Assessing The Impact
The E-band circuit’s PA power produced a peak output level of +24.2 dBm from a 600-μm periphery, corresponding to a power density of 440 mW/mm. It exceeded the 219 mW/mm power density reported for a 0.-1μm GaAs pHEMT amplifier with 640-μm output periphery and was similar in performance to the 415 mW/mm reported for a PA with smaller 100-μm output periphery. This suggested that the transistor combination used here had not resulted in significant power loss. These GaAs power densities were less than the values of 1400 to 1667 mW/mm reported for more expensive gallium-nitride-on-silicon-carbide (GaN-on-SiC) processes.
The PA was laid out with resistors in the drain bias supply lines as a conservative measure to provide additional stability margin for low-frequency operation. This reduced the drain efficiency of the PA’s last stage from an intrinsic value of approximately 35% to an extrinsic value of about 27%. The drain supply line resistors reduced the PAE by approximately two percentage points. The simulated performance for the PA was in good agreement with measured levels for both saturated output power and PAE (Fig. 5).
The simulated and measured data also agree rather well for the doubler and quadrupler circuits (Figs. 1 and 2), which is somewhat remarkable given the project team’s lack of familiarity with a new semiconductor process. Furthermore, considering that the models used for the simulation were based on a single model fit to a broad frequency range—in addition to a high dynamic range encompassing extremely linear conditions to highly nonlinear performance—the agreement in Fig. 1 is truly outstanding.
In particular, for the quadrupler this includes HEMTs operating in Class-A linear and saturated modes at K-band frequencies; a K-to-Q-band frequency-doubling HEMT device operating near pinchoff with low drain potential; HEMTs operating in Class-A linear and saturated modes at Q-band frequencies; the Q-to-E-band frequency-doubling HEMT device operating near pinchoff with low drain potential; and HEMTs operating in Class-A linear and saturated modes at E-band frequencies.
Considering the performance levels of these devices, the agreement shown in Fig. 2 is not only reasonable but also quite good in its own right. Note that a small discrepancy in the logarithmic output power simulation led to a larger discrepancy in the PAE being a linear metric. A similar instance applied to the frequency doubler in Fig. 1.
Figure 6 shows the measured output power for the PA in comparison with circuits at E- and W-band frequencies, with output power of 100 mW (+20 dBm) or more. The PA has a 3-dB power bandwidth limited by the measurement setup to 18 GHz (23%) compared with 13 GHz for the GaN amplifier documented in Fig. 6. The PA’s output-power performance compares with other semiconductor approaches, as shown in Fig. 6. Finally, Fig. 7 provides an indication of the DC power required for the amplifier, in terms of its PAE, with data points reported for the comparison processes.
In summary, the E-band circuit combined a frequency doubler, quadrupler, and PA with good results. The doubler offers broadband output power of better than +15 dBm while the quadrupler achieves maximum measured output power of +19.2 dBm. The PA, which exceeds the +23-dBm (200-mW) specification across the entire 15-GHz ETSI E-band bandwidth, offers maximum measured output power of +24.2 dBm (265 mW).
The PA’s measured PAE is above 8% across the ETSI E-band frequencies, which is the highest saturated output power and PAE for a PA spanning the full 71-to-86-GHz range of the ETSI E-band specification for any semiconductor system. The first-pass success of this design effort represents a major step forward in the industry’s ability to deliver 1 W radiated power from a single-chip solution in an E-band system.
Dr. Michael Heimlich, Technical Marketing Consultant
AWR Corp., 1960 E. Grand Ave., Ste. 430, El Segundo, CA 90245; (310) 726-3000, FAX: (310) 726-3005.
The author would like to thank the following for their contributions to the design effort and this article: Anthony E. Parker, Department of Engineering, Macquarie University; Melissa C. Rodriguez, Jabra Tarazi, Anna Dadello, Emmanuelle R.O. Convert, MacCrae G. McCulloch, Simon J. Mahon, Steve Hwang, Rodney G. Mould, Anthony P. Fattorini, Alan C. Young, and James T. Harvey of the Sydney Design Centre; MACOM Technology Solutions, Australia; and Wen-Kai Wang of WIN Semiconductor, Taiwan.