The negative impedance converter is a one-port op-ampcircuit acting as a negative load which injects energy into circuits in contrast to an ordinary load that consumes energy from them. This is achieved by adding or subtracting excessive varying voltagein series to the voltage drop across an equivalent positive impedance. This reverses the voltage polarity or the current direction of the port and introduces a phase shift of 180° between the voltage and the current for any signal generator. The two versions obtained are accordingly a negative impedance converter with voltage inversion and a negative impedance converter with current inversion. The basic circuit of an INIC and its analysis is shown below.
Basic circuit and analysis
INIC is a non-inverting amplifier with a resistor connected between its output and input. The op-amp output voltage is The current going from the operational amplifier output through resistor towardthe source is, and So the input experiences an opposing current that is proportional to, and the circuit acts like a resistor with negative resistance In general, elements,, and need not be pure resistances.
Application
By using an NIC as a negative resistor, it is possible to let a real generator behave like an ideal generator,. An example for a current source is shown in the figure on the right. The current generator and the resistor within the dotted line is the Norton representation of a circuit comprising a real generator and is its internal resistance. If an INIC is placed in parallel to that internal resistance, and the INIC has the same magnitude but inverted resistance value, there will be and in parallel. Hence, the equivalent resistance is That is, the combination of the real generator and the INIC will now behave like a composed ideal current source; its output current will be the same for any load. In particular, any current that is shunted away from the load into the Norton equivalent resistance will be supplied by the INIC instead. The ideal behavior in this application depends upon the Norton resistance and the INIC resistance being matched perfectly. As long as, the equivalent resistance of the combination will be greater than ; however, if, then the effect of the INIC will be negligible. However, when the circuit is unstable. In particular, the surplus current from the INIC generates positive feedback that causes the voltage driving the load to reach its power supply limits. By reducing the impedance of the load, the generator-NIC system can be rendered stable again. In principle, if the Norton equivalent current source was replaced with a Norton equivalent voltage source, a VNIC of equivalent magnitude could be placed in series with the voltage source's series resistance. Any voltage drop across the series resistance would then be added back to the circuit by the VNIC. However, a VNIC implemented as above with an operational amplifier must terminate on an electrical ground, and so this use is not practical. Because any voltage source with nonzero series resistance can be represented as an equivalent current source with finite parallel resistance, an INIC will typically be placed in parallel with a source when used to improve the impedance of the source.
The negative of any impedance can be produced by a negative impedance converter, including negative capacitance and negative inductance. NIC can further be used to design floating impedances - like a floating negative inductor.
