Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2001-05-24
2002-10-15
Patel, Rajnikant B. (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C327S538000
Reexamination Certificate
active
06465998
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of electronic circuits, and, more particularly, to a current source which may be supplied by a very low supply voltage (e.g., about 1.1 Volt) and which has reduced sensitivity to variations in supply voltage.
BACKGROUND OF THE INVENTION
Current sources are found in most integrated circuits. They are used for the biasing the various constituent parts of circuits. Integrated circuits are generally designed to be supplied by a wide range of supply voltages. By way of example, certain operational amplifiers may be supplied by a voltage between 2.7 Volts and 12 Volts. For such integrated circuits, it is important for their current sources to deliver currents that have little variance with respect to the supply voltage so that the operation of the integrated circuit is not influenced by the available supply voltage.
Furthermore, it is desirable for current sources to operate from a low supply voltage to reduce electrical consumption and to make the best use of the available power. This is particularly the case with devices powered by a battery, for example. The invention finds applications generally in the manufacture of electronic circuits, particularly integrated circuits, such as circuits intended for portable equipment.
One current source according to the prior art exhibiting substantial independence from the voltage supply includes a voltage generator delivering a regulated voltage and supplying a conventional current source at a constant voltage. Such generators, commonly referred to as bandgap generators, are described, for example, in Analysis and Design of ANALOG INTEGRATED CIRCUITS by Paul R. Gray, Robert G. Meyer, Third Edition, Ch. 4, A 4.3.2, pp. 345-346. These generators deliver a constant voltage of about 1.2 Volts and, therefore, require a supply voltage above this value. The minimum supply voltage required by bandgap generators is at least 1.3 to 1.5 Volts.
Another known current source may be seen in FIG.
1
. This is a so-called crossed source. The crossed source is constructed around four source transistors
10
,
12
, and
25
,
26
, connected in a master branch
14
and a slave branch
16
, respectively. A current fixing resistor
18
of a value R is connected in series with the first transistor
10
of the master branch. The base of each of the source transistors
10
and
12
of a given branch is connected respectively to the source transistor collector of the other branch. A current mirror
20
allows the current I circulating in the master branch to be copied to the slave branch. The current mirror
20
is constructed around two transistors
21
and
22
connected in the master branch and the slave branch, respectively. An output current for a load can be copied in an output branch (not shown) either from the master branch or from the slave branch.
The current I circulating in the master branch
14
is equal to
I
=
Δ
V
BE
R
where &Dgr;V
BE
is such that &Dgr;V
BE
=(V
BE26
+V
BE12
)−(V
BE25
+V
BE10
). In this expression, V
BE26
, V
BE12
, V
BE25
, and V
BE10
represent the base-emitter voltages of the transistors
26
,
12
,
25
and
10
, respectively.
One peculiarity of the current source of
FIG. 1
is that the current of the branches
14
,
16
evolves as a decreasing function of the supply voltage applied between the supply terminals
24
,
26
of the source. In other words, the source current tends to increase when the supply voltage falls. This characteristic is particularly advantageous when the current source is combined with other elements whose outputs evolve positively, i.e., as a growing function with the supply voltage.
To allow the operation of a current source such as that shown in
FIG. 1
, it is necessary to have available between the supply terminals
24
and
26
a voltage V
comin
equal to at least twice the base-emitter voltage V
be
of a bipolar transistor (source transistor and cascode stage transistor). To this the collector-emitter saturation voltage V
cesat
of a third transistor (current mirror) is added. In other words, V
comin
=2V
be
+V
cesat
. For typical bipolar silicon transistors such as those represented in
FIG. 1
, the minimum supply voltage is about 1.8 Volts. This voltage is comparable with that required by the source using the bandgap type generator.
A third example of a current source according to the prior art is shown in FIG.
2
. This is a simple cascoded source. To simplify the description, different elements of this current source, comparable with those of the current source in
FIG. 1
, are identified with the same numerical references. Reference may be made, for these elements, to the above description. Unlike the current source of
FIG. 1
, it may be seen that the bases of the source transistors
10
and
12
are connected to each other. The transistors
25
and
26
which are connected to the source transistors form a cascode stage. An output branch
30
includes a load
32
to be supplied by the output current and a copy transistor
34
controlled by the common bases of the transistors of the mirror stage
20
. The use of a cascode stage
25
,
26
makes it possible to obtain a high output impedance for the source and therefore a relatively low variation in output current.
By analogy with the current source of
FIG. 1
, it may be seen that the minimum supply voltage is still such that V
comin
=2V
be
+V
cesat?
1.8 Volts. With the current source of
FIG. 2
, in which an emitter surface ratio of source transistors is equal to 10, and in which the current fixing resistor has a value of 5 k &ohgr;, a master branch current sensitivity as low as 1.6% per volt can be obtained (the current sensitivity in the slave branch is then about 5.2% per volt).
A fourth prior art current source may be seen in FIG.
3
. This current source is commonly referred to as a emitter degeneration source and is further described, for example, in Analysis and Design of ANALOG INTEGRATED CIRCUITS by Paul R. Gray, Robert G. Meyer, Third Edition, Ch. 4, A 4.2.1, p. 276. The current source of
FIG. 3
still includes two branches
14
and
16
coupled by a current mirror
20
. The master branch
14
includes a first source transistor
10
in series with a current fixing resistor
18
. The slave branch includes a second source transistor
12
connected to the first transistor by its base.
Unlike the current sources described in the previous figures, the cascode stage has been eliminated from the current source of that of FIG.
3
. The source transistors are in fact connected directly to those of the current mirror
20
. On the other hand, the emitters of the bipolar transistors
21
,
22
used to form the current mirror
20
are connected to the upper supply terminal
24
by so-called degeneration resistors
41
,
42
, respectively. The values of these resistors will be referred to as R
3
and R
4
, respectively, hereafter. The minimum supply voltage now becomes, for example, V
comin
=V
be12
+V
cesat22
+R
4
I
2
. In this expression, V
be12
is the base emitter voltage of the source transistor of the slave branch
14
, V
cesat22
is the collector-emitter saturation voltage of the mirror transistor
22
, and I
2
is the current circulating in the slave branch
16
. The current circulating in the master branch is I
1
.
For a current source comparable with that of
FIG. 3
, the choice of low degeneration resistor values makes it possible to reduce the minimum supply voltage required for the operation of the source. On the other hand, these low values of the degeneration resistors increase the sensitivity of the output current to the supply voltage. This aspect will emerge more clearly in the following description.
SUMMARY OF THE INVENTION
An object of the invention is to provide a current source supplying an output current that is substantially independent of the supply voltage.
Another object of the invention is to provide such a current source that may be powered at a low supply voltage.
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Jorgenson Lisa K.
Patel Rajnikant B.
STMicroelectronics S.A.
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