Negative impedance converter

A negative impedance converter, or NIC, is a special type of active circuit. Most parts in an electrical circuit are called loads. These loads usually consume energy from the system. However, a NIC works in a very different way. It is an active circuit that injects energy into other circuits. It does this by adding or subtracting a varying voltage. This process happens in series with the voltage drop of a positive impedance.

This action can reverse the voltage polarity or the current direction. It also introduces a 180-degree phase shift between the voltage and the current. This shift is also known as an inversion.

There are two primary versions of this technology. The first is the voltage negative impedance converter, or VNIC. This version focuses on voltage inversion. The second is the current negative impedance converter, or INIC. An INIC is often built using an operational amplifier, also called an op-amp. It also uses a voltage divider. In a basic INIC circuit, a resistor connects the output of the op-amp back to its input.

This specific setup allows the circuit to mimic a resistor with negative resistance. The components used, such as $R_1$, $R_2$, and $R_f$, do not have to be pure resistances. They can actually be capacitors, inductors, or other impedance networks.

To understand how an INIC works, we must look at the current flow. The op-amp output voltage creates a specific current. This current flows from the op-amp output through the resistor toward the source. We can label this current as $i_f$. Because of the circuit design, the input experiences an opposing current. This current is proportional to the voltage.

As a result, the input behaves as if it is facing a negative resistance. The circuit effectively pushes current back toward the source rather than just letting it flow through.

Engineers use NICs to make real generators behave more like ideal generators. A real generator is not perfect. Its output often changes depending on the load connected to it. We can represent a real generator using a Norton representation. This includes a current generator and an internal resistance. If we place an INIC in parallel to that internal resistance, we can change the system.

For the best results, the INIC must have the same magnitude as the internal resistance. However, its resistance value must be inverted. When these two are in parallel, the equivalent resistance becomes infinite. This allows the system to act like an ideal current source.

In this ideal state, the output current stays the same regardless of the load. This happens because the INIC supplies any current that would normally be lost to the internal resistance. This is a very precise balancing act. The success of this application depends on the Norton resistance and the INIC resistance matching perfectly. If the INIC resistance is much larger than the Norton resistance, the effect is negligible. If the values are not matched, the circuit may struggle to perform its intended task.

Stability is a major concern when working with these circuits. If the circuit is unloaded, it can become unstable. This instability occurs because the surplus current from the INIC generates positive feedback. This feedback can cause the voltage to reach the limits of the power supply. To make the system stable again, an engineer can reduce the impedance of the load. By causing the load to draw more current, the generator-NIC system returns to a stable state.

While a VNIC could theoretically be used in a series circuit, it is not very practical. A VNIC implemented with an op-amp must terminate on an electrical ground. Because of this limitation, the INIC is the preferred choice. Since any voltage source with resistance can be turned into a current source, the INIC is more versatile. It is typically placed in parallel with a source to improve its performance.

This makes it a vital tool for managing how energy moves through a system.

Finally, the capabilities of a NIC extend to many different types of impedance. An INIC can produce the negative of any impedance. This includes creating negative capacitance or negative inductance.

It can even be used to design floating impedances. A floating negative inductor is one example of this advanced application. This ability to manipulate the fundamental properties of electricity makes the negative impedance converter a highly significant tool in electronic design.