Page 59 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

To achieve higher output power, recent work has high-efficiency and high-power, we continue to see
also investigated low-loss power combining from opportunities to continue improving the PAE
PA cells designed for high power. Fig. 11 illustrates towards 40% at power levels exceeding 20 dBm.
the 8-way combined power amplifier, where each While this work has not addressed trade-offs
PA is based on common-base, class-A stages [24]. As between power combining and device scaling in
opposed to the previous design where the power single-ended and differential PA designs, there
was combined through pseudo-differential stages remain significant research insights to be gathered
(Fig. 10), power combining across two CB HBTs is about the losses and area efficiency of the various
performed to prevent a large emitter length device approaches. The power combiners in Fig. 11 occupy
(lower left of Fig. 11). The output power combiner substantial area that might not satisfy the area
must be low loss for high PAE and compact for a constraints in a digital beam-former. Nonetheless,
small die area. While Wilkinson combiners are PA cells that are designed for a 50 Ohm load can be
broadband, an 8:1 Wilkinson combiner requires less risky than attempting to scale the device to
14 /4 transmission-lines with lossy, high- meet similar power requirements.
impedance lines. The proposed combiner is
designed for a 50-Ω load without including the
shunt inductive lines tuning Ccb. At the PA output,
short 50-Ω transmission line sections (TL1)
combine the outputs of the two 4×6um cells. At each
consecutive level of combining, the characteristic
impedance is divided in half, resulting in wider
transmission lines and lengths that are minimized
for smallest losses. A final impedance
transformation from 12.5Ω to 50Ω requires a
/4 line (TL3) having 35Ω characteristic impedance.

Fig. 12 - PAE and gain as a function of output power for VCC
= 2.43 V at 140 GHz. From [28].

7. CONCLUSION

This paper has reviewed the requirements for
power amplifiers in digital beam-forming arrays in
frequency bands between 100 and 300 GHz.
Efficiency will play a critical role in reducing the
thermal load for front-end packaging due to high
power density. We review the optimization of PAs
in gain-limited operation and available device
technologies above 100 GHz for PAs to construct
Fig. 11 - Power-combined, common-base InP HBT class-A PA PAE bounds on efficiency and compare recent
at 140 GHz. The area is 1.23mm× 1.09mm.
published work to these bounds to demonstrate the
Fig. 12 plots the PAE and gain as a function of Pout potential for future research. Recent
at 140 GHz at a class-A collector bias current density demonstrations of class-A and class-B power
of 1.14mA/um. The three-stage PA has 23 dBm peak amplifiers in the 120-140 GHz range have set
power with 17.8% Power Added Efficiency (PAE) records for efficiency at 20% and 30%, respectively,
and 16.5dB associated large-signal gain at 131GHz. which were substantial improvements over prior
At 131GHz, the small-signal gain is 21.9dB. The work. Further improvements in efficiency above
small-signal 3dB-bandwidth is 125.8-145.8GHz. 100 GHz are possible in all technologies and the
frequency bands between 100-300 GHz may be as
While the class-B PA and power-combined class-A energy efficient as lower millimeter-wave bands
PA offer state-of-the-art performance for while offering support of massive MIMO.

© International Telecommunication Union, 2021 47