Page 58 - 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

configurations indicate identical PAE, which points
to an invariance between CE and CB, that ranges
from 46% in deep class AB and drops to 35% at
class A. The gain of the PA at peak efficiency
demonstrates that the CB offers significantly more
gain than CE. Consequently, the CE has higher
collector efficiency under the range of quiescent
collector currents than CB but the higher gain of CB
results in similar PAE.
The schematic of the pseudo-differential, common-
base class-B PA is illustrated in Fig. 9 along with a
chip microphotograph of the stage. The class-B
voltage bias is provided to the emitter through the Fig. 9 – Pseudo-differential, common-base InP HBT class-B PA at
balun. The PA occupies an area of 0.4mm by 0.5mm 140 GHz. The area of the die on the right is 0.4 x 0.5 mm.
to easily fit with the 140 GHz the grid spacing. The peak PAE occurs at 0.3 dB higher output power.
The PA exhibits a peak gain of 7 dB which occurs at
Pout of 13 dBm with 1 dB of gain expansion due to
the class-B biasing. The output loss of the matching
network was measured through test structures to
be around 1dB which raises questions about why
the measured class-B gain is less than the maximum
available gain indicated in Fig. 7. The explanation is
partially explained when referring back to Fig. 8
which plotted the gain associated with the common-
base device under the matching conditions for high
efficiency. Here, the common-base device has only
around 9 dB of gain near the class-B bias,
corroborating the measured PA operating gain.

Fig. 8 - Gain, output power, and PAE at 140 GHz as a function
of the quiescent collector current for the common-emitter
and common-base amplifier for Vce = 2.5 V.
For CB HBTs, the ratio of impedance seen into the
collector and at the output of the emitter is related
directly to the power gain when IE ~ IC for
sufficiently large. To increase the output power,
two common-base stages are used as a pseudo-
differential output PA. A low-loss sub-quarter
wavelength balun was used at the input and the
output to match the pseudo-differential PA at the
input and output [7][23]. The balun places
constraints on the maximum gain that can be Fig. 10 - PAE and gain as a function of output power for VCC
realized from a CB amplifier. To provide 6 dB gain, = 2.5 V at 130 GHz. From [15].
we choose the impedance of emitter port to be close The PA achieves 32% peak PAE with 15.3-dBm
to 25 and for the 100-Ohm collector impedance. A saturated output power. The 1-dB power bandwidth
series inductor at the output transforms the load covers 122 GHz to 146 GHz and is consistent with the
impedance to 50 Ohms. measured 3-dB bandwidth from the S-parameters.
Fig. 10 plots the PAE and gain as a function of Pout The input power across the band is calibrated
at 130 GHz at a class-B collector bias current density between 8 dBm and 8.7 dBm with variation due to the
of 203uA/um. probe loss variation over the band.

46 © International Telecommunication Union, 2021