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An innovative negative group delay (NGD) theory based on a unity direct chain feedback (UDCF) circuit topology is developed in this paper. This NGD circuit is an active cell constituted by an operational amplifier in feedback with a four-port RC-network. This NGD circuit theory is developed based on the S-parameter model analytically established from the equivalent impedance matrix. The UDCF group delay frequency response is expressed as a function of the feedback RC-cell and the operational amplifier parameters. The NGD analysis of the developed UDCF cell is introduced. According to theoretical analysis, under a certain condition, the UDCF topology is susceptible to behave as a low-pass NGD function. The UDCF cell NGD characteristics are defined theoretically. The theoretical prediction is verified numerically and experimentally in both the frequency- and time-domain by designing and fabricating an active PCB prototype. The simulations and experimentations show that the UDCF circuit exhibits an NGD of approximately -38 ns with NGD cut-off frequency of about 5.5 MHz. More importantly, it is demonstrated in the time-domain that the low-pass NGD effect enables the UDCF cell to generate advanced output with sinc waveform input voltages.

Negative Group Delay Theory of a Four-Port RC-Network Feedback Operational Amplifier