efficiency
Fig. 1. Final-stage topology and output network of the novel digital PA
Fig. 2. Left: Novel digital PA MMIC design including efficient driver circuit and push-pull final-stage (V1/2); Right: Realized MMIC in digital PA module, chip area: 2.4 x 2.1 mm2
Fig. 3. Measured Pout, overall PAE and final-stage drain efficiency vs. power back-off for fc = 800 MHz and PWM input signal; Vdd = 30 V
5G wireless communication infrastructure demands for high flexibility, low cost, and high efficiency due to its approach with multiple in- and outputs (MIMO systems). This goes along with growing needs towards wideband, multi-band, and multi-standard features. As a consequence, increasing research efforts have been devoted to non-analog PA realizations in the last years on the way to a fully digital transmitter. One of the most crucial problems is, up to now, the low overall power-added efficiency (PAE) of digital microwave PAs vs. power back-off. The values achieved so far are not competitive with analog concepts like Doherty or envelope tracking.
Recently, FBH presented for the first time a GaN-based digital PA module that reaches high PAE over a wide range of power back-off at the International Microwave Symposium 2017 in Honolulu. This high efficiency is achieved by applying novel circuit approaches and principles known from low-frequency switching converters: By including a time gap between the ON-states of the final-stage transistors (see Fig. 1) no energy is wasted during charging/discharging of the output capacitances. This technique is known as zero-voltage switching which has been implemented for the first time in digital voltage-mode PAs and significantly enhances efficiency. Due to a very small voltage difference of only 5 V required in the upper driver (see Fig. 2, left), PAE is improved even more.
The proposed digital PA is optimized for the 800 MHz band, but can be used for other frequencies as well due to its broadband design. One only has to change the output filter.
Applying a single-tone PWM input signal with an encoded power back-off of 0…. 10 dB, the digital PA reaches a maximum Pout of 4.7 W and a peak PAE of 62% at a signal frequency of 800 MHz. At 6 dB and 10 dB power back-off the novel PA module reaches a PAE of 48% and 40%, respectively. Overall, the PAE decreases by only 19%-points within a 10 dB power back-off range. These values are the best ever reported for microwave digital PAs in the watt range and are comparable with state-of-the-art amplifiers like Envelope Tracking.
Publication
T. Hoffmann, A. Wentzel, F. Hühn, W. Heinrich, “Novel Digital Microwave PA with More Than 40% PAE Over 10 dB Power Back-Off Range,” IEEE MTT-S International Microwave Symposium Digest 2017, THIF2-12, Honolulu, USA