Design aspects of single-ended and differential SiGe low-noise amplifiers operating above fmax/2 in Sub-THz/THz frequencies
This article presents a single-stage single-ended (SE) and a multistage pseudo-differential cascode low-noise amplifiers (D-LNA) with their center frequencies at 235 and 290 GHz, respectively. Both low-noise amplifiers (LNAs) are designed beyond half of the maximum frequency of oscillation (fmax) in 130-nm SiGe BiCMOS technology with ft/fmax of 300 / 450 GHz. Implications of gain-boosting and noise reduction techniques in cascode structure are analyzed and it is observed that beyond fmax/2, these techniques do not provide desired benefits. The single-stage SE LNA is designed to ascertain the theoretical analysis, and the same analysis is further implemented in staggered tuned four-stage LNA. Single-stage SE LNA provides a small signal gain of 7.8 dB at 235 GHz with 50 GHz of 3-dB bandwidth by consuming 18 mW of power. Four-stage differential LNA gives 12.9 dB of gain at center frequency 290 GHz and 11.2 dB at 300 GHz by drawing 68 mA current from the 2-V supply. The 3-dB bandwidth of differential LNA is measured to be 23 GHz. Noise figure measurements of both LNAs are performed using a gain-method technique with their measured noise figure values of 11 and 16 dB, respectively. This work successfully demonstrates the possibility of using a Si-based process to implement amplifiers beyond fmax/2. To the authors’ best knowledge, the four-stage differential LNA achieves, without any gain-boosting technique, the highest gain at 2/3(fmax) with decent noise figure performance in SiGe technology.