These negative elements exist only theoretically; an equivalent circuit, for example,

negative impedance converter (NIC) or

negative impedance inverter (NII or NIV), is necessary to invert the passive capacitor to a negative capacitor or inductor.

As it was shown in [36], based on a classic cell (cross-connected inverter pair) it is possible to obtain an adjustable

negative impedance circuit, well suited for standing wave based distributions.

A common method to improve the meta-atoms is to use active circuits [15, 19-21], for example, by using

negative impedance converters.

1 consists of two operational amplifiers: A1 which is the Wien-bridge oscillator and A2, which acts as a

negative impedance converter (NIC) at higher levels of voltage on the capacitor C3, (see [15] for the in-depth of the functioning).

Negative impedance converter with reduced nonideal gain and parasitic impedance effects.

* [S.sub.11] and [S.sub.12] magnitude < 1 (otherwise there would be

negative impedance)

From the

negative impedance model, cavity type and DROs to the general feedback type, all oscillators" have one common property.

Non-Foster circuits, then, through the use of active circuitry in the form of a

negative impedance converter [85, 86], can achieve a

negative impedance slope.

Using a

negative impedance converter, the resistance of the coil can be compensated or the circuit can be converted to a Non-Foster circuit which has more bandwidth and a gyrator would allow to replace the capacitor by a second inductor (which would also couple to the magnetic field) [22-24].

Note that, for practical purposes, it may be preferable to avoid using a transmission line with a

negative impedance; however, the line with

negative impedance can be easily substituted with a positive impedance line and a dependent voltage source.

The use of serial resonators has been traditionally recommended for generators of

negative impedance.

To obtain information concerning the oscillator startup reliability, it is useful to simulate the

negative impedance of the circuit.