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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