The 'gyrator' is an
electric circuit which inverts an
impedance. In other words, it can make a
capacitive circuit behave
inductively, a
bandpass filter behave like a
band-stop filter, and so on. The concept was invented around 1948 by
B.D.H. Tellegen of
Philips Research Laboratories, Eindhoven ("The gyrator, a new electric network element", Philips Res. Rep. 3 (1948) pgs 81-101). It is primarily used in
active filter design.
Simulated inductor
Gyrator simulating Inductance. The two ''Zin'' have the same value
The primary use of a gyrator is to simulate an
inductive element in a small
electronic circuit or
integrated circuit. Before the invention of the
transistor,
coils of
wire with large
inductance might be used in
electronic filters. A real inductor can be replaced by a much smaller assembly containing a
capacitor,
operational amplifiers or
transistors, and
resistors. This is especially useful in
integrated circuit technology. Real capacitors are often much closer to "ideal capacitors" than real inductors are to "ideal inductors". Because of this, a synthetic inductor realized with a gyrator and a capacitor ''may'', for certain applications, be closer to an "ideal inductor" than any real inductor can be. Thus, use of capacitors and gyrators may improve the quality of filter networks that would otherwise be built using inductors. Also, the
Q factor of a synthesized inductor can be selected with ease.
Since gyrators use active components, they only function as a gyrator when both terminals of the simulated inductor are at voltages between the supply voltages of the active element. Hence gyrators are usually not very useful for situations requiring simulation of the 'flyback' property of inductors where a large voltage spike is caused when current is interrupted.
Operation of the circuit
The circuit works by inverting the effect of the capacitor. The desired effect is an impedance of the form
This is an ideal inductor ''L'' with a series resistance ''R
L''. From the diagram, it can be seen that the impedance of the simulated inductor is the desired impedance in parallel with the impedance of C and R.
If R is much greater than R
L, this comes close to
This is the same as a resistance R
L in series with an inductance L = R
LRC. It differs in function from a true inductor due to the parallel RC term, and because R
L is large compared to a real inductor. A real inductor has low internal resistance caused only by the wire it is made of. This limits the
Q factor, or accuracy, of
filters that can be made with the simulated inductor.
Applications
The primary application for a gyrator is to reduce the size and cost of a system by removing the need for bulky, heavy and expensive inductors.
Gyrator circuits are extensively used in telephony devices that connect to a
POTS system. This has allowed telephones to be much smaller, as the gyrator circuit carries the
DC part of the line loop current, allowing the transformer carrying the AC voice signal to be much smaller, due to the massively reduced current. Circuitry in telephone exchanges has also been affected with gyrators being used in
line cards.
There are many applications where it is not possible to use a gyrator to replace an inductor:
★ High Voltage systems (above working voltages of transistors/amplifiers)
★ RF systems (RF inductors are usually small anyhow)
★ Power conversion, where a coil is used as energy storage.
External links
★
Good description of this form of the simulated inductor — Elliot Sound Products
★
Another description, with the same circuit
★
LC filter design using equal value R gyrator, an alternative design
★
An alternative circuit
★
Another alternative circuit <-- 404 Error: Fixme
★
Discussion of the gyrator in general and a macro for Micro-Cap V
★
Java simulation of this circuit
★
Single transistor gyrator for telephony applications
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