A tri-band negative group delay (NGD) microwave circuit for multiband wireless applications is proposed and self-matched without the need for external matching networks. The frequency range can be influenced by the characteristic impedance of the microstrip lines. Under the condition that the microstrip circuit can be implemented with the common printed circuit board (PCB) fabrication technology, the frequency ratio of the highest NGD band to the lowest NGD band can vary between 3.8 and 10.9. For verification, a 1.2/3.5/5.8-GHz tri-band NGD circuit for Beidou B2, WiMax, and WLAN application is designed, fabricated, and measured. From the measured results, the NGD times are -1.08 ns, -1.19 ns, and -1.09 ns at three NGD central frequencies with insertion losses of 16.4 dB, 24.6 dB, and 18.9 dB, respectively. And the measured NGD bandwidths are 12.40% for the lower band, 8.60% for the center band, and 3.59% for the upper band, in which the return losses are greater than 16 dB.
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