Operational amplifier finite gain and bandwidth influence to non-foster impedance matching

Negative impedance converters (NIC) are widely used for impedance matching of electrically-small antennas (ESA). For example, impedance of electrically-small monopole antenna contains high capacitive component and negative capacitance can be used to compensate this. Such negative capacitance can be realized with the use of NIC circuit based on wideband and low noise operational amplifier. Most of the papers related to operational amplifiers based NIC analysis make assumptions on ideal op-amp behavior and concentrates at circuit design considerations. However, real world op-amps have finite gain and bandwidth frequency response. This paper utilizes low pass op-amp model with high gain at dc and a single-pole frequency response and presents a technique of negative capacitance circuit analysis. We first review model of NIC circuit build with non-ideal operational amplifier, and then present experimental results for the non-Foster impedance matching of electrically-small monopole. The measurements on ESA with NIC matching circuit demonstrate about 15 dB signal level improvement over working bandwidth and are in good agreement with the presented theory.