Electrical PrinciplesUse of two-port networks

Introduction Hi my name is Daniel Till, this

presentation on the use of two-port networks.

What is a Two-port network? To start, what a two-port network is: this type of network is

an electrical device or circuit that has two pairs of terminals as you can tell by the name. You can also get four port networks which will have 4 pairs of terminals. This network includes having a circuit or part of the circuit which is isolated and searching for its characteristic parameters. After this has been done then the part of the circuit you isolated will become what’s known as a ‘black box’ which will carry a set of certain properties which will allow us to take away its specific physical build-up which will then allow a more simple analysis of the circuit itself. But any type of circuit can be made into a two-port network is needed as long as it doesn’t have an independent source running to it.

Example of Two-port network The picture below is an example of the Two-port

network described:

The symbols on the image above are:-V1 – Input VoltageV2 – Output VoltageI1 – Input CurrentI2 – Output Current

The arrows on the image show the direction flow.

3 Main systems Impedance Matching is used in:- Transmission Lines Telephone systems Loudspeakers

Impedance matching Impedance matching is used to maximise the power

transfer and minimise the reflections from the load, you would do this by making the output impedance of a source equal the source of the input impedance of the load which are connected but this only works when both are linear devices. Generally the term Impedance Matching is used to mean “Choosing impedances that work well together” rather than “Making two impedances combine”. This includes impedance bridging which is where the load impedance is greater than the source impedance, these connections tend to be used to get the max voltage transfer rather than the power transfer.

Transmission Lines Transmission lines must have the impedance

matched because the length of the line is greater than the wavelength of the signal, this means the signal changes a lot compared to the time it takes to get from one end to the other. This means they must be the same at the beginning and at the end, which is why they must be matched. This also prevents reflections of the signal at the end of the line which causes echo, although some of the wave is reflected back some keeps moving onwards if there is only one boundary.

Telephone systems Telephone systems use matching

impedances, this reduces the echo that is made due to the long distances of the lines. By matching the impedances it enables the telephone hybrid coil work correctly (2 to 4 wire conversion).

Loudspeakers Loudspeaker impedances tends to be

higher because the amplifiers that are used have been designed to have a relatively low output impedance and this is normally as low as a fraction of an ohms. Modern amplifiers are generally called bridging devices which take an input voltage from an audio source and form an amplified image of that voltage at the output.

References http://

www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp

Two-port networks North Lindsey Moodle http://

www.ece.uci.edu/docs/hspice/hspice_2001_2-269.html

The End Thank you for looking at this

presentation, if you have any questions or queries please send me an email: Danny@live.co.uk