Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2000-08-08
2001-07-03
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S324000, C327S543000, C323S315000
Reexamination Certificate
active
06255887
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates in general to the field of integrated electronic devices, and more particularly, to a variable transconductance current mirror circuit.
BACKGROUND OF THE INVENTION
Current mirrors are generally used to provide an output current in proportion to an input current. For example, one type of current mirror may include an input P-channel field effect transistor and an output P-channel field effect transistor. The input current may be applied to a commonly connected gate and drain of the input transistor, which has its source connected to a voltage supply. The gates of the input and output transistors may be connected in common, and the source of the output transistor may also be connected to a voltage supply. The drain of the output transistor may be connected to provide the output current to a load device or other circuit. The input and output transistors may be sized to provide the output current a desired fraction greater than or less than the input current.
Prior art current mirror circuits, however, suffer several disadvantages. For example, prior art current mirror circuits are generally susceptible to breakdown of the gate oxide integrity of the input and output transistors. For example, the voltage signal to the input and output transistors may be greater than the gate oxide integrity of the input and output transistors. Where the gate and the drain of the input transistor are connected together, a source-to-gate voltage drop across the input and output transistors may exceed the gate oxide integrity of the input and output transistors. This is often possible due to transient circumstances and fault conditions that must be accounted for in the input signal received by the mirror circuit.
SUMMARY OF THE INVENTION
Accordingly, a need has arisen for an improved current mirror circuit. In accordance with the present invention, a variable transconductance current mirror circuit is provided which substantially eliminates or reduces disadvantages and problems associated with prior art current mirror circuits.
According to an embodiment of the present invention, a variable transconductance current mirror circuit includes a first field effect transistor having a gate, a source, and a drain, and a second field effect transistor having a gate, a source, and a drain. The gate of the second transistor is connected to the gate of the first transistor, and a current source is connected to the gates of the first and second transistors. The circuit also includes a voltage supply connected to the sources of the first and second transistors. The circuit further includes a first diode having an anode and a cathode. The anode of the first diode is connected to the gate of the first and second transistors, and the cathode of the first diode is connected to the source of the first and second transistors. The first diode comprises a zener diode having a reverse breakdown voltage operable to prevent gate oxide breakdown of the first and second transistors.
According to another embodiment of the present invention, a method for mirroring a variable transconductance current includes supplying a first voltage to a gate of a first field effect transistor and a gate of a second field effect transistor. The method includes supplying a second voltage to a source of the first transistor and a source of the second transistor. The method also includes providing a source-to-gate voltage drop across the first and second transistors and providing a source-to-drain voltage drop across the first transistor. The method further includes providing an input current from the first transistor which is to be mirrored to the second transistor and providing an increase in the source-to-gate voltage drop in response to an increase in the source-to-drain voltage drop to provide an increase in input current.
Technical advantages of the present invention include providing a current mirror circuit that prevents breakdown of the gate oxide integrity of the input and output transistors. For example, according to an embodiment of the present invention, the circuit prevents a source-to-gate voltage drop that is higher than the gate oxide integrity of the transistors.
Another technical advantage of the present invention includes providing a current mirror circuit capable of accurately mirroring over an expanded range of currents. For example, according to an embodiment of the present invention, the circuit provides variable transconductance of the input transistor as the source-to-drain voltage drop and the source-to-gate voltage drop across the input transistor varies.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.
REFERENCES:
patent: 4709216 (1987-11-01), Davis
patent: 5512857 (1996-04-01), Koskowich
patent: 5835994 (1998-11-01), Adams
patent: 5986411 (1999-11-01), Sueri et al.
patent: 6198343 (2001-03-01), Matsuoka
Adams Reed W.
Baldwin David J.
Brady III Wade James
Holmbo Dwight N.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Tran Toan
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