A 260-GHz Power Amplifier With 12.5-dBm $P_{\rm sat}$ and 21.4-dB Peak Gain Utilizing a Modified Coupled-Line-Balun Network

This article presents a prototype of a two-way differential power-combining sub-terahertz (sub-THz) power amplifier (PA) utilizing a coupled-line-balun network in a 0.13- $\mu$ m SiGe BiCMOS process with $f_{t}/f_{\text{max}}$ $=$ 350/450 GHz. To achieve optimum load impedance matching and power combining simultaneously in a sub-THz band, a modified design method based on the characteristic impedance and electrical length of a coupled line is proposed. This method employs offset coupled lines and additional compensated transmission lines (T-lines) to obtain a wide single-ended-to-differential impedance transformation range. Furthermore, the influence of bypassed capacitances on common-mode performance is also analyzed, and the metal–oxide–metal (MOM) capacitor is used in the balun design. The proposed power combiner and splitter occupy a compact area of less than 125 $\times$ 90 $\mu \text{m}^2$ . The sub-THz four-stage PA utilizing the proposed techniques exhibits a peak gain of 21.4 dB at 259 GHz, a 3-dB small-signal bandwidth of 33 GHz (242–275 GHz), and a 3-dB saturated power bandwidth of 41 GHz (241–282 GHz). A measured maximum saturated power of 12.5 dBm and the corresponding power-added efficiency (PAE) of 2.26% at 265 GHz verifies the effectiveness of the proposed modified design method.