GaN PAs Deliver 100 W, 27–31 and 32–38 GHz
Qorvo’s new enhanced mmWave Spatium® amplifiers deliver unparalleled combining efficiency and bandwidth in a compact envelope for high performance applications such as satellite ground stations, 5G infrastructure, radar and electronic warfare. Using its latest GaN MMICs, Qorvo has developed two mmWave Spatium products, both in volume production:
- QPB2731, which provides 100 W output power from 27 to 31 GHz.
- QPB3238, which provides 100 W output power from 32 to 38 GHz.
These solid-state power amplifiers (SSPA) handle continuous wave (CW), pulsed and modulated RF waveforms, with two biasing options to best suit the application.
Like previous incarnations of Spatium technology, the mmWave Spatium employs a laminate-based antenna structure to spatially combine 16 elements in a quasi-transverse electromagnetic (TEM) coaxial environment. Theoretically, this results in an SSPA with 12 dB more power than the power from each MMIC.
Before building the amplifier, Qorvo designed a passive structure to assess the performance of the combining network - constructed to duplicate the RF path of the amplifier without the MMICs. At the input, the RF signal enters through a 2.92 mm female connector, then physically expands in an oversized coaxial region to a laminate, end-fire antenna array that splits the RF into 16 equal signals, each with a microstrip transmission line connecting to the input of the MMIC (see Figure 1). In the passive structure, though, the microstrip line connects to another laminate antenna array that reverses the transitions at the input. In most cases, the output of a high-power amplifier (HPA) requires an output connection with suitable power-handling capability, such as WR28 waveguide. With this passive test structure, a 2.92 mm female connector was used at the output to make a broadband connection with a standard network analyzer.
Each laminate was bonded to a silver-plated copper blade, forming a 1/16th wedge when viewed along the major axis. AGC Taconic TLY-5Z was chosen as the core laminate material, as it exhibits a low permittivity (εr = 2.2) and relatively low loss tangent (tanδ = 0.0015 at 10 GHz), which contribute to the broadband, low loss performance of the antenna structures. Although the metallization on the passive laminate was covered with an IPC-compliant, immersion silver to prevent oxidation of the copper, the amplifier designs generally use gold metallization to be compatible with gold wire bond attachment to the MMICs.
The passive structure was designed and analyzed using ANSYS HFSS. The simulated insertion loss and return loss compared to the measured passive structure assembly are shown in Figure 2. While small deviations between the measured (red lines) and simulated (blue lines) are seen, there is good correlation overall, and the observed performance is acceptable for amplifier development. The measured insertion loss reflects the loss of the entire RF path, i.e., both input and output. In the SSPA, the MMIC is placed toward the output, so more than 50 percent of the microstrip line is prior to the MMIC. Thus, a reasonable estimate of the output insertion loss is 0.67 dB, which yields a combining efficiency of 86 percent.
For the QPB2731 design, Qorvo’s QPA2211 GaN MMIC was selected as the HPA. The QPA2211 is specified to provide 14 W saturated and 5 W linear CW output power, with 34 percent power-added efficiency (PAE) in CW operation (see Figure 3). For the QPB3238N design, the TGA2222 GaN MMIC was selected as the RF HPA; it has a specified CW output power of 40.2 dBm saturated with 22.3 percent PAE from 32 to 38 GHz and a die backside temperature of 25°C (also shown in Figure 3). Both devices use Qorvo’s 0.15 µm GaN on SiC process (QGaN15).
Figure 4 shows the output power and drain efficiency measurements of the QPB2731 and QPB3238 SSPAs at a “clamp” temperature of 25°C. Note the different thermal reference points between the die and SSPA data. Operating at saturated power with a CW signal, self-heating may raise the backside temperature of the die more than 50°C above the clamp temperature of the SSPA. Therefore, Figures 3 and 4 do not show “apples to apples” comparisons; nonetheless, the figures show the mmWave Spatium platform performs well combining the power from the individual MMICs.
For ease of system integration, Qorvo offers two bias options for the mmWave Spatium products. The first has a separate bias card, operating remotely from the SSPA, containing a microcontroller to fully customize the amplifier for its operating environment (see Figure 5a). The second option is more integrated, achieving a more compact physical profile (see Figure 5b). Both solutions provide the required sequencing and gate voltage control for the GaN MMICs, so the user can power the SSPA by simply applying the prime voltage at the power connector.
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