Current source able to operate at low supply voltage and...

Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...

Reexamination Certificate

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Reexamination Certificate

active

06590371

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to current sources, and more particularly, to a current source that operates at a low supply voltage and with quasi-null current variation in relation to a supply voltage.
BACKGROUND OF THE INVENTION
Current sources that operate at a low supply voltage and with quasi-null current variation in relation to a supply voltage are used, in particular, for polarizing circuits such as operational amplifiers, for example. These circuits are intended to operate over wide voltage ranges.
For example, one can consider portable devices that may be supplied either from a battery or from a main power supply. These devices can be radio devices, and devices for reading or sound reproduction. When these devices operate on a battery, the supply voltage is relatively low, on the order of 3 volts for example, and diminishes when the battery drains down to about 2 volts or less. When these devices operate from a main power supply, the supply voltage is on the order of 5 volts. There can be a ratio of 2 or even 3 between the two supply voltages.
At present the current sources used in this type of application are such as that shown in FIG.
1
. This source of current, produced in this example using bipolar technology, is connected between two supply terminals. Terminal
20
is connected to a high potential V
CC
and the other terminal
21
is connected to a low potential V
ee
, which is generally ground.
The current source comprises a core C and a current mirror M mounted in series between the two supply terminals
20
,
21
. The core C is the part of the current source which controls an equation corresponding to the source current. In this case, it concerns a so-called V
BE
/R source. The core C comprises a transistor Q
1
, a resistance R for setting the current and possibly an additional transistor Q
2
. The core C is connected to one of the supply terminals
21
, in this case the terminal
21
at the potential V
ee
. The transistors Q
1
and Q
2
of the core are of the same type, in this case of the n-p-n type.
In the description below, a voltage V
BE
represents a base-emitter voltage and a voltage V
CE
represents a collector-emitter voltage. The current mirror M comprises a pilot transistor Q
5
and at least one recopy transistor Q
4
. The mirror M is linked to the other supply terminal
20
, in this example, the potential V
CC
. The mirror transistors Q
4
, Q
5
are of the same type, in this case of the p-n-p type, and are complementary to those of the core C. They are produced at the same time and are thus identical.
The transistor Q
1
is connected between the supply terminal
21
and the recopy transistor Q
4
of the mirror M. These two transistors Q
1
, Q
4
form a slave branch
22
between the two supply terminals
20
,
21
. The base of the transistor Q
1
is connected to a first end of the resistance R for current setting. The second end of the resistance R is connected to the supply terminal
21
at the potential V
ee
. The first end of the resistance R is also connected to the pilot transistor Q
5
of the mirror M via the additional transistor Q
2
. The resistance R for setting the current, the additional transistor Q
2
and the pilot transistor Q
5
form a pilot branch
23
between the two supply terminals
20
,
21
. The transistor Q
1
is configured as a diode, that is, its base is connected to its collector via the additional transistor Q
2
. The mirror M is connected to the other supply terminal
20
, in this case at the potential V
CC
.
The recopy transistor Q
4
of the mirror M has its emitter connected to the supply terminal
20
at the potential V
CC
, its collector connected to the transistor O
1
of the core C and its base connected to the base of the pilot transistor Q
5
of the mirror M. The pilot transistor Q
5
of the mirror M has its base connected to the base of the recopy transistor Q
4
of the mirror M and to its collector. It is configured as a diode. Its connector is also linked to the resistance R of the core C via the additional transistor Q
2
. The emitter of the pilot transistor Q
5
is connected to the supply terminal
20
at the potential V
CC
.
The biasing current of the source is accessible at the level of the collector of an output transistor Q
6
, which is configured as a recopy transistor relative to the mirror M. Its emitter is connected to the supply terminal
20
at the potential V
CC
, and its base to the base of the pilot transistor Q
5
of the mirror M. The output transistor Q
6
is identical to the pilot transistor Q
5
. This biasing source is described on page 324 of the work “Analysis and Design of Analog Integrated Circuits” by P R GRAY and R. G. MEYER, 3rd Edition.
One can assume that in the core C, the current I crossing the resistance R, and which corresponds to the collector current of the transistor Q
2
, is the same as that circulating in the branch
22
by current mirror effect. Thus, one has:
I
=(
V
T
/R
×
1
n
(
I/I
S
)
where the thermal voltage V
T
equals kT/q, k is Boltzmann constant, T the temperature in degrees Kelvin and q the charge of the electron. I
S
represents the saturation current of the transistor Q
2
.
If I is known, this makes it possible to determine the expression of the polarization current Ic(Q
6
) of the source at the level of the output transistor Q
6
:
Ic
(
Q
6
)=
I
×(1
+V
CE
(
Q
6
)/
V
EA
(
Q
6
)/1
+V
CE
(
Q
5
)/
V
EA
(
Q
5
))
where V
EA
(Q
6
) and V
EA
(Q
5
) are respectively the Early voltages of the transistors Q
6
and Q
5
. They are equal, since the transistors Q
6
and Q
5
are of the same p-n-p type and are identical. The voltage V
CE
(Q
5
) is equal to V
BE
(Q
5
) because the pilot transistor Q
5
is configured as a diode. The voltage V
BE
(Q
5
) remains relatively constant while V
CC
varies.
The current Ic(Q
6
) varies in the same direction as the potential difference between the two supply terminals
20
,
21
since V
CE
(Q
6
) varies in the same direction as this potential difference. In the rest of the description below, this potential difference is assimilated to V
CC
since it has already been assumed that the supply terminal
21
is at a ground potential.
To obtain a biasing current in the opposite direction from the current Ic(Q
6
), that is, complementary to the current Ic(Q
6
), one can add a second output transistor Q
3
configured as a current mirror with the Q
1
transistor of the core. In this second mirror, the transistor Q
1
is the pilot transistor and the transistor Q
3
is a recopy transistor.
This recopy transistor Q
3
has its base connected to the base of the transistor Q
1
, its emitter connected to the first supply terminal
21
at the potential V
ee
and its collector forms another source output. The collector current of the transistor Q
3
is given by:
Ic
(
Q
3
)=
I
×(1
+V
CE
(
Q
3
)/
V
EA
(
Q
3
))/1
+V
CE
(
Q
1
)/
V
EA
(
Q
1
))
Ic
(
Q
3
)=
I
×(1
+V
CE
(
Q
3
)/
V
EA
(
Q
3
))/1
+V
BE
(
Q
1
)
+V
BE
(
Q
2
))/
V
EA
(
Q
1
))
V
EA
(Q
3
) and V
EA
(Q
1
) are Early voltages of the Q
3
and Q
1
transistors respectively. They are equal and correspond to the Early voltages of n-p-n transistors since Q
1
and Q
3
are identical n-p-n transistors. In this case again V
BE
(QL) and V
BE
(Q
2
) remain relatively constant while V
CC
varies, but V
CE
(Q
3
) varies in the same direction as V
CC
, and thus I
C
(Q
3
) varies in the same direction as V
CC
.
The properties of electronic circuits biased by a current source are intrinsically linked with the current consumption of their components. For example, the gain of a transistor increases as the current passing therethrough increases. To have properties as constant as possible to control electronic circuits, the biasing current should be as constant as possible regardless of the value of the supply voltage.
The biasing current source of
FIG. 1
is not completely satisfactory from this point of view. In addition, this biasing current source only starts up when the supply voltage Vcc reaches a relativ

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