Miscellaneous active electrical nonlinear devices – circuits – and – External effect – Temperature
Reexamination Certificate
2001-06-07
2002-10-01
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
External effect
Temperature
C327S362000
Reexamination Certificate
active
06459326
ABSTRACT:
The present invention concerns generally the field of current generator circuits. More particularly, the present invention relates to a method for generating a substantially temperature independent current and a device allowing implementation of the same.
Current generator circuits, commonly known by the name of “current sources” or “current sinks” are important elements in the design of numerous electric and electronic circuits.
FIG. 1
shows an example of a current generator circuit of the prior art globally designated by the reference numeral
10
. This current generator circuit
10
constitutes a voltage controlled current generator circuit.
Current generator circuit
10
typically includes amplifying means formed of an operational amplifier or differential amplifier
11
, a transistor
12
and a resistor
13
. Operational amplifier
11
includes a positive input terminal (non inverting input)
11
a
at which is applied an input voltage designated Vin, a negative input terminal (inverting input)
11
b
and an output
11
c
. Amplifying means
11
supplies a voltage at its output
11
c
in response to a difference between the voltages applied respectively to its first and second input terminals
11
a
and
11
b.
Transistor
12
is formed in this example of an n-MOS field effect transistor whose gate
12
c
is connected to the output
11
c
of operational amplifier
11
. Source
12
a
of transistor
12
is connected to negative input
11
b
of operational amplifier
11
and to a first terminal of resistor
13
. The other terminal of resistor
13
is connected to a supply potential or reference potential Vss. This reference potential Vss is typically defined as the most negative potential of the circuit or the circuit's earth at 0 volts. Another supply potential Vdd (not illustrated in
FIG. 1
) is also provided. Potentials Vss and Vdd constitute supply voltages for the circuit, and particularly for operational amplifier
11
.
According to the current generator circuit of
FIG. 1
, a current designated
11
passes through the drain-source branch
12
a
-
12
b
of MOS transistor
12
. The analysis of this circuit is direct. Operational amplifier
11
modifies the voltage at its output
11
c
such that the voltage present at its negative input
11
b
is substantially equal to the voltage present at its positive input
11
a
, i.e. substantially equal to input voltage Vin. The voltage across the terminals of resistor
13
is thus substantially equal to input voltage Vin, such that current
11
passing through the drain-source branch of MOS transistor
12
is given by:
I1
=
Vin
R
(
1
)
where R is the value of resistor
13
. Generated current I
1
is thus proportional to input voltage Vin applied at positive input
11
a
of the operational amplifier.
Current generator circuit
10
of
FIG. 1
forms a “current sink”, i.e. a current I
1
is drained from drain
12
b
of transistor
12
towards the most negative potential Vss. A modification of circuit
10
of
FIG. 1
allows a current source to be formed.
FIG. 2
illustrates a generator circuit designated
20
showing such a modification. Identical reference numerals are used to indicate those elements which have already been presented, i.e. operational amplifier
11
, MOS transistor
12
and resistor
13
.
In addition to the elements already mentioned, generator circuit
20
of
FIG. 2
typically includes a current mirror
30
formed of first and second p-MOS field effect transistors respectively designated
31
and
32
. Sources
31
a
and
32
a
of transistors
31
and
32
are connected to the most positive supply potential Vdd. Gate
31
c
and drain
31
b
of transistor
31
are connected together to drain
12
b
of transistor
12
and gate
32
c
of transistor
32
is connected to gate
31
c
of transistor
31
.
Current mirror
30
thus operates so as to “copy” current
11
and generate a current which is the image of current I
1
in the drain-source branch of transistor
32
. In accordance with what is typically known in the field, a proportionality factor can be introduced into the mirror by a suitable choice of the channel width to length ratios W/L of MOS transistors
31
,
32
in order to multiply or divide current I
1
.
Circuit
20
of
FIG. 2
may of course be further modified so that the current mirror includes other branches, for example a third MOS field effect transistor
33
as indicated in
FIG. 2
in order to generate a third current
13
.
One problem of the current generator circuits illustrated in
FIGS. 1 and 2
lies in particular in the temperature dependence of the currents generated. Typically, a temperature stable voltage such as a reference bandgap voltage approximately equal to 1.2 volts is used as input voltage Vin. This reference bandgap voltage has a relative low temperature dependence of the order of 50 ppm/°C.
In order to make resistor
13
, it is also sought to use a resistor whose temperature coefficient is relatively low. For design reasons, it is also sought to make resistor
13
in an integrated form and to avoid using a resistor external to the circuit. Various solutions exist in CMOS technology to design integrated resistors. It can however be noted that the temperature coefficients of these integrated resistors remains relatively high with respect to the temperature stability of a reference bandgap voltage. By way of example, an integrated resistor of the Rpoly type, i.e. an integrated resistor formed of a polysilicon layer, typically has a temperature coefficient of the order of +0.07%/°C., namely a temperature coefficient which remains substantially significant with respect to the stability of a reference bandgap voltage.
Those skilled in the art quickly note that there is no satisfactory way available, in CMOS technology, of making integrated resistors with sufficiently low temperature coefficients. With the aim of making a current generator circuit of the aforementioned type, the current generated by means of such a circuit will thus have a temperature dependence essentially due to the temperature dependence of the integrated resistor used.
A general object of the present invention is thus to propose a method for generating a substantially temperature independent current by means of a current generator circuit of the aforementioned type.
Another object of the present invention is to propose a device allowing the aforementioned method to be implemented, namely a current generator circuit overcoming the drawbacks encountered with the use of integrated resistors and arranged to generate a substantially temperature independent current.
A further object of the present invention is to propose a solution which involves only a few modifications to the current generator circuit and which consequently proves simple and inexpensive to manufacture with respect to the already existing solutions.
In order to answer these objects, the present invention first concerns a method for generating a substantially temperature independent current the features of which are listed in claim
1
.
The present invention also concerns a current generator circuit the features of which are listed in claim
5
.
The present invention relies on the observation by the inventor of the possibility of compensating for the temperature dependence of the current due to the resistor used by acting on the geometry of the differential pair of transistors of the operational amplifier used, in order to intentionally generate an offset voltage between the input terminals of the operational amplifier, this offset voltage being adjusted to have a temperature dependence compensating for the temperature dependence of the resistor used.
Indeed, the inventor was able to observe that by arranging the operational amplifier so as to create a geometric imbalance between the two transistors of the differential pair of said amplifier, an offset voltage between the input terminals of the amplifier was generated, this offset voltage having a substantially linear temperature dependence able to be adjusted by working with the geometry of th
Cox Cassandra
EM Microelectronic-Marin SA
Griffin & Szipl, P.C.
Wells Kenneth B.
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