Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
1999-06-07
2001-05-08
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S081000, C327S103000
Reexamination Certificate
active
06229351
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a current measuring arrangement for providing at an output terminal an voltage proportional to a current to be measured, said current measuring device comprising a first field effect device and a second field effect device.
The present invention also relates to a voltage measuring device and a telephone terminal using a current measuring device according to the invention.
A current measuring device according to the preamble is known from the Unexamined Japanese Patent Application, Laid Open No. 59-221672.
Such current measuring devices find widespread use in applications in which a voltage should be provided representing a current flowing in a part of an electronic circuit. An example of such an application is a telephone terminal in which it is advantageous to make several transmission parameters dependent on the line current. Examples of such parameters are the gain of microphone and earpiece amplifiers. Also the control of the side tone balance can be made dependent on the DC line current to ensure a correct side tone suppression for all line lengths.
In the current measuring arrangement according to the above mentioned Japanese Patent Application, the current to be measured is applied to the source of a first field effect device. The drain of the first field effect device is connected to the source of a second field effect device. The drain of the second field effect device is connected to a power supply. The gates of the first and second field effect devices are connected to a corresponding reference voltage.
The voltage at the source of the second field effect device will change with the current to be measured because the second field effect device will adjust its gate voltage source to a value corresponding to the input current. This source voltage is applied to a source follower circuit which makes the output signal available at the output terminal.
Because the relation between the gate-source voltage and the drain current of a field effect device is non-linear, the relation between the current to be measured and the output voltage will also be non-linear. In general such a non-linear relationship is undesired in such a measuring arrangement.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a current measuring arrangement according to the preamble in which a linear relation between the current to be measured and the output voltage is obtained.
To achieve said objective, the present invention is characterized in that the current measuring device comprises a current sensing resistor having a first terminal and a second terminal, a gate electrode of the first field effect device being coupled to the first terminal of the current sensing resistor, a source electrode of the first field effect device being coupled to a second terminal of the current sensing resistor, a drain electrode of the first field effect device being coupled to a current coupling device for causing a current dependent on the drain current of the first field effect device to flow in the second field effect device, and an output coupling device for supplying to an output terminal an output voltage derived from the voltage between a gate and a source terminal of the second field effect device.
By converting the voltage over a current sensing resistor into a drain current of the first field effect device, and by converting this drain current into a source-gate voltage of a second field effect device, an output voltage is obtained which is proportional to the voltage across the current sensing resistor. The non-linear of relation between the gate-source voltage and the drain current of the first field effect device is compensated by the same non-linearity of the second field effect device.
An embodiment of the arrangement according to the invention is characterized in that the current measuring device comprises an offset voltage source for offsetting the gate voltage of the first field effect device with respect to the source voltage of the first field effect device.
By offsetting the gate voltage of the first field effect device with respect to the source voltage of the first field effect device, it becomes possible to measure currents resulting in voltages across the sense resistor lower than the threshold voltage of the first field effect device. If the offset voltage is selected in a suitable way, it is possible to measure currents having values starting from 0.
A further embodiment of the arrangement according to the invention is characterized in that the current measuring device comprises a further offset source for offsetting the output voltage of the current measuring device with respect to the voltage between the gate and the source terminal of the second field effect device.
This additional measure makes it possible to obtain an output voltage is obtained which is independent of the value of the offset voltage. This makes it possible to use an offset voltage which is allowed to vary with temperature. This kind of offset voltage can be easily generated on an integrated circuit.
A still further embodiment of the invention is characterized in that the current coupling device comprises a cascode field effect device coupled between the drain terminal of the first field effect device and the source terminal of the second field effect device, the gate of the cascode field effect device being coupled to a reference source.
The introduction of the cascode field effect device has as a result that the output impedance of the first field effect is increased drastically. This results in a reduced influence of the supply voltage on the drain current of the first field effect device. This has as a consequence an improved accuracy of the measuring arrangement.
REFERENCES:
patent: 3892983 (1975-07-01), Okada et al.
patent: 4064377 (1977-12-01), Regan
patent: 4721957 (1988-01-01), Buttle
patent: 5430401 (1995-07-01), Shtulman
patent: 5654661 (1997-08-01), Kammiller
Unexamined Japanese Patent Application Laid Open No. 59-221672 (Abstract).
Callahan Timothy P.
Franzblau Bernard
Luu An T.
U.S. Philips Corporation
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