H. Choi, Y. Jeong, C. D. Kim, and J. S. Kenney, “Efficiency enhancement of feedforward
amplifiers by employing a negative group-delay circuit,” IEEE Trans. Microw. Theory
Tech., vol. 58, no. 5 PART 1, pp. 1116-1125, 2010,doi: 10.1109/TMTT.2010.2045576.