Graphical Approach to Optimization of Maximally Efficient-Gain-Boosted Feedback Amplifiers

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.