Telephonic communications – Line equalization or impedance matching
Reexamination Certificate
1998-01-30
2001-05-15
Chan, Wing F. (Department: 2643)
Telephonic communications
Line equalization or impedance matching
C379S387020
Reexamination Certificate
active
06233334
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a telecommunication circuit having an input impedance and containing current sources.
The present invention also relates to a telephone circuit, telephone and telecommunication device comprising such a telecommunication circuit.
Such telecommunication circuits containing current sources are known in the art of telephone apparatus from U.S. Pat. No. 5,226,078 in which they are described as being a useful tool in controlling the line current in a wired connection between the telephone exchange and the subscriber. At the subscriber end, a communication device, such as a telephone or facsimile or the like, has to have line transmission circuitry which has to perform a number of functions. Examples of required functions, apart from the actual transmission and reception, are:
termination of the line with a correct definite impedance,
deriving of power from the line for feeding internal and/or peripheral devices and
modulating the line current with an AC-signal to be transmitted. Details of these requirements may vary from country to country. These requirements should be layed down so as to form standards. However, even the standards are changed frequently. One of the features varying per country is the value of the terminating line impedance. The above-mentioned US patent mentions off-chip components that are apparently necessary for performing the adjustments required by the varying standards.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a telecommunication circuit that can be used to cope with at least the varying line impedance in a way which allows smooth control thereof without jeopardizing the desired degree of integration of circuit components on the chip. To this end, the telecommunication circuit according to the invention is characterised in that at least two current sources are controlled current amplifiers, which are connected in such a way that their respective current transfer ratios determine the input impedance of the circuit. This enables the required input impedance to be adjusted simply by influencing the current transfer ratio of the controlled current amplifier(s). Influencing the transfer ratio of a controlled current amplifier takes place by means of controlling a control signal supplied to the controlled current amplifier. By means of this control signal, the input impedance for a particular country is set to the desired value, without any presettings or preadjustments in the telecommunication circuit being necessary. This even allows the input impedance to be controlled by programming it to the desired value.
A very simple embodiment of the telecommunication circuit in accordance with the invention is characterised in that the at least two controlled current amplifiers have outputs that are connected to one another.
A further embodiment of the telecommunication circuit according to the invention is characterised in that the telecommunication circuit comprises a further current amplifier having an input, and the two outputs of the at least two controlled current amplifiers and the input of the further current amplifier form a current summing node. Such a further embodiment is particularly suited for use in a telephone set and for controlling the telephone line current.
A still further embodiment is characterised in that at least the further current amplifier has two outputs, and, in particular, momentarily only one of the outputs conducts an output current. This still further embodiment of the telecommunication circuit according to the invention is particularly useful for providing a supply point as indicated in the aforementioned second function, which supply point can be used to electrically feed internal and peripheral circuitry, such as for hands free facilities, listening-in features, dialling features, loudspeaker features etcetera.
Another embodiment of the telecommunication circuit according to the invention is characterised in that the circuit comprises an AC-current source which is at least connected to the at least two controlled current amplifiers. This satisfies the requirements of the aforementioned third function as the AC-current source is capable of modulating the line current with the AC-signal to be transmitted, resulting in a flat frequency characteristic of the line voltage despite a complex line termination impedance. Desired replacement schemes of various telephone circuits in a variety of countries can be simulated in an embodiment of the telecommunication circuit according to the invention, which is characterised in that the controlled current amplifiers current transfer ratio of at least one of the controlled current amplifiers is frequency-dependent.
REFERENCES:
patent: 5226078 (1993-07-01), Coulmance et al.
patent: 5515434 (1996-05-01), Cotreau
Chan Wing F.
U.S. Philips Corporation
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