The very first commercially viable GaN devices from Japan began hitting the shores of the USA in 2004. After more than 22 years, GaN devices continue to drive new SWaP-C metrics for RF, microwave and millimeter wave transmission equipment. GaN amplifiers are a mainstay in many commercial and defense applications and take on many forms such as highly integrated monolithic microwave integrated circuits (MMICs), multi-chip modules (MCMs), Systems-in-Packages (SIPs), and chip-and-wire hybrid modules (Hybrids).
Achieving Wideband Performance by Cascading SMT Components
This article focuses on applying Mini-Circuits’ GNA-252-5W+1 GaN MMIC amplifier in a balanced configuration2 using surface mount technology (SMT) components that are all of Mini-Circuits’ origin, combined with construction techniques that are also envisioned to be traditionally SMT.
The GaN power amplifier (PA) module is designed to deliver approximately 20 W (+43 dBm) across the 500-3300 MHz band when operated in saturation. The following paragraphs show performance based on simulation results and calculated from empirical data for various RF input power levels. All simulated results and those based on empirical test data are shown for +25⁰C operation.
The Amplifier Device Lineup
The 20 W 500-3300 MHz GaN balanced amplifier lineup is shown in Figure 1. The input stage, or more commonly referred to in a three-stage amplifier as the predriver, is comprised of the Mini-Circuits’ LHA-83W+3 pHEMT MMIC amplifier. This device exhibits 16.47 ± 0.14 dB of gain and a NF that ranges between 2.65 and 3 dB over the 500-3300 MHz band, as shown in the test data for the part. With a P1dB of over +23 dBm, the LHA-83W+ is more than capable of driving the amplifier deep into compression before it, too folds up. The heavily damped series notch that follows the predriver may look out-of-place, but it is centered at approximately 400 MHz and has a depth of 6 dB. The upper skirt of the notch (500 MHz and up) serves to equalize the 4 dB of gain runup found in the combination of the driver (0.7 dB) and final (3.2 dB) stages, which flattens the small signal gain curve to the extent shown by the upper trace in the ADS simulation results shown in Figure 2. Modelithic’s models were used in the simulation for all of the following passives: the Coilcraft 0402DC-series 27 nH inductor, the Passive Plus 0402N-series 5.6 pF capacitor, and the KOA RK73-series 50Ω resistor. The active models were the Mini-Circuits’ S2P files. The 30 dB+ input return loss and 20 dB of reverse isolation of the LHA-83W+ predriver buffers the RF input from the mismatch of the notch, yielding the excellent S11 of Figure 2.
Worthy Device Pairing on Balance
Downstream from the predriver and notch/equalizer is the QCH-392+4 90⁰ quadrature hybrid, or more commonly the “quad”. This multi-section, broadside-coupled 3 dB splitter easily covers the 500-3300 MHz band and can accommodate up to 130 W, provided the termination resistor is sized accordingly. The balanced amplifier is designed by having two pairs of driver-final cascades enclosed within a back-to-back pair of QCH-392+ quads, as shown in Figure 1. The driver is the PMA5-63-2W+5 distributed 2 W GaAs MMIC and the final is the GNA-252-5W+6 8 W GaN MMIC power amplifier. These quadrature-combined, cascaded stages work in tandem to deliver approximately 20 W PSAT at the RF output port. The PMA5-63-2W+ driver can saturate the GaN finals by driving +30 dBm into each of the GNA-252-5W+ MMICs across virtually the entire band, before reaching P1dB, and has the headroom to drive up to +33 dBm into each of the finals. Insertion phase differences between each of the driver-final cascades is generally so small as to have very little impact on splitting and recombining loss. The 50Ω output termination resistor, the Kyocera/AVX CA12525T0050J, is sized for 20 W, or full reflected saturated RF output power.
The good S22 (> 12 dB) of the GNA-252-5W+ devices and the isolation provided by the QCH-392+ output quad yield the overall S22 curve shown in the small signal ADS simulation of Figure 2. This full 2-port simulation uses the Mini-Circuits’ S2P models for the active devices and Modelithic’s models for the passives. The RF circuit achieves an S22 of 12 dB or more as shown in Figure 2, which is excellent for an RF power amplifier. The balanced configuration also protects the final stage MMICs from RF output load mismatch due to the isolation of the output quad and the 20W power handling capability of it termination resistor.
RF Output Power vs. Input Drive Level in Compression – Large Signal
Large signal performance for this 20 W 500-3300 MHz balanced amplifier configuration was modeled based upon a simple cascade analysis spreadsheet which includes gain, P1dB and IP3, F and NF for all stages combined with the actual test data provided under the “View Data” selection on the Mini-Circuits’ dashboard for each component in the lineup. Where exact power levels or frequencies were not available under the “View Data” section, simple linear interpolation between two data points was utilized.
Figure 3 makes use of this empirical approach to determine what happens as the amplifier is driven nonlinear and compresses significantly. The lower trace in Figure 3 is the RF output power vs. frequency with a constant 0 dBm RF input drive level.
This RF output power level is of particular note, since a comparison to the simulated small signal gain of Figure 2 reveals just how compressed the amplifier is at each frequency. As with any RF input drive level (except for extreme saturation), compression occurs in the final stage. Roughly speaking, small signal gain is 45 dB, and the RF output level at 0 dBm RF input is 42 dBm, which is essentially P3dB. There is some additional headroom in the final stage, since the RF output power continues to increase as the RF input level is increased to 3 dBm. Finally, with 6 dBm at the RF input port (which corresponds to nearly +30 dBm at the input to each of the GaN MMIC finals), saturation is reached, and the PA module delivers +43 dBm (20 W) or greater across the vast majority of the 500-3300 MHz band.
RF Output Power and Power Added Efficiency (PAE) vs. Final Stage Drive Level
Another way to examine the compression characteristics of the amplifier is to utilize the exact drive level and associated current consumption of the final stage devices to determine RF output power, but more importantly, to examine the DC-to-RF efficiency. Note that since the amplifier has a significant amount of gain (even 40 dB when very compressed), DC-to-RF efficiency will converge to power added efficiency (PAE), so the easiest of the two nomenclatures was chosen. Figure 4 is based on IDD vs. PIN for the published test data for the GNA-252-5W+ final stage MMIC amplifiers. Three different levels of IDD are utilized for the PMA5-63-2W+ driver stage MMICs, 650 mA at P1dB, which corresponds to the power level needed from the driver to reach +30 dBm at the GaN MMIC final, 525 mA to drive the final stage devices with +26 dBm, and 400 mA to drive the finals at +22 dBm. Figure 4 shows RF output power in Watts and PAE in percentage on the same scale for the three different final stage drive levels of +22, +26, and +30 dBm. Perhaps the most interesting traces of all in Figure 4 are the RF output power at +30 dBm final stage drive and associated PAE in hard saturation. Interestingly enough, the entire amplifier, predriver, quads, drivers and finals achieves PAE levels in the low to mid 30% range across the majority of the band, even out to 3300 MHz, and naturally in the mid to upper 30% range at the lower end of the band. Saturated RF output power is very flat across the band.
Even when the drive level to the GaN MMIC finals is backed off to +22 dBm, the RF output power remains relatively flat, the runup in power from midband (~10W or +40 dBm) to the low end of the band (~15W or +41.8 dB) is less than 2 dB.
Cascade Analysis – Noise Figure
There can be no cascade analysis spreadsheet without a discussion of noise figure (NF). Figure 5 shows the NF for the entire amplifier cascade across the 500-3300 MHz band. The excellent performance is due of course to the LHA-83W+ predriver stage, because it is the first amplifier in the cascade, but the PMA5-63-2W+ is no slouch, achieving 3 to 4 dB for the upper half of the operational band. While there is a runup in NF at the low end of the band, an examination of Figure 5 shows that the runup has been mitigated so well that the whole amplifier NF peaks at just slightly over 4 dB at 500 MHz.
Mechanical Mockup – An Artist’s Impression
No discussion of an amplifier module would be complete without a discussion of component layout and SWAP-C considerations. A mockup, or artist’s rendition of the 20W balanced amplifier is shown in Figure 6. The RF input port is the lower lefthand connector and the RF output is in the upper righthand corner. At the RF input is the LHA-83W+ amplifier, followed by the notch/equalizer which is comprised of all 0402 chip components. The first QCH-392+ quad splits the amplified input signal so that it feeds each of the driver-final, GaAs-GaN cascades. The two PMA5-63-2W+ GaAs parts drive the GNA-252-5W+ GaN amplifier outputs, which are then combined in the output QCH-392+ quad to deliver approximately 20W to the RF output connector.
Note that a 50Ω, 20 W termination resistor is designed to be installed on the isolated port of the output quad so that the design can withstand full reflected power.
The RF circuit board in the mockup measures 2.58 x 1.51 inches [65.54 x 38.45 mm] without the connector protrusions, and if designed as a drop-in module (without connectors), the footprint dimensions are reduced to approximately 2.38 x 1.17 inches [60.57 x 29.72 mm].
SWaP-C Considerations
Technologies that compete with traditional SMT solutions in the SWaP-C space are high-power MMICs and hybrids. High-power MMICs incorporating distributed architectures integrated directly into the die can offer equivalent or even greater RF output power. With footprints of less than half a square inch, they also appear significantly smaller at first, but their gain is often equivalent to the final stage alone, and additional RF amplifiers are still required to achieve full gain, which increases net size of the high-power MMIC solution. While providing only final stage gain, the high-power MMICs themselves come at a price that is often more than twice that of the entire traditional SMT solution. Additionally, distributed power amplifiers do not exhibit nearly the level of robustness into mismatched loads, such as antennas, that is inherent to balanced amplifiers.
Chip-and-wire hybrids have been around for decades, and many GaN amplifiers have been built using this miniaturization technology. Scrubbing or bonding GaN die to a plated aluminum enclosure provides excellent thermal management. Connecting components in die form with bond wires is no doubt compact. While nontrivial, the quads can be integrated directly into the substrate. With careful design and layout, the entire module can be designed to fit into an aluminum enclosure that measures just 3.0 x 2.0 inches [76.2 x 50.8 mm]. Unfortunately, despite its SWaP benefits, the cost of hybrid technology is inordinately high when compared to traditional SMT. An equivalent amplifier constructed from hybrid technology would cost 4 to 5 times that of the traditional SMT PA module described in this article. Note that the 3.0 x 2.0-inch hybrid module in an aluminum enclosure is a size estimate, while an existing product, which is twice as large was used for the hybrid cost comparison.
Power Down
In this article, we demonstrated how we could take all Mini-Circuits’ parts and design and simulate a compelling, wideband balanced PA module with a pair of Mini-Circuits GNA-252-5W+ GaN devices as the centerpiece. We studied the amplifier’s architecture as well as its small and large signal characteristics, even calculating the NF resulting from the cascade analysis.
In our closing paragraphs, we showed a mockup of the balanced amplifier and discussed its merits when comparing it to a high-power MMIC distributed module which has less than half the gain, costs twice as much, and isn’t nearly as robust for mismatched loads. We also compared the architecture and mockup to a full-up chip-and-wire design, which was comparable in size but significantly higher in cost than the SMT balanced PA module. It’s now time to power down, and don’t forget to remove the drain voltage first.
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