Information
-
Patent Grant
-
6407615
-
Patent Number
6,407,615
-
Date Filed
Friday, April 14, 200024 years ago
-
Date Issued
Tuesday, June 18, 200222 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Tran; Toan
- Nguyen; Hai L.
-
CPC
-
US Classifications
Field of Search
US
- 327 513
- 327 362
- 327 419
- 327 538
- 323 312
- 323 313
- 323 314
- 323 315
-
International Classifications
-
Abstract
A temperature compensation circuit converts a control signal (IG) that has an undesirable temperature coefficient to a temperature compensated control signal (I32) having a desirable temperature coefficient. In one embodiment, four transistors (60, 64, 68, and 72) are configured to convert the control signal (IG) having an undesirable temperature coefficient to the temperature compensated control signal (I32) having the desired temperature coefficient. Additional embodiments use components to refine the temperature compensation process.
Description
In general, this invention relates to a temperature compensation circuit. Specifically, this invention provides for a temperature compensation circuit and method that controls the temperature coefficient of an output signal.
Temperature compensation is often employed in situations where a control signal provided by another semiconductor device or circuit has a particular temperature coefficient and the control signal needs to be converted to a different temperature coefficient. For example, in a typical Radio Frequency (RF) application, a gain control signal is produced by a microprocessor. This gain control signal typically has an undesirable temperature coefficient, in that the gain control curve, e.g. voltage versus decibels, is subject to unwanted anomalies with temperature variation.
Prior art temperature compensation circuits, particularly those found in cellular or cordless phones, are typified by the presence of Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and an operational amplifier connected to a reference voltage for controlling the transfer characteristics of gain control input over temperature. These types of prior art circuits typically use voltage to current converters, where the reference and input voltages have an undesirable temperature coefficient and the reference and output currents have a desired temperature coefficient. One drawback of the prior art temperature compensation circuits is that the transfer characteristic does not produce a sufficiently linear result. Furthermore, the transfer characteristic produces a gain control curve where the minimum voltage is the threshold voltage (V
T
) of the MOSFET device, not zero. This is undesirable because the full control range is limited due to the threshold voltage. Also, the requirement for the operational amplifier adds complexity and cost to the circuit.
Therefore, a need exists to provide a temperature compensation circuit that produces an approximately linear output signal that is capable of a full range of control.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a circuit diagram of a temperature compensation circuit;
FIG. 2
is a circuit diagram of another embodiment of the temperature compensation circuit; and
FIG. 3
is a circuit diagram of still another embodiment of the temperature compensation circuit.
DETAILED DESCRIPTION OF THE DRAWINGS
Bipolar circuits have transistor base-emitter voltages and, in particular, base-emitter voltage differences that are Proportional To Absolute Temperature (PTAT). The present invention provides a circuit and method for interfacing a bipolar circuit to an external circuit having a different temperature coefficient.
FIG. 1
illustrates a four transistor model of a temperature compensation circuit. A current source
62
is connected between a power conductor that receives a voltage V
cc
and the collector of a transistor
60
, where current source
62
provides an input current. The emitter of transistor
60
is connected to a power conductor that receives ground potential. The base of transistor
60
is connected to an emitter of a transistor
64
. Transistor
60
conducts a current I
1
, which is the input signal having an undesirable temperature coefficient.
Transistor
64
has a collector connected to the power conductor that receives the voltage V
cc
and an emitter connected through a current source
66
to the power conductor that receives ground potential. Current source
66
provides a current I
2
having the desired temperature coefficient. The base of transistor
64
and the base of a transistor
68
are connected to each other and further connected to the collector of transistor
60
. The collector of transistor
68
is connected to the power conductor that receives the voltage V
cc
and an emitter connected through a current source
70
to the power conductor that receives ground potential. Current source
70
provides a current I
3
, which is a function of current I
2
. Current I
3
has the same undesirable temperature coefficient as current I
1
. A transistor
72
has a base connected to the emitter of transistor
68
, an emitter connected to the power conductor that receives ground potential, and a collector connected to an output terminal. Transistor
72
conducts a current I
4
, which is a function of the current I
1
and has a desired temperature coefficient. Thus, the current supplied by transistor
72
at the output terminal is the temperature compensated output signal.
In operation, temperature compensation circuit operates as follows. The circuit voltages are a function of the transistor base-emitter voltages (V
BE
) and, more particularly, the V
BE
of each transistor in relation to other transistors. Summing the V
BE
for the transistors results in the following relationship:
V
BE60
+V
BE64
=V
BE68
+V
BE72
, (Equation 1)
where V
BE60
is the V
BE
of transistor
60
, V
BE64
is the V
BE
of transistor
64
, V
BE68
is the V
BE
of transistor
68
, and V
BE72
is the V
BE
of transistor
72
. Note that V
BE
is equal to (kT/q) * In(I
c
/I
s
), where kT/q is the thermal voltage of the device, current I
c
is the relevant collector current, and current I
s
is the saturation current of the transistor. Thus, converting equation 1 to currents, the product of the current I
1
and the current I
2
is equal to the product of current I
3
and the current I
4
.
I
1
*I
2
=I
3
*I
4
(Equation 2)
where I
1
is the current conducted by transistor
60
, I
2
is the current conducted by transistor
64
, I
3
is the current conducted by transistor
68
, and I
4
is the current conducted by transistor
72
.
Isolating for the temperature compensated output current I
4
yields the following:
I
4
=(I
1
*I
2
)/I
3
(Equation 3)
Current I
2
was chosen with a desirable temperature coefficient. Currents I
2
and I
3
are chosen to be nominally equal at a known temperature. Current I
1
has an undesirable temperature coefficient that is canceled by the undesirable temperature coefficient for the current I
3
(see equation 3). Thus, current I
4
supplied at output terminal
36
is equal to the current I
1
, but whereas input current I
1
has an undesirable temperature coefficient, output current I
4
has the desirable temperature coefficient. Furthermore, current ratios other than 1:1 between currents I
4
and I
1
are possible by simply providing an alternate ratio for currents I
2
and I
3
as, for example, changing the physical dimensions of the transistor emitter areas with respect to each other. It should be noted that currents I
1
and I
2
are interchangeable, where current I
1
is the input signal and current I
2
is chosen with the desirable temperature coefficient.
FIG. 2
illustrates another embodiment of a temperature compensation circuit. In this embodiment, a current source
47
supplies a current I
G
to the emitter of a transistor
16
and to the base of a transistor
22
. The collector of transistor
16
is connected to a power conductor that receives a voltage V
cc
. The collector of transistor
22
is connected to an emitter of a transistor
24
and further connected to a base of a transistor
28
. The base and collector of transistor
24
are connected through a current source
26
to the power conductor that receives a voltage V
cc
. The collector of transistor
28
is connected to the power conductor that receives the voltage V
cc
. The emitter of transistor
28
is connected to the base of a transistor
32
and to the power conductor that receives the ground potential through a current source
33
. The collector of transistor
32
is connected to an emitter of a transistor
34
. The collector of transistor
34
is connected to a temperature compensated output terminal
36
. The base terminals of transistors
34
and
38
are connected to the base of transistor
24
. The collector of transistor
38
is connected to the power conductor that receives the voltage V
cc
. The emitter of transistor
38
is connected through a current source
40
to the power conductor that receives the ground potential and to the base of transistor
16
. It should be pointed out that transistor
34
may be removed from the circuit configuration.
The equations from above are modified consistent with the operation of the temperature compensation circuit. Summing the V
BE
for transistors
32
,
28
,
24
,
38
,
16
, and
22
results in the following:
V
BE32
+V
BE28
+V
BE24
=V
BE38
+V
BE16
+V
BE22
(Equation 4)
where V
BE32
is the base-emitter voltage of transistor
32
, V
BE28
is the base-emitter voltage of transistor
28
, V
BE24
is the base-emitter voltage of transistor
24
, V
BE38
is the base-emitter voltage of transistor
38
, V
BE16
is the base-emitter voltage of transistor
16
, and V
BE22
is the base-emitter voltage of transistor
22
. Transistors
22
and
24
conduct the same current and, therefore, the V
BE22
of transistor
22
is the same as the V
BE24
of transistor
24
because transistors
22
and
24
share the same current I
22
. Thus, equation 4 is simplified to:
V
BE32
+V
BE28
=V
BE38
+V
BE16
(Equation 5)
The currents for transistors
32
,
28
,
38
, and
16
can be represented by the product of currents I
32
and I
28
being equal to the product of currents I
38
and I
16
.
I
32
*I
28
=I
38
*I
16
, (Equation 6)
where I
32
is the current conducted by transistor
32
, I
28
is the current conducted by transistor
28
, I
38
is the current conducted by transistor
38
, and I
16
is the current conducted by transistor
16
.
Isolating for current I
32
, i.e., the temperature compensated output current, provides the following equation.
I
32
=(I
38
*I
16
)/I
28
(Equation 7)
In the preferred embodiment, transistors
16
,
38
,
28
,
32
,
24
, and
22
are bipolar transistors with similar sizing. Transistors
16
,
38
,
28
, and
32
are devices used in the basic operation of the circuit as described above in
FIG. 1
, while transistors
22
and
24
are included to improve the performance of the temperature compensation circuit.
This embodiment produces a temperature compensated output current at terminal
36
that is a function of the variable input current I
G
, but with a different temperature coefficient. By way of example, current I
G
may be received as a PTAT current but desired as having a zero temperature coefficient. The temperature compensation circuit illustrated in
FIG. 2
converts the input PTAT current I
G
to an output current I
32
having the zero temperature coefficient. In this embodiment, current I
40
is chosen as having a zero temperature coefficient and the output current I
32
will have the same temperature coefficient as the current I
40
. Thus, the current I
32
supplied at output terminal
36
is equal to current I
G
at a given temperature, but having a zero temperature coefficient.
FIG. 3
illustrates another embodiment of the temperature compensation circuit. It should be pointed out that like elements in the figures are denoted by the same reference numerals. The temperature compensation circuit receives a control voltage V
G
from a voltage source
12
. A microprocessor, microcontroller, or other device capable of producing a variable voltage may supply the voltage V
G
. Alternatively, the voltage V
G
may be generated on the same integrated circuit as the temperature compensation circuit. The voltage V
G
received at one terminal of resistor
13
is converted to a current I
G
. The other terminal of resistor
13
is commonly connected to the emitter of transistor
16
, a collector of a transistor
52
, and a base of transistor
22
. A reference voltage generator
42
is connected to one terminal of a resistor
44
. The other terminal of resistor
44
is connected to the base and collector of a transistor
46
, and to the base of transistors
52
and
30
. The emitters of transistors
46
,
52
and
30
are connected to the power conductor that receives a ground potential. In this embodiment, a resistor
31
connects the power conductor that receives a ground potential to the common connection that includes the emitter of transistor
28
, the base of transistor
32
, and the collector of transistor
30
. In the preferred embodiment, resistors
13
,
31
, and
44
have matching resistance values.
Equations 4, 5, 6 and 7 set forth above are applicable to the embodiment of the temperature compensation circuit illustrated in FIG.
3
. This embodiment of the temperature compensation circuit produces a temperature compensated output current at terminal
36
that is a function of the variable input current I
G
, but with a different temperature coefficient. The temperature compensation circuit illustrated in
FIG. 3
compares the input voltage V
G
to the reference voltage V
R
that is received having an unknown temperature coefficient and a current I
32
is supplied at output terminal
36
having a desired and known temperature coefficient.
By now it should be appreciated that a circuit is provided that receives a signal having a particular temperature coefficient and generates an output signal having a different temperature coefficient.
Claims
- 1. A temperature compensation circuit, comprising:a first transistor having an emitter coupled to a first power conductor and a collector for conducting a temperature compensated output current; a second transistor having an emitter directly connected to a base of the first transistor, an emitter coupled through a first current source to the first power conductor, and a collector coupled to a second power conductor; a third transistor having a base and a collector coupled through a second current source to the second power conductor, and an emitter coupled to the base of the second transistor; a fourth transistor having a base coupled to the base of the third transistor, an emitter coupled through a third current source to the first power conductor, and a collector coupled to the second power conductor; a fifth transistor having a base coupled to the emitter of the fourth transistor, an emitter coupled through a fourth current source to the first power conductor, and a collector coupled to the second power conductor; and a sixth transistor having a base coupled to an emitter of the fifth transistor, a collector coupled to the emitter of the third transistor, and an emitter coupled to the first power conductor.
- 2. The temperature compensation circuit of claim 1, further comprising:a seventh transistor having a collector coupled to the base of the sixth transistor and an emitter coupled to the first power conductor; and a eighth transistor having a base coupled to a base of the seventh transistor, a collector coupled to the base of the first transistor, and an emitter coupled to the first power conductor.
- 3. The temperature compensation circuit of claim 2, further comprising a first resistor that couples the base of the first transistor to the first power conductor.
- 4. The temperature compensation circuit of claim 3, further comprising a second resistor having a first terminal coupled for receiving a signal and a second terminal coupled to the base of the sixth transistor.
- 5. The temperature compensation circuit of claim 4, further comprising:a third resistor having a first terminal coupled for receiving a reference signal; and a ninth transistor having a commonly coupled collector and base coupled to a second terminal of the third resistor and further coupled to the base of the seventh and eighth transistors.
- 6. The temperature compensation circuit of claim 5, further comprising a tenth transistor having a base coupled to the base of the third transistor, a collector coupled to the output terminal, and an emitter coupled to the collector of the first transistor.
US Referenced Citations (8)