Gyrator

About 2 pages (734 words)

The gyrator or positive impedance inverter 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^[1]. It is primarily used in active filter design and miniaturization.

1 Simulated inductor
2 Applications
3 External links
4 References

Simulated inductor

Gyrator simulating Inductance.

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. Additionally, 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. Note: The image to the right shows the op amp circuitry used to generate the transfer function <math>A_v = \frac{V_o}{V_i} = \frac{sCR}{sCR+1}</math> with <math> Z_{in\ op\ amp} = \frac{R_L(sCR+1)}{(sCR_L+1)} ;s=j\omega </math>

which is the input impedance seen by a connected source.
This is not the same as <math>Z_{in}</math> of the eqivalent RL network shown below the op amp circuit.
If we multiply the transfer function of the op amp by <math>\frac{R_L}{R_L}</math> we get <math>\frac{{sCR}R_L} </math>
The transfer function of a simple RL high pass network would be <math>\frac{V_o}{V_i} = \frac{sL}{sL+R_L}</math>
If we substitute <math>L=CRR_L</math>, the output of the op amp simulates a simple RL high pass network.
The point is, the op amp circuit is used to simulate a voltage source being connected to an RL circuit, without using a coil, but using an op amp circuit with the appropriate transfer function.

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. For examples, L-C-R bandpass filter characteristics can be realized with capacitors, resistors and operational amplifiers without using inductors. Thus Hi-Fi graphic equalizers can be achieved with capacitors, resistors and operational amplifiers without using inductors because of the invention of "gyrator". 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. Gyrators are also widely used in Hi-Fi Graphic Equalizers, Parametric Equalizers, discrete Bandstop and Bandpass filters, such as rumble filters and FM pilot tone filters.

There are many applications where it is not possible to use a gyrator to replace an inductor: