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 , 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
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  for the in-depth of the functioning).
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
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.