Negative impedance converter

The negative impedance converter (NIC) is an active circuit 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 voltage in 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° (inversion) between the voltage and the current for any signal generator.

The two versions obtained are accordingly a negative impedance converter with voltage inversion (VNIC) and a negative impedance converter with current inversion (INIC).

The basic circuit of an INIC and its analysis is shown below.

INIC is a non-inverting amplifier (the op-amp and the voltage divider

, and the circuit acts like a resistor with negative resistance In general, elements

need not be pure resistances (i.e., they may be capacitors, inductors, or impedance networks).

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

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

The ideal behavior in this application depends upon the Norton resistance

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 (i.e., by causing the load to draw more current), the generator–NIC system can be rendered stable again.

In principle, if the Norton equivalent current source was replaced with a Thévenin 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.