Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-03-04
2001-01-09
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
Reexamination Certificate
active
06172556
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to circuits employed for the sinking/sourcing a reference current, and is particularly directed to a new and improved multi-transistor current interface circuit that is operative to increase the gate-source voltage of a current sink/supply output MOSFET, in response to a drop in drain-source voltage of the MOSFET, that would otherwise cause its operation to shift from a saturation region to a linear region of its drain-to-source current versus drain-source voltage characteristic. This increase in gate-source voltage of the output MOSFET effectively shifts the saturation-linear transition region to a lower drain-to-source voltage range, thereby reducing the amount of headroom voltage required of a given sink/source current at the output terminal.
BACKGROUND OF THE INVENTION
Bipolar, CMOS and biCMOS transistor current mirror circuits are widely used throughout the electronics industry to source or sink a current that is to be interfaced with one or more signal processing circuits of an integrated circuit architecture. For proper operation, a current interface circuit should ideally be insensitive to changes in its power supply voltage. This has been conventionally accomplished by making the voltage supply rail differential large enough to accommodate powering the integrated circuit of interest, and still leave sufficient voltage ‘headroom’ for the current supply/sink circuit, in the presence of some variation in the power supply's output.
Unfortunately for the circuit designer, the ongoing microminiaturization of electronic products, such as, but not limited to wireless communication circuits, has been and is expected to be continued to be accompanied by a reduction in the size of the power supply. This means that the circuit designer is faced with the task of obtaining the same or even more performance from a circuit that is to be powered by an ever shrinking supply voltage differential (e.g., currently on the order of two volts or less).
As a non-limiting example, in a communication signal processing application employing an IF amplifier circuit having a bipolar transistor configured peak detector input stage, the associated current sink (e.g., an N-channel MOSFET circuit) may forced to operate with an extremely low overhead voltage (dependent upon the IF amplifier's AGC setting), for example, on the order of less than 0.2 V at a low V
CC
supply rail value and low temperature, due to relatively large base-emitter voltages required of the peak detector circuit.
SUMMARY OF THE INVENTION
In accordance with the present invention, this problem is addressed by a new and improved low voltage MOSFET-configured current sink/source, that couples a gate-source voltage control feedback circuit in a feedback path with the output MOSFET of an output current mirror circuit. The feedback circuit includes a feedback control MOSFET that is coupled to the output MOSFET, and is turned on in response to a drop in drain-source voltage of the output MOSFET that would otherwise cause the output MOSFET to shift from its saturation region to its linear region of operation.
When the feedback MOSFET is turned on an associated feedback control current mirror circuit mirrors the drain current in the feedback control MOSFET through a gate-coupling resistor of the output current mirror circuit. This produces a voltage drop across the gate-coupling resistor that increases the gate-source voltage of the output MOSFET to a value that effectively shifts the saturation-linear transition region of the drain-to-source current versus drain-source voltage of the output MOSFET to a lower drain-to-source voltage range.
For the application of the invention as a current sink, the circuit's output node is coupled to the drain of the output MOSFET, which is coupled in a current mirror circuit configuration with a like channel polarity reference current MOSFET. The geometries of these two output current mirror MOSFETs are ratioed to achieve the desired current mirror effect in the output MOSFET. The current-sinking output MOSFET has its source electrode coupled to first (e.g., ground) power supply rail, and its gate electrode coupled through a voltage-dropping element (resistor) to the gate electrode of the reference current MOSFET. The source electrode of the reference current MOSFET is also coupled to the ground supply rail. The drain electrode of the reference current MOSFET is coupled in common (diode-connected) with its gate electrode and is further coupled to receive a reference current from a current source that is coupled in circuit with a second power supply rail (V
CC
).
The drain electrode of the output MOSFET is further coupled to the source electrode of a third V
GS
-feedback control device, e.g., a like polarity channel MOSFET contained within a feedback circuit that also includes a further current mirror circuit. This third MOSFET has its gate electrode coupled in common with the gate electrode of the reference current MOSFET. The V
GS
feedback control MOSFET has its drain electrode coupled to the commonly connected drain and gate of a fourth, opposite polarity channel MOSFET, which is connected in current mirror configuration with a fifth opposite polarity channel MOSFET of the feedback current mirror circuit.
Normally, the third V
GS
-feedback control MOSFET is in its off state, since its V
GS
is less than its threshold voltage V
Th
, and no reference current is supplied to or mirrored by the further current mirror circuit. Current flow through the feedback control MOSFET and thereby through the further current mirror circuit is initiated when the drain-source voltage V
DS
of the output MOSFET drops below its threshold voltage V
Th
. Since there is no other gate current applied to either of the first and second MOSFETs, their gate-coupling resistor does not change the value of V
GS
of the output MOSFET.
The gate electrodes of the further current mirror's MOSFETs are connected in common, while their source electrodes are coupled to the second power supply rail. The drain electrode of the further current mirror's mirror MOSFET, which serves as the output current node of the further current mirror circuit, is coupled to the common connection of the gate-coupling resistor and the gate electrode of the output MOSFET. As will be described, the output current generated by the further current mirror circuit serves as a V
GS
feedback control current, by causing a voltage drop across the gate-coupling resistor, and thereby increases the gate-source voltage V
GS
of the output MOSFET in response to a drop in the drain-source voltage V
DS
of the output MOSFET.
Operation of the circuit is based upon relationships among various voltage and current relationships of a MOSFET, and the transition between the saturation and triode regions of a MOSFET when its drain-source voltage V
DS
satisfies the relationship V
DS
=V
GS
−V
Th
, where V
GS
is its gate-source voltage, and V
Th
its threshold voltage. Since V
GD
=V
GS
−V
DS
then, then at the transition region or ‘knee’ in the output MOSFET's current−voltage characteristic, V
GD
=V
Th
. As the drain-source voltage V
DS
of the output MOSFET decreases to the point that output MOSFET is no longer in its saturation region, the V
GS
-feedback control MOSFET begins to turn on, causing the flow of drain current in the feedback control MOSFET.
The current mirror circuit mirrors the drain current through the feedback MOSFET and applies this drain current through the gate-coupling resistor. This produces a voltage drop across the gate-coupling resistor, so that the value of gate-source voltage applied to the output MOSFET is modified (e.g., increased), since the V
GS
of the output MOSFET equals the sum of V
GS
of its associated mirror MOSFET and the voltage drop across the gate-coupling resistor.
The effect of this increase in the value of V
GS
of the output MOSFET for a reduced value of its drain-source voltage V
DS
is to shift the knee or
Intersil Corporation, Inc.
Stanford Gary R.
Tra Quan
Tran Toan
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