HIGH-POWER BROADBAND PA DESIGN
For broadband high-power applications, a single high-power device may be used to design a single-ended PA to meet specifications. However, using a single high-power device presents challenges with bandwidth, PAE, harmonics, thermal management and cost. To minimize harmonics, lowpass filters (LPFs) are needed to suppress the harmonics generated by the PA. LPF components must support the appropriate power levels, which may increase their size and the space required on the PCB. This adds to the loss and cost of the overall design. Additionally, achieving better PAE and output power with a single PA often requires sacrificing output return loss. Standard designs using a single high-power PA also typically achieve poor second harmonic levels.
Alternatively, using two smaller PAs in a push-pull topology and combining them with a 90-degree hybrid, as shown in Figure 3, can yield higher Pout, improved PAE, better harmonic performance and excellent input and output return loss due to the inherent characteristics of the hybrid. This approach enhances overall performance and addresses the efficiency issues associated with single high-power device designs. This topology also simplifies thermal management efforts since power dissipation is distributed across four active devices rather than one in the single-device HPA. One of the drawbacks of this push-pull topology is cost, as it requires additional baluns and hybrid combiners at the input and output.
Table 3 summarizes the comparison of a single PA configuration versus two small push-pull PAs. Table 4 summarizes the comparison of a single HPA versus two push-pull PAs combined with a 90-degree hybrid.
Overall, the push-pull topology with 90-degree hybrids offers a more efficient and higher performing solution for high-power broadband PA design despite the increased design complexity and potential for higher costs.
PUSH-PULL AMPLIFIER REFERENCE DESIGNS
To illustrate the practical application of the push-pull PA design approach, this section presents 15 W and 65 W push-pull PA module reference designs.
15 W Push-Pull PA Module
Figure 4 shows a 15 W push-pull amplifier design using TagoreTech’s TA9310E GaN-on-SiC power transistors and baluns. This amplifier is designed to operate from 30 to 512 MHz. The two GaN HEMTs are biased at an 18 V drain supply with a total quiescent current of 80 mA. The input side of the module uses a balun rated at 1 W, while the output uses a balun from Mini-Circuits rated at 15 W.
Figure 4 15 W push-pull PA module.
Figure 5 15 W PA module gain and PAE.
The push-pull design effectively minimizes second-order harmonics to -35 dBc without sacrificing output power or PAE. The desired levels of power and PAE can be achieved by operating the push-pull amplifier at drain voltages below the maximum rating for the GaN PA. The reduction of second-order harmonics makes this design particularly advantageous for tactical radio applications, especially within the 30 to 512 MHz frequency band. The GaN power transistors shown in Figure 4 are mounted on an evaluation board developed by TagoreTech. This module delivers an average power output of approximately 15 W. Figure 5 shows the measured gain and PAE versus output power for the 15 W PA module. Figure 6 shows the performance of the second-order harmonics versus the output power of the 15 W amplifier module.
Figure 6 15 W PA module second-order harmonics.
Figure 7 65 W push-pull PA module.
65 W Push-Pull PA Module
Figure 7 showcases a 65 W push-pull amplifier design developed using two of TagoreTech’s TA9410E 40 W GaN-on-SiC power transistors in conjunction with an in-house coaxial balun design. This amplifier is designed to operate from 135 to 870 MHz, driven by a 50 V drain supply. For optimal performance, a balun rated at 4 to 5 W is utilized on the input side, while a balun rated at 75 W is employed at the output.
The amplifier’s push-pull topology effectively reduces second-order harmonics to -30 dBc. This ensures that output power and PAE are not compromised. Operating at a maximum voltage of 50 V, this design meets the high-power requirements essential for dash-mount or vehicular LMR/PMR radio applications.
The suppression of second-order harmonics makes this amplifier particularly suitable for LMR applications, especially across very high frequencies (VHF), ultra-high frequency 1 (UHF1), UHF2 and the 800 MHz band. Table 5 shows a summary of the 65 W push-pull amplifier performance in these bands. The evaluation board in Figure 7 delivers an average power output of 63 to 70 W in the LMR frequency range. Figure 8 shows the measured gain and PAE versus output power for the 65 W PA module. Figure 9 shows the performance of the second-order harmonics versus the output power of the 65 W amplifier module.
Figure 8 65 W PA module gain and PAE.
Figure 9 65 W PA module second-order harmonics.
SUMMARY
This article has explored the advantages of push-pull amplifier topology, particularly when combined with GaN technology, for wideband PA design. By pairing two or more amplifiers, push-pull amplifiers effectively address the challenges of achieving high power output, wide bandwidth and low distortion in broadband PA designs. GaN technology offers superior properties such as wide bandwidth, high efficiency, superior thermal conductivity and high breakdown voltage, making it an ideal choice for these applications.
The article has compared push-pull configurations to PA designs using a single device. This analysis has highlighted the superior harmonic performance, efficiency and thermal distribution of the push-pull architecture. Despite the increased design complexity and potentially higher component count, push-pull configurations provide significant advantages in bandwidth, efficiency and harmonic control.
Combining push-pull topology with GaN technology results in high performance, wideband amplifiers that meet stringent requirements for efficiency, distortion, power handling and thermal management. This makes them a compelling choice for RF applications where performance, size and cost are critical.
