Online Spotlight: Reconfigurable Dual-Band Power Amplifier for Telemetry Applications
A reconfigurable, dual-band, high efficiency power amplifier (PA) employing 0.25 μm GaN technology simultaneously covers two distinct telemetry bands: L (1.435 to 1.55 GHz) and C (5.0 to 5.15 GHz). The uniqueness of this design is its three modes. Either one of the bands can be eliminated simply by applying a bias voltage to a barium strontium titanite (BST) varactor. High efficiency, high output power and high gain are achieved.
RF PAs are used in a wide variety of applications including cell phones, wireless communications, radar and electronic warfare (EW). Reconfigurability, where the same hardware can be used for different frequency bands, is important to meet the needs of future wireless systems. To efficiently manage spectrum use and meet future Department of Defense (DoD) requirements, agile reallocation of wireless assets to less congested bands is becoming essential.
In the United States, specific frequency bands are allocated for telemetry.1 For this development effort, the L and C bands are the ones of interest. The DoD is auctioning off the L (1.435 to 1.85 GHz) and S (2.2 to 2.395 GHz) bands for commercial applications and moving to C-Band (4.4 to 6.7 GHz). Most existing systems, however, still use the legacy L and/or S bands. To ensure compatibility with both the legacy and new systems, it is preferred to use a dual-band design, as the telemetry transmitters must be interchangeable between bands and require no modifications to the aircraft. The PAs are key components, so proper PA design to accommodate multi-band operation is a requirement.
The challenge of multi-band and multi-standard operation can be met with reconfigurable technologies. Various approaches using CMOS switches,2 MEMS switches,3 PIN diodes,4 thin film BST capacitors,5 high Q varactor diodes,6 GaN RF switches7 and integrated piezoelectric actuators8 have been used for reconfigurable PA designs. In most of these earlier research efforts, the reconfigurable PAs were designed for applications other than telemetry. The design described here addresses the demand and importance of reconfigurable PAs in the L and C telemetry bands used for EW applications.9 The unique feature of the design is its simplicity, where tunable components are used only in the input matching network to select one of the bands of the concurrent dual-band PA.
The increasing need for high power at RF/microwave frequencies led to the development of compound (III-V) semiconductor GaAs and GaN transistors, which are useful for microwave PAs because of their high electron mobilities.10,11 Because of their exceptionally high power density and breakdown voltage, GaN high electron mobility transistors (HEMT) are being rapidly adopted for various applications.12 The high thermal conductivity of SiC efficiently dissipates the excess heat of the high power density devices, preventing extreme channel temperature and reduced reliability. For this work, a Qorvo 5 W GaN on SiC HEMT was used.
The reconfigurable behavior of the PA was accomplished by exploiting the unique structure of a BST varactor, in which a thin BST layer is sandwiched between two metal plates. Ferroelectric materials like BST are dielectrics with complex functionalities. Some of their physical properties, such as permittivity and polarization, change with variations in the external electromagnetic field, temperature and mechanical strain, and these functionalities can be used for various applications. In this design, a unique coplanar waveguide BST varactor13 was used to achieve dual- and single-band operation.
The design approach was a concurrent dual-band PA operating in the L and C telemetry bands, with a notch to reject one of the bands (see Figure 1). Two BST varactors are used to create rejection notches at the specified frequencies, with the selected frequency depending on the capacitance values of the varactors. The capacitance values are controlled by the applied bias voltages, which are switchable to select one of the two bands (i.e., L or C).
Figure 2 shows the electrical model of the BST thin film varactor. Biased with a fixed voltage, the varactor is modeled using lumped elements as a conventional parallel-plate capacitor, where the capacitor C1 represents the BST capacitor, and the parallel resistor R1 represents the dielectric loss of the ferroelectric material. The series resistance R represents the parasitic conductor and interconnect (i.e., electrode) resistance, and the series inductor L1 represents the parasitic presented by the shunt line in the varactor.
With no bias voltage applied, the varactor is “off,” its effective capacitance is highest, and the signal is shunted to ground instead of appearing at the output port. As DC bias is applied, the varactor turns “on,” and its capacitance decreases, which allows more signal to pass from the input to the output. This feature is used to create a notch filter to reject one of the bands.
Simulation of the BST varactor with bias is shown in Figure 3, with |S21| and |S11| values of -0.88 and -10 dB, respectively, at 3 GHz. The simulated |S21| and |S11| without bias is shown in Figure 4. In this case, the |S21| and |S11| values are -7.2 and -1.71 dB, respectively, at 3 GHz.
The PA circuit uses one transistor assembled on a Rogers TMM4 substrate, which has a thickness of 15 mils, a dielectric constant of 4.5 and a copper metal thickness of 0.7 mils.9 The 5 W Qorvo GaN transistor selected for the design has a breakdown voltage of 100 V, maximum drain voltage rating of 40 V and maximum drain current of 1.25 A.14
The dual-band PA was designed using Cadence® AWR® Microwave Office® and implemented in a hybrid configuration. A Modelithics transistor model was used for the Qorvo 5 W device, and load-pull analysis determined the optimal power and efficiency of the transistor.15,16 Figure 5 shows the load-pull contours at 1.5 GHz, with power-added efficiency (PAE) shown in blue, output power (Pout) in pink; the contours at 5 GHz are shown in Figure 6. For these tests, the device was biased at VDS = 28 V and VGS = -2.7 V. For optimal efficiency at 1.5 GHz, Figure 5 show a load reflection coefficient ΓL = 0.3 at an angle of 40.8 degrees, resulting in a PAE of 66.9 percent and an associated Pout of 38 dBm. The load-pull analysis at 5 GHz yields ΓL = 0.5 at an angle of 89.1 degrees, resulting in a PAE of 66.2 percent with an associated Pout of 37 dBm. However, for maximum Pout, ΓL = 0.11 at an angle of 50.7 degrees yields Pout = 38.7 dBm at 1.5 GHz; ΓL = 0.3 at an angle of 102.4 degrees gives the maximum Pout at 5 GHz: 37.9 dBm.
The BST varactor layout is shown in Figure 7. The overlapping area that produces the capacitive effect is sized at 12.5 μm x 12.5 μm, and the varactor is layered such that the bottom layer metal thickness is 750 nm, the BST layer 250 nm and the top gold 2.63 μm. A photoresist layer provides passivation to prevent oxidation. The varactors are diced from a wafer and wire bonded to the PA board.
Figure 8 shows a 3D view, the entire layout and the fabricated PA circuit. Both varactors are placed on the input side of the PA. Two additional DC blocking capacitors separate the transistor bias from the varactor bias. Figure 9 shows the simulated S11 of the output matching network, highlighting the two band of interest: 1.5 and 5 GHz. Load-pull analysis15 determined the optimal power and efficiency of the 1.25 mm GaN device, and the ΓL data was used as the initial starting point for the design of the concurrent dual-band PA.
The on and off simulations of |S11| and |S21| for the first and second varactor sections are presented in Figures 10 and 11, respectively. For the first varactor section (see Figure 10), with 0 V bias, the minimum |S21| is –36.4 dB at 4.4 GHz, representing a capacitance of 4 pF. With 8 V bias, the minimum |S21| is –18.4 dB at 8.0 GHz, representing a capacitance of 0.8 pF. For the second varactor section, with 0 V bias, the minimum |S21| is –14.7 dB at 1.7 GHz, representing a capacitance of 4 pF. Biased on, the minimum |S21| is -16.6 dB at 3.5 GHz, representing a capacitance of 0.8 pF. The varactors create notches at different frequencies, determined by the bias voltage. This characteristic, along with placement of the varactors in the input matching section of the circuit, enables the desired band rejection.
The entire input network and its simulated S22 are shown in Figure 12.
DUAL-BAND PA PERFORMANCE
Figure 13 shows the simulated small-signal performance of the dual-band design. To achieve dual-band performance, both varactors are biased so their capacitance values are 0.8 pF. At this bias, the small-signal gain is 14.1 dB at 1.4 GHz and 15.4 dB at 5 GHz, with the input return loss better than 7 dB at L-Band and 6 dB at C-Band. The output return loss is better than 13 dB for both bands.
Figure 14 shows the simulated large-signal performance in dual-band operation, i.e., with both varactors biased to 0.8 pF. At L-Band, the simulation predicts a PAE of 51.9 percent, Pout of 33.8 dBm and a compressed gain of 10.8 dB. At C-Band, the simulation predicts a simultaneously PAE of 62.8 percent, Pout of 36.2 dBm and compressed gain of 13.4 dB. The PA achieves good performance in both bands.
Figures 15 and 16 show the simulated large-signal performance with only one of the two bands selected, i.e., the other rejected.
L-Band operation is shown in Figure 15, with a simulated peak PAE of 53.1 percent, an output power of 34.2 dBm and associated gain of 11.1 dB. The gain at C-Band is less than 3 dB, with PAE less than 10 percent and output power less than 22 dBm.
Figure 16 shows the simulated performance with C-Band operation. PAE is better than 50 percent, with output power of 34.8 dBm and associated gain of 11.8 dB. At L-Band, the PAE, output power and gain are low: 14.1 percent, 25.4 dBm and 2.9 dB, respectively.
To reject one of the two bands — C-Band for example (see Figure 15) — the capacitance for the second varactor is set to 4 pF, and the first varactor’s capacitance is 0.8 pF. To reject L-Band (see Figure 16), the capacitance values are flipped: the first varactor’s capacitance is set to 4 pF, the second varactor’s to 0.8 pF. Recall that the capacitance required to operate in both bands simultaneously with good gain and efficiency, i.e., concurrent dual bands, is 0.8 pF for both varactors. To reject one of the bands, the capacitance value is set to 4 pF for one of the two varactors. This requires a varactor tuning ratio of 5:1.
MEASURED vs. SIMULATION
Measured |S21| and |S11| of the BST varactors used in the reconfigurable PA board is compared with the simulated performance for two bias states: 0 and 6 V (see Figures 17–18). The varactor current was limited to 1 μA, which determined the maximum bias voltage applied: 6 V with these devices. The measured variation in capacitance agrees well with the simulated capacitance value of 2.8 pF for 0 V bias and 1.5 pF for 6 V bias, a 1.9:1 tuning ratio.
Figure 19 compares the measured data with the small-signal simulation with both varactors biased to 1.5 pF capacitance, i.e., both varactors on. The measured gain generally matched the predictions with both varactors biased to 1.5 pF capacitance; discrepancies occurred because the varactor capacitance values did not meet the design requirements.
Figure 20 shows the simulated efficiency, output power and gain with both varactors biased to 1.5 pF capacitance. To construct this plot, measured parameters matching the small-signal simulated data were used, e.g., 1.5 pF varactor capacitance, 44 mA device drain current. The PA is predicted to have a PAE of 47.4 percent, Pout of 30.7 dBm and associated gain of 7.6 dB at L-Band, and a PAE of 62.9 percent, Pout of 32.8 dBm and associated gain of 10.3 dB at C-Band.
This article presented the design of a reconfigurable, concurrent, dual-band PA operating at the L and C telemetry bands. Using a 0.25 μm GaN HEMT and tunable components solely in the input matching section predicts high efficiency in both bands, which makes this design unique. Small-signal measured data verifies the dual-band performance, and large-signal simulation using measured small-signal data predicts efficiency above 45 percent for L-Band and above 60 percent for C-Band with a varactor capacitance of 1.5 pF.
Better small-signal response — higher gain, better input and output return loss — and better large-signal response — higher PAE and output power — can be achieved with varactors having 5:1 capacitance tuning or with values closer to the design values of 4 pF at no bias and 0.8 pF with bias. Achieving the desired capacitance values and tuning ratios, the PA can achieve its design goals. BST varactors with higher tunability are being fabricated to verify the simulations.
To the best of the authors’ knowledge, this design represents the first high efficiency, reconfigurable, concurrent dual-band PA suitable for telemetry applications.
This research is supported by the Air Force Office of Scientific Research (Program Officer: Dr. Michael Kendra). We would like to thank Dr. Paul Watson, Mr. Tony Quach, Ms. Aji Mattamana, Mr. Will Gouty and Mr. Steve Dooley for their technical support and advice. The authors would also like to thank Modelithics for the transistor models used in this design, which were provided as part of Modelithics' University Program.
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