In this paper, a compact negative-group-delay (NGD) microstrip bandpass filter is proposed. The NGD characteristic is achieved by coupling a resistor-loaded microstrip line to a square open-loop resonator. To improve the selectivity, the square open-loop resonator is loaded with an open-circuited stub for realizing two transmission zeros (TZs) in the upper stopband. To verify the proposed method, an NGD microstrip bandpass filter with a size of 0.58λg × 0.35λg is designed and fabricated. From the measured results, the NGD time of -1.08 ns at the center frequency of 1.995 GHz is obtained with the NGD bandwidth of 34 MHz (1.977-2.011 GHz), in which the insertion loss is less than 7.5 dB, and the return loss is greater than 20 dB. Furthermore, three TZs at 1.520, 2.495, and 2.735 GHz are achieved with good stopband attenuation.
2. Yang, G., Q. Liu, S. Liu, and Y. Chang, "A compact wideband filtering power divider," Progress In Electromagnetics Research Letters, Vol. 81, 71-76, 2019.
3. Du, R.-N., Z.-B. Weng, and C. Zhang, "A miniaturized filtering 3-dB branch-line hybrid coupler with wide suppression band," Progress In Electromagnetics Research Letters, Vol. 73, 83-89, 2018.
4. Ravelo, B., "Theory of coupled line coupler-based negative group delay microwave circuit," IEEE Trans. Microwave Theory Tech., Vol. 64, No. 11, 3604-3611, 2016.
5. Chaudhary, G. and Y. Jeong, "Negative group delay phenomenon analysis using finite unloaded quality factor resonators," Progress In Electromagnetics Research, Vol. 156, 55-62, 2016.
6. Liu, G. and J. Xu, "Compact transmission-type negative group delay circuit with low attenuation," Electron. Lett., Vol. 53, No. 7, 476-478, 2017.
7. Wang, Z., Y. Cao, T. Shao, S. Fang, and Y. Liu, "A negative group delay microwave circuit based on signal interference techniques," IEEE Microwave Wireless Compon. Lett., Vol. 28, No. 4, 290-292, 2018.
8. Noto, H., K. Yamauchi, M. Nakayama, and Y. Isota, "Negative group delay circuit for feed-forward amplifier," IEEE MTT-S Int. Microwave Symp. Dig., 1103-1106, Honolulu, HI, USA, 2007.
9. Ahn, K. P., R. Ishikawa, and K. Honjo, "Group delay equalized UWB InGaP/GaAs HBT MMIC amplifier using negative group delay circuits," IEEE Trans. Microwave Theory Tech., Vol. 57, No. 9, 2139-2147, 2009.
10. Joeng, J., G. Chaudhary, and Y. Jeong, "Efficiency enhancement of cross cancellation power amplifier using negative group delay circuit," Microw. Opt. Technol. Lett., Vol. 61, No. 7, 1673-1677, 2019.
11. Mirzaei, H. and G. V. Eleftheriades, "Realizing non-Foster reactive elements using negative-group-delay networks," IEEE Trans. Microwave Theory Tech., Vol. 61, No. 12, 4322-4332, 2013.
12. Mirzaei, H. and G. V. Eleftheriades, "Arbitrary-angle squint-free beamforming in series-fed antenna arrays using non-foster elements synthesized by negative-group-delay networks," IEEE Trans. Antennas Propag., Vol. 63, No. 5, 1997-2010, 2015.
13. Choi, H., Y. Jeong, C. D. Kim, and J. S. Kenney, "Efficiency enhancement of feedforward amplifiers by employing a negative group-delay circuit," IEEE Trans. Microwave Theory Tech., Vol. 58, No. 5, 1116-1125, 2010.
14. Choi, H., Y. Jeong, C. D. Kim, and J. S. Kenney, "Bandwidth enhancement of an analog feed-back amplifier by employing a negative group delay circuit," Progress In Electromagnetics Research, Vol. 105, 253-272, 2010.
15. Chaudhary, G., Y. Jeong, and J. Lim, "Microstrip line negative group delay filters for microwave circuits," IEEE Trans. Microwave Theory Tech., Vol. 62, No. 2, 234-243, 2014.
16. Qiu, L., L. Wu, W. Yin, and J. Mao, "Absorptive bandstop filter with prescribed negative group delay and bandwidth," IEEE Microwave Wireless Compon. Lett., Vol. 27, No. 7, 639-641, 2017.