It is challenging to design high-gain amplifiers near the maximum oscillation frequency (f_max) of the transistors. This paper presents a comprehensive graphical approach to maximize the gain of feedback amplifiers with maximally efficient gain (G_ME) conception at near-fmax frequency. The complex gain-plane and the reflection-coefficient-plane are utilized to provide clear insights into both the gain and stability states of the two-port device while boosting G_ME. An efficient flowchart to synthesize feedback amplifiers is given, which optimizes the G_ME of a two-port device while ensuring the stability. A 210 GHz power amplifier in 40 nm CMOS was designed and optimized based on the proposed approach. The feedback circuit of the transistor pushes it to become potentially unstable and boosts GME. The measured peak small-signal gain was 10.48 dB at 195.33 GHz. The measured saturation output power and large-signal gain at 210 GHz were 3.04 dBm and 7.08 dB, respectively. The presented method could facilitate terahertz amplifier design.

The power gain and output power of millimeter-wave (mm-Wave) and terahertz (THz) amplifiers are critical performance metrics, particularly when the operating frequencies of amplifiers are near to the maximum oscillator frequency ($f_{max}$) of the transistor. This paper presents the design of a 110 GHz amplifier based on the quasi-linear method. The power gain can be boosted to maximum achievable gain ($G_{max}$) using a linear, lossless, reciprocal feedback network, though this leads to a simultaneous decrease in output power. Based on quasi-linear analysis, for an amplifier with $G_{max}$ gain, when the K-factor is equal to 1, the output power is zero. To avoid the very low output power of amplifiers, a new approach is proposed to balance power gain and output power. A 110 GHz six-stage feedback amplifier was designed using the proposed approach and fabricated using 40 nm CMOS technology. The measured power gain is 26.5 dB, and the saturation output power is 13 dBm at 110 GHz.