Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-01-10
2002-05-14
Zweizig, Jeffrey (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
06388507
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to current source circuits and, more particularly, to a voltage controlled current source circuit.
FIG. 1
illustrates a conventional voltage controlled current source circuit
100
for generating a current in response to a voltage. Such circuits are also known in the art as voltage-to-current converters. The voltage controlled current source circuit
100
of
FIG. 1
includes a transistor
101
and a load impedance
102
. The transistor
101
of voltage controlled current source circuit
100
receives a control voltage V
in
and produces an output current I
0
. The transistor
101
may be an NMOS transistor, for example. The current at the output is given by the equation:
I
o
=
(
V
i
n
-
V
th
)
R
,
where I
0
is the output current, V
in
is the control voltage, V
th
is the threshold voltage of the NMOS transistor, and R is the resistance of the load impedance.
Another conventional voltage controlled current source circuit
200
is shown in FIG.
2
. The voltage controlled current source circuit
200
of
FIG. 2
includes a transistor
201
and an active load impedance
202
. The load impedance used in voltage controlled current source
200
is a NMOS transistor bias in the triode region of operation. The transistor
201
of voltage controlled current source circuit
200
receives a control voltage V
in
and produces an output current I
0
. The transistor
201
may be an NMOS transistor, for example.
One major problem with voltage controlled current sources, such as those in
FIGS. 1 and 2
is that the load impedance is typically implemented using integrated circuit technology. For example, the resistor in
FIG. 1
may be implemented using doped polysilicon, for example. However, typical polysilicon resistors can achieve a tolerance on only about 20% across process. Additionally, process and temperature variations may cause deviations in the circuits of
FIGS. 1 and 2
by as much as 60%. Accordingly, voltage controlled current sources utilizing integrated circuit resistors can suffer deviations in the voltage to current relations that are unacceptable in many applications.
Accordingly, a voltage controlled current source that reduces the variation in the voltage to current relationship across process and temperature variations is desired.
SUMMARY OF THE INVENTION
A voltage controlled current source circuit, in accordance with one embodiment of the present invention, includes a first precision reference current coupled to a voltage control node, a first voltage controlled impedance circuit having a current input and a first voltage control input, and a first current mirror having a first current terminal coupled to the current input of the first voltage controlled impedance and a second current terminal, wherein the first current mirror generates a second reference current on the second current terminal. The first precision reference current and the second reference current are coupled together at the voltage control node. Additionally, the first voltage control input is coupled to the voltage control node.
A voltage controlled current source circuit, in accordance with another embodiment of the present invention, includes a first resistor, a first current mirror having a current terminal coupled to a first terminal of the first resistor, wherein the first current mirror generates a first reference current, a first voltage controlled impedance circuit having a current input and a first voltage control input, a second current mirror having a current terminal coupled to the current input of the voltage controlled impedance, wherein the second current mirror generates a second reference current. The first reference current and the second reference current are coupled together at a voltage control node and the first voltage control input is coupled to the voltage control node.
A voltage controlled current source circuit, in accordance with another embodiment of the present invention, includes a first transistor having a control terminal coupled to a control voltage, a first terminal, and a second terminal, a first resistor, a first current mirror having a current terminal coupled to a first terminal of the first resistor, wherein the first current mirror generates a first reference current, a first voltage controlled impedance circuit having a current input and a first voltage control input, a second current mirror having a current terminal coupled to the current input of the voltage controlled impedance, and a second voltage controlled impedance circuit having a current input coupled to the second terminal of the first transistor, and a second voltage control input, wherein the second current mirror generates a second reference current. The first reference current and the second reference current are coupled together at a voltage control node and the first voltage control input is coupled to the voltage control node.
According to one embodiment, the first resistor is an external resistor for generating a reference current.
According to one embodiment, the present invention includes a method of controlling a current. The method comprises generating a first current through a resistor, generating a second current at a current input of a voltage controlled impedance, providing reproductions of the first current and the second current at a voltage control node to generate a first control voltage at the voltage control node, and coupling the first control voltage at the voltage control node to a voltage input of the voltage controlled impedance, wherein the first control voltage corresponds to the difference between the first current and the second current.
The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the present invention.
REFERENCES:
patent: 5349286 (1994-09-01), Marshall et al.
patent: 6107868 (2000-08-01), Diniz et al.
patent: 6201436 (2001-03-01), Hur et al.
Elwan Hassan Osama
Hwang Chanku
Hitachi America Ltd.
Townsend and Townsend / and Crew LLP
Walsh Chad R.
Zweizig Jeffrey
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