non-Foster Negative Capacitor Research Articles

How does the operating bandwidth of non-foster negative capacitor affect its stability properties?

How does the operating bandwidth of non-foster negative capacitor affect its stability properties?

Physically sound model of a non-Foster negative capacitor

ABSTRACTIn both engineering and physics communities, it is believed that approximation of a realistic non-Foster negative capacitor (within its operating bandwidth) with an ideal dispersionless negative capacitor is acceptable for practical purposes. However, the ideal negative capacitor is not causal and, therefore, not compatible with basic physics. Its use in the design process is misguiding since it often predicts entirely non-physical behaviour. Here, we show that a realistic negative capacitor can always be modelled as a dispersive voltage-controlled source, the internal impedance of which is an ordinary positive capacitor. This equivalent circuit clearly explains the origin of negative conductance that always accompanies negative capacitance, as well as the background physics of previously reported counter-intuitive phenomena in non-Foster metamaterials. Theoretical analysis was verified by simulations and measurements on an experimental low-frequency (100 Hz–25 kHz) negative capacitor demonstrator. Measurement results agreed well with theoretical predictions, showing that the proposed model is indeed physically sound.

Superluminal wave propagation in a non‐Foster negative capacitor loaded transmission line

Superluminal wave propagation phenomenon achieved from a periodic structure in which a transmission line (TL) is loaded with non-Foster negative capacitors is presented. The loading decreases the effective capacitance and the effective permittivity of the dielectric substrate of the TL. An experimental investigation of the superluminal phase and group velocity in the non-Foster loaded TL is reported. The phase and group velocities are extracted from measured S-parameters. To do so, three non-Foster negative capacitors operating over [50–350 MHz] frequency band are integrated in a TL. Superluminal broadband effects are obtained from 50 to 220 MHz. The negative capacitors loaded TL paves the way to innovative broadband leaky wave antennas, active metamaterial surfaces and cloaking devices.

Stability Analysis of Superluminal Metamaterial Transmission Line with Realistic Non-Foster Negative Capacitors

Recently, it has been shown possible to go around basic dispersion energy constraints that limit bandwidth of every passive metamaterial and construct a broadband active Epsilon-Near-Zero superluminal transmission line. A basic building block of this unusual transmission line is an active ‘tank circuit’ that contains both conventional (positive) capacitor and non-Foster negative capacitor. Published theoretical studies revealed that such a ‘tank circuit’ is stable if an overall capacitance is positive. These studies assumed lossless host transmission line periodically loaded with ideal dispersionless negative capacitors. However, a possible influence of the imperfections of a realistic negative capacitor (its dispersion and loss/gain) on the stability has not been investigated so far. Here, stability analysis of realistic superluminal transmission line is performed in Laplace domain. The obtained results are in a good agreement with those obtained in recent experiments on active transmission line developed at University of Zagreb.

Negative capacitor paves the way to ultra-broadband metamaterials

Experimental demonstration of the overcoming of basic dispersion-energy constraints in metamaterials with the help of active non-Foster negative capacitors is reported. The experimental metamaterial operates in RF regime, and it is based on air transmission line loaded with negative capacitors. Measurement results clearly show almost dispersionless Epsilon-Near-Zero behavior, accompanied with superluminal both phase and group velocities, over a bandwidth of more than four octaves (2 MHz-40 MHz). The principle of periodic loading of transmission line with negative capacitors may find applications in ultra-broadband active metamaterials for antennas and cloaking technology.