Current source with low temperature dependence

Details Current source with low temperature dependence Current source with low temperature dependence

2001-05-24

2003-04-01

Han, Jessica (Department: 2832)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C323S907000

Reexamination Certificate

active

06541949

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of electronic circuits, and, more particularly, to a current source with a low coefficient of temperature dependence.
BACKGROUND OF THE INVENTION
A coefficient of temperature dependence is a parameter which, for an electronic device, relates the variations in the device's output characteristics (i.e., its output current) to the variations in its operating temperature. The operating temperature may be especially influenced by ambient temperature. The temperature dependence coefficient may be defined both for a device in its entirety and for its constituent parts.
The present invention finds applications, for example, in the manufacture of electronic integrated circuits and in circuits including a current source. In particular, the invention may be useful for the manufacture of integrated circuits or circuit components requiring a current source having very little sensitivity to variations in temperature, such as oscillators, for example. Oscillators may be used in portable transceivers that are powered by battery and may be used at highly variable temperatures, for example.
A prior art current source with low temperature dependence is shown in FIG.
1
. The current source of
FIG. 1
includes a so-called reference current source
10
, a bandgap type reference voltage generator
16
that receives a reference current from the reference current source, and a transconductor
18
for converting the reference voltage of the generator
16
into an output current. The current source
10
has two branches
12
,
14
. These branches provide a reference current which is copied to the reference voltage generator
16
by a double cascoded current mirror
20
.
The reference voltage generator
16
includes a resistor
22
connected in series with a bipolar transistor
24
(PNP). The base of this transistor is connected to the collector and to a terminal
26
with a reference potential (e.g., ground). Its emitter is connected to the resistor
22
. The voltage V
bg
of the generator
16
, which is measured between a terminal
25
and the terminal
26
, may be expressed in the form V
bg
=V
EB
+R
1
I. In this expression, V
EB
is the emitter-base voltage of the transistor
24
, R
1
is the value of the resistor
22
, and I is the value of the current copied by the mirror
20
from the reference current source to the reference voltage generator
16
.
The transducer
18
includes an amplifier
27
and of a transistor
28
of the metal-oxide semiconductor (MOS) type. It delivers a current I
out
in a load resistor
29
having a value R
2
such that I
out
=V
bg
/R
2
. Thus, for a bipolar transistor such as the transistor
24
, the base-emitter voltage is a negative temperature function (i.e., a negative temperature dependence coefficient). On the other hand, the values R
1
and R
2
of the resistors
22
,
29
, as well as the current I copied from the reference generator
10
, evolve positively with the temperature.
By appropriately choosing the values of R
1
and I and summing the terms V
EB
and R
1
I it is possible to obtain, at the terminal
26
, a reference voltage generator with a temperature dependence coefficient able to compensate for the temperature drifts of the load resistor
29
and of the transconductor
18
. Thus, the output current I
out
may be rendered substantially insensitive to temperature. A more comprehensive description of the output source of
FIG. 1
may be found in Analysis and Design of Analog Integrated Circuits, Paul R. Gray/Robert G. Meyer, 3
rd
edition, p. 345 (FIG. 4.50).
The current source of
FIG. 1
provides very good temperature stability. Yet, it includes a large number of components and has a high power consumption. These characteristics do not lend themselves to integration of the current source in a high density integrated circuit or reduced circuit cost. Indeed, the chip surface required for such a current source integration is too great for many applications.
Another current source according to the prior art having a smaller number of components is illustrated in FIG.
2
. The current source of
FIG. 2
combines two individual current sources having opposite thermal behavior. The first individual source
30
is a current source with two branches coupled together by a current mirror. Such a source is known per se and delivers a current that varies in proportion to the temperature. More precisely, the current I
a
is such that:
Ia
=
kT
qR
a
⁢
ln
⁢
 
⁢
S
2
S
1
=
Δ
⁢
 
⁢
V
BE
R
a
,
where k, T, q, R
a
, S
1
and S
2
respectively represent the Boltzmann constant, the temperature, the electron charge, the value of a source current fixing resistor
34
, and the surfaces of emitters of bipolar transistors
31
,
32
,
33
and
35
(being respectively in two branches of the source). The term &Dgr;V
BE
represents a magnitude such that &Dgr;V
BE
=(V
BE33
+V
BE32
)−(V
BE34
+V
BE31
), where V
BE33
, V
BE32
, V
BE34
and V
BE31
respectively indicate the base-emitter voltages of the transistors mentioned above.
The second individual source
40
includes a bipolar transistor
42
connected in series with a current fixing resistor
44
having a value R
b
. It is further connected in parallel to the first current source
30
. A current I
b
delivered by the second source is such that I
b
=
I
b
=
V
BE
R
b
,
where V
BE
is the base-emitter voltage of the bipolar transistor
42
. The current I
b
is inversely proportional to the temperature, i.e., to
1
T
.
Transistors
51
,
52
, combined with resistors
53
,
54
, connect the two sources
30
,
40
to a first supply terminal
56
, connected to a first potential (V
cc
), and to a second supply terminal
58
, connected to a second potential (V
ee
). The transistors
51
,
52
have their bases respectively connected to biasing lines
61
,
62
which may be used to copy the current of the sources
30
,
40
to loads (not shown). That is, they are current mirror control transistors, also not shown.
By adjusting the values R
a
and R
b
of the current fixing resistors of the two individual sources
30
,
40
(and possibly the surfaces of the transistors
31
,
32
,
33
,
35
and
42
), it is possible to set the amount of current each current source contributes to the total current passing through the control transistors
51
and
52
. It is also possible to set the amount of current each individual source contributes to the thermal drift of the overall source combining the two sources.
Thus, the thermal drifts of the individual sources
30
,
40
are respectively proportional to the temperature (positive coefficient) and inversely proportional to the temperature (negative coefficient). As discussed previously, this is due to the fact that one of the sources is of the
Δ
⁢
 
⁢
V
BE
R
type and the other source is of the
V
BE
R
type. It is therefore possible to obtain at least a partial compensation for the drifts of the two sources, and therefore an overall source with a low temperature dependence coefficient. A more comprehensive discussion of the current source of
FIG. 2
may be found in Evolution of High-Speed Operational Amplifier Architectures by Doug Smith et al., IEEE J. of SSC., Oct. 1994, vol. 29, no. 10.
FIGS. 3
,
4
and
5
respectively show the temperature behavior of the first and second individual sources
30
,
40
and the overall source resulting from their combination. These figures respectively show, in graphical form, the current (shown on the ordinate) as a function of the temperature (shown on the abscissa). The evolution of the current is given for two different values of the supply voltage (2.7 and 5.5 V) measured between the supply terminals. On each graph, the letters A and B respectively show the curves obtained at 2.7 and 5.5 Volts. The currents are expressed as 10
−4
A and the temperatures are expressed in ° C.
It can be seen in
FIG. 3
that the curves A and B have a positive slope. This is characteristic of a positive temper

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