U.S. patent application number 09/854139 was filed with the patent office on 2002-04-25 for generation of a voltage proportional to temperature with accurate gain control.
Invention is credited to Chowdhury, Vivek.
Application Number | 20020047696 09/854139 |
Document ID | / |
Family ID | 9891524 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020047696 |
Kind Code |
A1 |
Chowdhury, Vivek |
April 25, 2002 |
Generation of a voltage proportional to temperature with accurate
gain control
Abstract
A circuit for generating an output voltage which is proportional
to temperature with a required gradient is disclosed. The circuit
relies on the principle that the difference in the base emitter
voltage of two bipolar transistors with differing areas, if
appropriately connected, can result in a current which has a
positive temperature coefficient, that is a current which varies
linearly with temperature such that as the temperature increases
the current increases. It is important to maintain a stable
internal line voltage in the face of significant variations in a
supply voltage to the circuit. This is achieved herein by providing
control elements appropriately connected to a differential
amplifier. The stable internal supply voltage can be used to power
a subsequent stage of the circuit for fine control of the gradient
of the voltage proportional to temperature.
Inventors: |
Chowdhury, Vivek; (Warfield,
GB) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, PC
FEDERAL RESERVE PLAZA
600 ATLANTIC AVENUE
BOSTON
MA
02210-2211
US
|
Family ID: |
9891524 |
Appl. No.: |
09/854139 |
Filed: |
May 11, 2001 |
Current U.S.
Class: |
323/316 |
Current CPC
Class: |
G05F 3/30 20130101; Y10S
323/907 20130101 |
Class at
Publication: |
323/316 |
International
Class: |
G05F 003/16 |
Foreign Application Data
Date |
Code |
Application Number |
May 12, 2000 |
GB |
0011545.1 |
Claims
1. A circuit for generating an output voltage proportional to
temperature with a required gradient, the circuit comprising: first
and second bipolar transistors with different emitter areas having
their emitters connected together and their bases connected across
a bridge resistive element, wherein the collectors of the
transistors are connected to an internal supply line via respective
matched resistive elements such that the voltage across the bridge
resistive element is proportional to temperature; a differential
amplifier having its inputs connected respectively to said
collectors and its output connected to a control terminal of a
first control element having a controllable path connected between
a first power supply rail and a control node; a second control
element having a controllable path connected between the control
node and a second power supply rail; and a third control element
having a control terminal connected to the control node and a
controllable path connected between the second power supply rail
and AN internal supply line, whereby the differential amplifier and
the first, second and third control elements cooperate to maintain
a stable voltage on the internal supply line despite variations
between the first and second power supply rails.
2. A circuit according to claim 1, wherein the current flowing
through the bridge resistive element is a temperature dependent
current which is also supplied through a first resistive chain to
generate at an output node of the circuit a voltage proportional to
temperature with a predetermined gradient determined by the first
resistive chain.
3. A circuit according to claim 2, which comprises first and second
bipolar transistors of opposite polarity connected in series
between the internal supply line and the output node which serve to
set the voltage on the internal supply line.
4. A circuit according to claim 3, wherein the first and second
transistors cooperate with a current supply element to generate a
supply current for the differential amplifier.
5. A circuit according to any preceding claim, wherein the first,
second and third control elements are bipolar transistors with the
base constituting the control terminal and the collector emitter
path constituting the controllable path.
6. A circuit according to any preceding claim which comprises a
second stage which has a second differential amplifier connected to
receive the output voltage proportional to temperature and a second
input connected to receive a feedback voltage which is derived from
an output signal of the differential amplifier whereby the gain of
the output voltage can be adjusted.
7. A circuit according to claim 6, wherein the second differential
amplifier is powered by the stable voltage on the internal supply
line.
8. A circuit according to claim 2 or 3, wherein the required
gradient is programmable through variation of the resistance of the
first resistive chain.
9. A circuit according to claim 6 or 7, wherein the feedback
voltage in the second stage is derived from the output signal of a
differential amplifier via an offset circuit which introduces an
offset voltage such that the output signal of a differential
amplifier provides at an output node said output voltage which has
a negative variation with negative temperature.
10. A circuit according to claim 9, wherein the offset circuit
comprises a bipolar transistor connected in series with a resistive
element.
11. A circuit according to claims 2 and 10, wherein the temperature
dependent current from the circuit is mirrored into the second
stage to flow through the resistive element of the offset circuit.
Description
[0001] The present invention relates to a circuit for generating an
output voltage which is proportional to temperature with a required
gradient.
[0002] Such circuits exist which rely on the principle that the
difference in the base emitter voltage of two bipolar transistors
with differing areas, if appropriately connected, can result in a
current which has a positive temperature coefficient, that is a
current which varies linearly with temperature such that as the
temperature increases the current increases. This current, referred
to herein as Iptat, can be used to generate a voltage proportional
to absolute temperature, Vptat, when supplied across a
resistor.
[0003] One such practical difficulty is the need to accurately
control the required gradient of variation of the voltage with
respect to temperature. In a circuit of the type mentioned above,
this can be done by controlling the value of resistance through
which the current proportional to absolute temperature Iptat is
supplied. However, this may not give adequate control of the
gradient and it is desirable therefore to incorporate a second
stage which allows the finer adjustment of the gradient to be made.
It is an aim of the present invention to incorporate such a second
stage in an environment with good line regulation for the first and
second stages.
[0004] The present invention provides a circuit for generating an
output voltage proportional to temperature with a required
gradient, the circuit comprising: a first stage arranged to
generate a first voltage which is proportional to temperature with
a predetermined gradient, the first stage comprising: first and
second bipolar transistors with different emitter areas having
their emitters connected together and their bases connected across
a bridge resistive element, wherein the collectors of the
transistors are connected to an internal supply line via respective
matched resistive elements such that the voltage across the bridge
resistive element is proportional to temperature; a differential
amplifier having its inputs connected respectively to said
collectors, and its output connected to stabilisation circuitry
connected between first and second power supply rails and an
internal supply line which cooperates with the differential
amplifier to maintain a stable voltage on the internal supply line
despite variations between the first and second power supply rails;
and a second stage which comprises a gain circuit connected to
receive the first voltage for altering the predetermined gradient
to match the required gradient, the gain circuit having as its
voltage supply said stable voltage on the internal supply line.
[0005] For a better understanding of the present invention and to
show how the same may be carried into effect reference will now be
made by way of example to the accompanying drawings in which:
[0006] FIG. 1 represents circuitry of the first stage;
[0007] FIG. 2 represents construction of a resistive chain;
[0008] FIG. 3 represents circuitry of the second stage;
[0009] FIG. 4 is a graph illustrating the variation of temperature
with voltage for circuits with and without use of the present
invention; and
[0010] FIG. 5 represents circuitry of another form of second
stage.
[0011] The present invention is concerned with a circuit for the
generation of a voltage proportional to absolute temperature
(Vptat). The circuit has two stages which are referred to herein as
the first stage and the second stage. In the first stage, a "raw"
voltage Vptat is generated, and in the second stage a calibrated
voltage for measurement purposes is generated from the "raw"
voltage.
[0012] FIG. 1 illustrates one embodiment of the first stage. The
core of the voltage generation circuit comprises two bipolar
transistors Q0,Q1 which have different emitter areas. The
difference .DELTA.Vbe between the base emitter voltages
Vb(Q1)-Vb(Q0) is given to the first order by the equation (1): 1
Vbe = KT q ln Ic 1 Is 0 Ic 0 Is 1 ( 1 )
[0013] where K is Boltzmanns constant, T is temperature, q is the
electron charge, Ic.sub.0 is the collector current through the
transistor Q0, Ic.sub.1 is the collector current through the
transistor Q1, Is.sub.0 is the saturation current of the transistor
Q0 and Is.sub.1 is the saturation current of the transistor Q1. As
is well known, the saturation current is dependent on the emitter
area, such that the ratio Is.sub.0 divided by Is.sub.1 is equal to
the ratio of the emitter area of the transistor Q0 to the emitter
area of the transistor Q1. In the described embodiment, that ratio
is 8. Also, the circuit illustrated in FIG. 1, is arranged so that
the collector currents Ic.sub.1 and Ic.sub.0 are maintained equal,
such that their ratio is 1, as discussed in more detail in the
following. Therefore, to a first approximation, 2 Vbe = KT q ln 8
(1a)
[0014] The difference .DELTA.Vbe is dropped across a bridge
resistor R2 to generate a current proportional to absolute
temperature Iptat, where:
Iptat=.DELTA.Vbe/R2 (2)
[0015] This current Iptat is passed through a resistive chain Rx to
generate the temperature dependent voltage Vptat at a node N1. A
resistor R3 is connected between R2 and ground.
[0016] With R2 equal to 18 kOhms, substituting the values in
equations (1) and (2) above, Iptat is in the range 2.5 .mu.A to 3
.mu.A over a temperature range of -20 to 100.degree. C. The
temperature dependent voltage Vptat is given by: 3 Vptat = Iptat
.times. ( R2 + R3 + Rx ) = KT q ln 8 ( R2 + R3 + Rx ) R2 ( 3 )
[0017] To get a relationship of the temperature dependent voltage
Vptat variation with temperature, we differentiate the above
equation to obtain: 4 Vptat T = K ln 8 ( R2 + R3 + Rx ) q .times.
R2 ( 4 )
[0018] With the values indicated above R2=18K, R3=36K, Rx=85K, the
variation of voltage with temperature is 4.53 mV/.degree. C.
[0019] Before discussing how Vptat is modified in the second stage,
other attributes of the circuit of the first stage will be
discussed.
[0020] The collector currents Ic.sub.1, Ic.sub.0 are forced to be
equal by matching resistors R0, R1 in the collector paths as
closely as possible. However, it is also important to maintain the
collector voltages of the transistors Q0,Q1 as close to one another
as possible to match the collector currents. This is achieved by
connecting the two inputs of a differential amplifier AMP1 to the
respective collector paths. The amplifier AMP1 is designed to hold
its inputs very close to one another. In the described embodiments,
the input voltage Vio of the amplifier AMP1 is less then 1 mV so
that the matching of the collector voltages of the transistors
Q0,Q1 is very good. This improves the linearity of operation of the
circuit.
[0021] Vddint denotes an internal line voltage which is set and
stabilised as described in the following. A transistor Q4 has its
emitter connected to Vddlnt and its collector connected to the
amplifier AMP1 to act as a current source for the amplifier AMP1.
It is connected in a mirror configuration with a bipolar transistor
Q6 which has its base connected to its collector. The transistor Q6
is connected in series to an opposite polarity transistor Q8, also
having its base connected to its collector.
[0022] The bipolar transistors Q8 and Q6 assist in setting the
value of the internal line voltage V.sub.ddint at a stable voltage
to a level given by, to a first approximation,
V.sub.ddint-Iptat(R3+R2+Rx+Rz)+Vbe(Q6)+Vbe(Q8) (5)
[0023] According to the principal on which bandgap voltage
regulators are based, as Vptat increases with temperature, the Vbe
of transistors Q6 and Q8 decrease due to the temperature dependence
of Vbe in a bipolar transistor. Thus, V.sub.ddint is a reasonably
stable voltage because the decrease across Q6 and Q8 with rising
temperature is compensated by the increase in Vptat.
[0024] The amplifier AMP1 has a secondary purpose, provided at no
extra overhead, to the main purpose of equalising the collector
voltages Q0 and Q1, discussed above. The secondary use is for
stabilising the line voltage V.sub.ddint. Imagine if V.sub.ddint is
disturbed by fluctuating voltage or current due to excessive
current taken from the second stage (discussed later) or noise or
power supply coupling onto it. The voltage on line V.sub.ddint will
go up or down slightly. If V.sub.ddint goes higher, then the
potential at resistor R2 and R3 will rise. Icl will increase
slightly more than IcO and the difference across AMP1 increases.
AMP1 is a transconductance amplifier and as the Vic increases more
current is drawn through Q2, i.e. Ic2 increases. Q3 is starved of
base current and switches off allowing V.sub.ddint to recover by
current discharge through the resistor bridge. The opposite occurs
when V.sub.ddint goes low in which case AMP1 supplies less current
to the base of Q2 therefore the current Ic2 decreases and mor
ecurrent from Q9 can go to the base of Q3 allowing more drive
current lc3 to supply V.sub.ddint. In effect there is some
stabilisation.
[0025] The base of a transistor Q9 connected between the transistor
Q2 and V.sub.supply is connected to receive a start-up signal from
a start-up circuit (not shown). The transistor Q9 acts as a current
source for the transistor Q2. An additional bipolar transistor Q5
is connected between the common emitter connection of the voltage
generating transistors Q0,Q1 and has its base connected to receive
a start-up signal from the start-up circuit. It functions as the
"tail" of the Vptat transistors Q0,Q1.
[0026] The temperature dependent voltage Vptat generated by the
first stage illustrated in FIG. 1 has a good linear variation at
the calculated slope.apprxeq.4.53 mV/.degree. C. However, the
internal line voltage V.sub.ddint limits the swing in the upper
direction, and also Vptat cannot go down to zero.
[0027] It will be appreciated that the resistive chain Rx
constitutes a sequence of resistors connected in series as
illustrated for example in FIG. 2. The slope of the temperature
dependent voltage is dependent on the resistive value in the
resistive chain Rx and thus can be altered by tapping off the
voltage at different points P1,P2,P3 in FIG. 2.
[0028] FIG. 3 illustrates the second stage of the circuit which
functions as a gain stage. The circuit comprises a differential
amplifier AMP2 having a first input 10 connected to receive the
temperature dependent voltage Vptat at node N1 from the first stage
and a second input 12 serving as a feedback input. The output of
the differential amplifier AMP2 is connected to a Darlington pair
of transistors Q10, Q11. The emitter of the second transistor Q11
in the
[0029] Darlington pair supplies an output voltage Vout at node 14.
The amplifier AMP2 and the first Darlington transistor Q10 are
connected to the stable voltage line V.sub.ddint supplied by the
first stage. The second Darlington transistor is connected to
V.sub.supply.
[0030] The output voltage Vout is a voltage which is proportional
to temperature with a required gradient and which can move negative
with negative temperatures.
[0031] The adjustment of the slope of the temperature versus
voltage curve is achieved in the second stage by a feedback loop
for the differential amplifier AMP2. The feedback loop comprises a
gain resistor R4 connected between the output terminal 14 at which
the output voltage Vout is taken and the base of a feedback
transistor Q12. The collector of the feedback transistor Q12 is
connected to ground and its emitter is connected into a resistive
chain Ry, the value of which can be altered and which is
constructed similarly to the resistive chain Rx in FIG. 2. A
resistor R5 is connected between the resistor R4 and ground. The
gain of the feedback loop including differential amplifier AMP2 can
be adjusted by altering the ratio: 5 R4 + R5 R5 ( 6 )
[0032] This allows the slope of the incoming temperature dependent
voltage Vptat to be adjusted between the gradient produced by the
first stage at N1 and the required gradient at the output terminal
14. In the described example, the slope of the temperature
dependent voltage Vptat at N1 with respect to temperature is 4.53
mV/.degree. C. This is altered by the second stage to 10
mV/.degree. C. This is illustrated in FIG. 4 where the crosses
denote the relationship of voltage and temperature at N1 and the
diamonds denote the relationship of voltage to temperature for the
output voltage at the output node 14.
[0033] As has already been mentioned, the voltage Vptat at the node
N1 cannot move into negative values even when the temperature moves
negative. The second stage of the circuit accomplishes this by
providing an offset circuit 22 connected to the input terminal 12
of the differential amplifier AMP2. The offset circuit 22 comprises
the resistor chain Ry and the transistor Q12. Together these
components provide a relatively stable bandgap voltage of about
1.25 V. The resistive chain Ry receives the current Iptat mirrored
from the first stage via two bipolar transistors Q13, Q14 of
opposite types which are connected in opposition and which
cooperate with the transistors Q6 and Q8 of the first stage to act
as a current mirror to mirror the temperature dependent current
lptat. As lptat increases with temperature, Vbe(Q12) decreases.
This offset circuit 22 introduces a fixed voltage offset at the
input terminal 12, thus shifting the line of voltage with respect
to temperature. This shift can be seen in FIG. 4, where the curve
of the output voltage Vout at node 14 can be seen to pass through
zero and move negative at negative temperatures.
[0034] From the above description it can be seen that the "bridge"
network in the first stage performs a number of different
functions, as follows. Firstly, it provides a temperature related
voltage Vptat at the node N1. Secondly, it assists in providing a
relatively fixed internal supply voltage V.sub.ddint even in the
face of external supply variations, thus giving good line
regulation for the gain circuit of the second stage. Thirdly, it
provides in conjunction with the current mirror transistors Q4,Q6
current biasing for the amplifier AMP1 of the first stage.
Fourthly, it provides, through the mirroring of transistors Q6,Q13
current biasing for the resistive chain Ry in the offset circuit 22
of the second stage.
[0035] Table 1 illustrates the operating parameters of one
particular embodiment of the circuit. To achieve the operating
parameters given in Table 1, adjustment can be made using the
resistive chain Rx implemented in the manner illustrated in FIG. 2
to adjust the slope of Vptat in the first stage.
[0036] Alternatively, the slope may be adjusted in the second stage
by altering the gain resistors R4,R5.
1TABLE 1 Parameter Conditions Min Typ Max Units Accuracy T = 25 C.
+/-2 deg C. -30 < T < 130 C. Sensor Gain -30 < T < 130
C. 10 mv/deg C. Load Regulation 0 < Iout < 1 mA 15 mV/mA Line
Regulation 4.0 < VCC < 11 V +/-0.5 mV/V Quiescent current 4.0
< VCC < 11 V 80 uA T = 25 C. Operating supply range 4 11 V
Output voltage offset 0 V
[0037] FIG. 5 represents an alternative second stage which includes
a differential amplifier AMP2 in a feedback loop as in the circuit
of FIG. 3. However, the second stage illustrated in FIG. 5 differs
from that in FIG. 3 in that there is no offset circuit. Instead,
the transistor Q12 is connected via a current mirror CM1 to the
supply line V.sub.Supply. This second stage allows the gradient of
the temperature dependent voltage at node N1 to be altered but does
not allow it to move negative with negative temperatures. CM2
denotes a second current mirror in the circuit of FIG. 5. The
second stage of FIG. 5 nevertheless still makes use of the stable
internal voltage supply line V.sub.ddint to supply the differential
amplifier AMP2. Table II illustrates the operating parameters of an
embodiment of the invention using the stage of FIG. 5.
2TABLE II Parameter Conditions Min Typ Max Units Accuracy -30 <
T < 130 C. +/-2 deg C. Sensor Gain -30 < T > 100 C. 10
mv/deg C. Load Regula- 0 < Iout < 1 mA +/-15 mV/mA tion Line
Regula- 4.0 < VCC < 10 V +/-0.5 mV/V tion Quiescent 4.0 <
VCC < 10 V 80 uA current Operating sup- 4.5 11 V ply range
Output voltage 0.81 V offset
[0038] For the circuit of FIG. 5, -10.degree. C.=0.71 V,
-20.degree. C.=0.61 V, -30.degree. C.=0.51 V, 100.degree. C.=1.81
V.
[0039] FIG. 5 represents an alternative second stage which includes
a differential amplifier AMP2 in a feedback loop as in the circuit
of FIG. 3. However, the second stage illustrated in FIG. 5 differs
from that in FIG. 3 in that there is no offset circuit. Instead,
the transistor Q12 is connected via a current mirror CM1 to the
supply line V.sub.supply. This second stage allows the gradient of
the temperature dependent voltage at node N1 to be altered but does
not allow it to move negative with negative temperatures. CM2
denotes a second current mirror in the circuit of FIG. 5. The
second stage of FIG. 5 nevertheless still makes use of the stable
internal voltage supply line V.sub.ddint to supply the differential
amplifier AMP2. Table II illustrates the operating parameters of an
embodiment of the invention using the stage of FIG. 5.
3TABLE II Parameter Conditions Min Typ Max Units Accuracy -30 <
T < 130 C. +/-2 Deg C. Sensor Gain -30 < T > 100 C. 10
mv/deg C. Load Regula- 0 < Iout < 1 mA +/-15 mV/mA tion Line
Regula- 4.0 < VCC < 10 V +/-0.5 mV/V tion Quiescent 4.0 <
VCC < 10 V 80 uA current Operating sup- 4.5 11 V ply range
Output voltage 0.81 V offset
[0040] For the circuit of FIG. 5, -10.degree. C.=0.71 V,
-20.degree. C.=0.61 V, -30.degree. C.=0.51 V, 100.degree. C.=1.81
V.
* * * * *