Information
-
Patent Grant
-
6642772
-
Patent Number
6,642,772
-
Date Filed
Tuesday, October 15, 200221 years ago
-
Date Issued
Tuesday, November 4, 200320 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 327 478
- 327 482
- 323 315
-
International Classifications
-
Abstract
Current mirror circuits that are parts of a first circuit and a second circuit, respectively, allow the same constant current to flow through the input side and the output side. Therefore, the base-emitter voltages of transistors Tr1 and Tr4, which tend to vary due to a temperature variation, can be set identical and hence can cancel out each other sufficiently. The same is true of the base-emitter voltages of transistors Tr5 and Tr8. Therefore, an input signal can be converted by a function having reference voltages as change points without being affected by temperature. Desired function circuits can be obtained by combining first circuits and second circuits in various manners.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a function circuit for converting an input signal into an output signal by a prescribed function. In particular, the invention relates to a function circuit that is less prone to be affected by temperature.
2. Description of the Related Art
FIG. 5
is a circuit diagram of a conventional function circuit.
FIG. 6
shows an input/output characteristic of the circuit of FIG.
5
.
The function circuit of
FIG. 5
is composed of three resistors R
1
, R
2
, and R
3
, two diodes D
1
and D
2
, and two reference supply voltages V
1
and V
2
. As shown in
FIG. 5
, the resistor R
2
, the diode D
1
, and the reference supply voltage V
2
are connected to each other in series and the resistor R
3
, the diode D
2
, and the reference supply voltage V
1
are also connected to each other in series. The resistor R
1
is connected to the resistors R
2
and R
3
. One end of the resistor R
1
is an input terminal IN and the other end (connecting point) is an output terminal OUT of the function circuit. The diode D
2
is opposite in direction to the diode D
1
. An input signal Vs is input to the input terminal IN. For example, the reference supply voltage V
1
is 2 V and the reference supply voltage V
2
is 3 V.
In the input/output characteristic shown in
FIG. 6
, the horizontal axis represents the input signal Vs that is input to the input terminal IN and the vertical axis represents the output signal Vout at the output terminal OUT of the function circuit. In
FIG. 6
, each of Vs and Vout is in the range of 0 V to 5 V. As shown in
FIG. 6
, as the voltage level of the input signal Vs increases gradually, two change points α and β where linear lines having different slopes are connected to each other smoothly appear in the vicinity of the voltages 2 V and 3 V (reference supply voltages V
1
and V
2
), respectively. A generally S-shaped curve can be formed that is bent at the change points α and β that are in the vicinity of 2 V and 3 V.
The output signal Vout shown in
FIG. 5
can be given by the following formulae, where Vd is the forward voltage of the diodes D
1
and D
2
:
When Vs≧V
1
+Vd (in the vicinity of the high-temperature-side change point),
Vout≡{R
2
/(
R
1
+
R
2
)}(
Vs−V
1
−
Vd
)+
V
1
+
Vd.
(1)
When Vs≦V
2
−Vd (in the vicinity of the low-temperature-side change point),
Vout≡{R
1
/(
R
1
+
R
3
)}(
V
2
−
Vd−Vs
)+
Vs
(2)
When V
1
<Vs<V
2
,
Vout≡Vs
(3)
because the output resistance of the function circuit is rendered in a high-impedance state.
FIG. 7
is a circuit diagram of another conventional function circuit.
FIG. 8
shows an input/output characteristic of the function circuit of FIG.
7
.
The function circuit of
FIG. 7
is mainly composed of a first circuit including an npn transistor Q
1
and a pnp transistor Q
2
and a second circuit including a pnp transistor Q
3
and an npn transistor Q
4
. In the first circuit, the base terminal of the transistor Q
1
and the emitter terminal of the transistor Q
2
are connected to each other. In the second circuit, the base terminal of the transistor Q
3
and the emitter terminal of the transistor Q
4
are connected to each other. The emitter terminal of the transistor Q
1
and the emitter terminal of the transistor Q
3
are connected to each other via resistors R
2
and R
3
that have the same resistance (R
2
=R
3
). One end of a resistor R
1
is connected to the connecting point P
1
of the resistors R
2
and R
3
. The other end of the resistor R
1
serves as an input terminal IN to which an input signal Vs is input. A reference supply voltage V
1
(2 V) is applied to the base terminal of the transistor Q
2
, and a reference supply voltage V
2
(3 V) is applied to the base terminal of the transistor Q
4
. The connecting point P
1
also serves as an output terminal OUT.
In the second function circuit of
FIG. 7
, the potential of the emitter terminal of the transistor Q
2
, that is, the base potential of the transistor Q
1
, is set higher than the reference supply voltage V
1
(2 V) that is applied to the base terminal of the transistor Q
2
by the base-emitter voltage Vbe of the transistor Q
2
. The potential of the emitter terminal of the transistor Q
1
is set lower than the emitter potential of the transistor Q
2
by the base-emitter voltage Vbe of the transistor Q
1
. Therefore, the base-emitter voltage Vbe of the transistor Q
2
and the base-emitter voltage Vbe of the transistor Q
1
are in a relationship that they cancel out each other. The potential of the base terminal of the transistor Q
2
and the potential of the emitter terminal of the transistor Q
1
are set identical. As a result, as shown in
FIG. 8
, the function circuit of
FIG. 7
has an input/output characteristic having a curve that is centered at 2.5 V (Vcc/2) and is bent in the vicinity of the reference voltage V
1
(change point α) and the reference voltage V
2
(change point β).
The output signal Vout is given by the following formulae:
When Vs≧V
2
,
Vout≡{R
1
/(
R
1
+
R
3
)}(
V
2
−
Vs
)+
Vs
(4)
When Vs≦V
1
,
Vout≡{R
2
/(
R
1
+
R
2
)}(
Vs−V
1
)+
V
1
(5)
When V
1
<Vs<V
2
,
Vout≡Vs
(6)
because both of the transistors Q
1
and Q
3
are rendered off, that is, they are in a high-impedance state.
However, the function circuit of
FIG. 5
uses the diodes D
1
and D
2
. In general, diodes have a characteristic that the forward voltage Vd tends to vary with temperature. As seen from Formulae (1) and (2), the formula representing the output signal Vout includes the forward voltage Vd. Therefore, errors indicated by hatching in
FIG. 6
occur in the ranges of Vs≧V
1
+Vd and Vs≦V
2
−Vd because the diode forward voltage Vd varies being affected by a temperature variation.
Further, since the voltages of the change points are shifted from the respective reference voltages V
1
and V
2
by the diode forward voltage Vd, designing should take the forward voltage Vd into consideration and hence is complicated.
On the other hand, in the other function circuit of
FIG. 7
, in general, since a base current Ib
2
flowing through the transistor Q
2
and a base current Ib
1
flowing through the transistor Q
1
are different from each other in magnitude, the base-emitter voltage Vbe
2
of the transistor Q
2
and the base-emitter voltage Vbe
1
of the transistor Q
1
may be different from each other in magnitude; a relationship Vbe
1
−Vbe
2
=0 does not necessarily hold. That is, the two base-emitter voltages Vbe may not cancel out each other sufficiently. As a result, as hatched in
FIG. 8
, influences of variations in the transistor base-emitter voltages Vbe due to a temperature variation tend to arise in the ranges of Vs≦V
1
and Vs≧V
2
though in a lower degree than in the function circuit of FIG.
5
.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems, and an object of the invention is therefore to provide a function circuit that is less prone to be affected by temperature.
The invention provides a function circuit for converting an input signal by a prescribed function, comprising a first transistor; a second transistor; voltage dividing means connected to the first transistor, for dividing the input signal with a prescribed division ratio; a reference voltage source for applying a prescribed reference voltage to a base terminal of the second transistor; and a current mirror circuit that is connected to the first transistor and the second transistor so that the same constant current flows between a collector terminal and an emitter terminal of the first transistor and between those of the second transistor.
For example, a first function circuit is such that the first transistor is a pnp transistor and the second transistor is an npn transistor.
A second function circuit is such that the first transistor is an npn transistor and the second transistor is a pnp transistor.
A function circuit may be formed by using at least one pair of the first function circuit and the second function circuit, at least one first function circuit, or at least one second function circuit.
According to the invention, the use of the current mirror circuit makes it possible to allow the same base current to flow through the paired npn transistor and pnp transistor. Therefore, their base-emitter voltages Vbe can be made identical and can cancel out each other sufficiently even with a temperature variation. As a result, the function circuit is not affected by temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a circuit diagram of a function circuit according to the invention;
FIG. 2
shows an input/output characteristic of the function circuit of
FIG. 1
;
FIG. 3
is a circuit diagram of a combination of function circuits shown in
FIG. 1
;
FIG. 4
shows an input/output characteristic of the function circuit of
FIG. 3
;
FIG. 5
is a circuit diagram of a conventional function circuit;
FIG. 6
shows an input/output characteristic of the circuit of
FIG. 5
;
FIG. 7
is a circuit diagram of another conventional function circuit; and
FIG. 8
shows an input/output characteristic of the function circuit of FIG.
7
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be hereinafter described with reference to the drawings.
FIG. 1
is a circuit diagram of a function circuit
30
according to the invention.
FIG. 2
shows an input/output characteristic of the function circuit of FIG.
1
.
The function circuit
30
of
FIG. 1
is mainly composed of a first circuit
31
and a second circuit
32
.
The first circuit
31
is composed of transistors Tr
2
and Tr
3
that constitute a current mirror circuit K
1
, an npn transistor Tr
1
that is provided on the input side of the current mirror circuit K
1
, a pnp transistor Tr
4
that is provided on the output side of the current mirror circuit K
1
and serves as an active load, a resistor R
3
that is connected to the emitter terminal of the transistor Tr
1
, and a reference supply voltage V
1
that is applied to the base terminal of the transistor Tr
4
.
On the other hand, the second circuit
32
is composed of transistors Tr
6
and Tr
7
that constitute a current mirror circuit K
2
, a pnp transistor Tr
5
that is provided on the input side of the current mirror circuit K
2
, an npn transistor Tr
8
that is provided on the output side of the current mirror circuit K
2
and serves as an active load, a resistor R
2
that is connected to the emitter terminal of the transistor Tr
5
, and a reference supply voltage V
2
that is applied to the base terminal of the transistor Tr
8
.
The resistor R
3
of the first circuit
31
and the resistor R
2
of the second circuit
32
are connected to each other, and an input signal Vs is applied to the connecting point P
1
of the resistors R
2
and R
3
via a resistor R
1
.
The operation of the function circuit
30
will be described below. More specifically, an exemplary operation of the function circuit
30
will be described with an assumption that the supply voltage Vcc is set at 5 V and the change point reference voltages V
1
and V
2
are set at 2 V and 3 V, respectively.
(1) Vs≦V
1
Since the reference voltage V
1
=2 V is always applied to the base terminal of the transistor Tr
4
, the potential of the emitter terminal of the transistor Tr
4
and the potential of the base terminal of the transistor Tr
1
are set higher than the reference voltage V
1
by the base-emitter voltage Vbe
4
of the transistor Tr
4
. The potential of the emitter terminal of the transistor Tr
1
is set lower than the base potential of the transistor Tr
1
by the base-emitter voltage Vbe
1
of the transistor Tr
1
. Therefore, the emitter potential of the transistor Tr
1
is approximately equal to the base potential of the transistor Tr
4
.
If 1 V is input as an input signal Vs, an emitter current flows through the transistor Tr
1
via the resistors R
3
and R
1
and hence a similar constant current I
1
flows through the input side of the current mirror circuit K
1
. According to a characteristic of the current mirror circuit K
1
, if the constant current I
1
flows through the input side, a constant current I
2
that is the same in magnitude as the constant current I
1
flows through the output side, that is, through the transistors Tr
3
and Tr
4
(I
1
=I
2
). Since I
1
=I
2
, a base current Ib
4
of the transistor Tr
4
and a base current Ib
1
of the transistor Tr
1
are set identical (Ib
1
=Ib
4
). Therefore, the base-emitter voltage Vbe
4
of the transistor Tr
4
and the base-emitter voltage Vbe
1
of the transistor Tr
1
can be set identical (Vbe
1
=Vbe
4
). Since the base-emitter voltage Vbe
1
of the transistor Tr
1
can sufficiently cancel out the base-emitter voltage Vbe
4
of the transistor Tr
4
, the potential of the emitter terminal of the transistor Tr
1
can be made equal to the base potential of the transistor Tr
4
.
Even if a temperature variation has occurred, a variation in the base current Ib
4
of the transistor Tr
4
and a variation in the base current Ib
1
of the transistor Tr
1
can be made approximately equal to each other and hence the relationship Vbe
4
=Vbe
1
can be maintained. Since Vbe
1
and Vbe
4
can cancel out each other sufficiently without being affected by temperature, the emitter potential of the transistor Tr
1
can always be made equal to the base potential of the transistor Tr
4
.
The output signal Vout of this function circuit
30
is given by the following Formula (7):
When Vs≦V
1
,
Vout={R
1
/(
R
1
+
R
3
)}(
V
1
−
Vs
)+
Vs
(7)
For example, if R
1
=R
3
, Vs=1 V, and V
1
=2 V, the output voltage Vout of the function circuit
30
becomes equal to 1.5 V as indicated by point α
1
in the graph of FIG.
2
.
In this state, in the second circuit
32
, the transistor Tr
5
is off, that is, in a high-impedance state. Therefore, the second, circuit
32
does not cause any influences on the output signal Vout of the function circuit
30
.
(2) Vs≧V
2
As shown in
FIG. 1
, the transistors Tr
5
and Tr
8
of the second circuit
32
are a pnp transistor and an npn transistor, respectively.
Since the reference supply voltage V
2
=3 V is always applied to the base terminal of the transistor Tr
8
, the transistor Tr
8
is always on. Therefore, the potential of the emitter terminal of the transistor Tr
8
and the potential of the base terminal of the transistor Tr
5
are set lower than the base potential of the transistor Tr
8
by the base-emitter voltage Vbe
8
of the transistor Tr
8
. The potential of the emitter terminal of the transistor Tr
5
is set higher than the base potential of the transistor Tr
5
by the base-emitter voltage Vbe
5
pof the transistor Tr
5
. Therefore, the emitter potential of the transistor Tr
5
is set approximately equal to the base potential (3 V) of the transistor Tr
8
.
If an input signal Vs (≧V
2
) is applied, a current
13
flows through the collector terminal of the transistor Tr
5
via the resistors R
1
and R
2
and hence a similar current
13
flows through the input side of the current mirror circuit K
2
. Therefore, a constant current
14
that is the same in magnitude as the constant current
13
flows through the output side of the current mirror circuit K
2
, that is, through the transistors Tr
8
and Tr
7
(I
3
=I
4
) Since I
3
=I
4
, a base current Ib
8
of the transistor Tr
8
and a base current Ib
5
of the transistor Tr
5
are set identical (Ib
8
=Ib
5
). Therefore, the base-emitter voltage Vbe
5
of the transistor Tr
5
and the base-emitter voltage Vbe
8
of the transistor Tr
8
can be set identical (Vbe
5
=Vbe
8
). Since the base-emitter voltage Vbe
8
of the npn transistor Tr
8
can sufficiently cancel out the base-emitter voltage Vbe
5
of the pnp transistor Tr
5
, the potential of the emitter terminal of the transistor Tr
5
can be made equal to the base potential of the transistor Tr
8
.
Even if a temperature variation has occurred, a variation in the base current Ib
8
of the transistor Tr
8
and a variation in the base current Ib
5
of the transistor Tr
5
can be made approximately equal to each other and hence the relationship Vbe
8
=Vbe
5
can be maintained. Since Vbe
8
and Vbe
5
can cancel out each other sufficiently without being affected by temperature, the emitter potential of the transistor Tr
5
can always be made equal to the base potential of the transistor Tr
8
.
The output signal Vout of this function circuit
30
is given by the following Formula (8):
When Vs≧V
2
,
Vout={R
2
/(
R
1
+
R
2
)}(
Vs−V
2
)+
V
2
(8)
For example, if R
1
=R
2
, Vs=4 V, and V
2
=3 V, the output voltage Vout of the function circuit
30
becomes equal to 3.5 V as indicated by point β
1
in the graph of FIG.
2
.
In this state, in the first circuit
31
, the transistor Tr
1
is off, that is, in a high-impedance state. Therefore, the first circuit
31
does not cause any influences on the output signal Vout of the function circuit
30
.
(3) V
1
<Vs<V
2
In this case, both of the transistor Tr
1
of the first circuit
31
and the transistor Tr
5
of the second circuit
32
are set off, that is, rendered in a high-impedance state, and hence the input signal Vs becomes the output signal Vout of the function circuit
30
as it is (Vout=Vs).
In the function circuit
30
, an input signal Vs that is in the range between the reference voltages V
1
and V
2
can be output as it is (Vout=Vs). By setting the reference voltages V
1
and V
2
, in the ranges of Vs≦V
1
and Vs≧V
2
, output signals Vout that satisfy Formulae (7) and (8) can be generated.
Further, in the ranges of Vs≦V
1
and Vs≧V
2
, the slopes of the straight lines of Formulae (7) and (8) can easily be set in accordance with the ratio among the resistances R
1
, R
2
, and R
3
.
Since the transistor base-emitter voltages Vbe can cancel out each other sufficiently, no influences are caused by variations in the diode forward voltages Vd or the transistor base-emitter voltages Vbe due to a temperature variation.
FIG. 3
is a circuit diagram of a function circuit
40
that is a combination of function circuits shown in FIG.
1
.
FIG. 4
is an input/output characteristic of the function circuit
40
of FIG.
3
.
The function circuit
40
of
FIG. 3
is such that two circuits each being the main circuit of the function circuit
30
shown in
FIG. 1
, except for the voltage source circuit, are connected to each other. More specifically, a third circuit
41
that is the same as the first circuit
31
and a fourth circuit
42
that is the same as the second circuit
32
are connected to the function circuit
30
. However, reference voltages V
3
and V
4
of the third circuit
41
and the fourth circuit
42
are different from the reference voltages V
1
and V
2
of the first circuit
31
and the second circuit
32
, respectively. For example, the reference voltages V
3
and V
4
are set at 1 V and 4 V, respectively.
The resistance division ratios R
2
/(R
1
+R
2
) and R
1
/(R
1
+R
3
) are set at prescribed values.
As shown in
FIG. 4
, in this function circuit
40
, change points a
2
and b
2
can be set at Vs=1 V and Vs=4 V in addition to the change points a1 and b1 that are located at Vs=2 V and Vs=3 V, respectively. This makes it possible to obtain a desired function.
The number of change points can be increased by combining a plurality of circuits each being the main part of the function circuit
30
of
FIG. 1
in the above-described manner. An arbitrary function circuit can be obtained by connecting linear functions at those change points.
Although the above function circuits employ the first circuit and the second circuit in the form of a pair, the invention is not limited to such a case. Only a plurality of first circuits or only a plurality of second circuits may be combined together. Even in the case of combining first circuits and second circuits, the first circuits and the second circuits need not be used in the same number. Desired function circuits can be formed by combining first circuits and second circuits in various manners.
As described above, according to the invention, an input signal can be converted into an output signal by a desired function circuit without being affected by temperature.
Claims
- 1. A function circuit for converting an input signal by a prescribed function, comprising:a first transistor; a second transistor; voltage dividing means connected to the first transistor, for dividing the input signal with a prescribed division ratio; a reference voltage source for applying a prescribed reference voltage to a base terminal of the second transistor; and a current mirror circuit that is connected to the first transistor and the second transistor so that the same constant current flows between a collector terminal and an emitter terminal of the first transistor and between those of the second transistor, wherein the first transistor is a pnp transistor and the second transistor is an npn transistor.
- 2. A function circuit for converting an input signal by a prescribed function, comprising:a first transistor; a second transistor; voltage dividing means connected to the first transistor, for dividing the input signal with a prescribed division ratio; a reference voltage source for applying a prescribed reference voltage to a base terminal of the second transistor; and a current mirror circuit that is connected to the first transistor and the second transistor so that the same constant current flows between a collector terminal and an emitter terminal of the first transistor and between those of the second transistor, wherein the first transistor is an npn transistor and the second transistor is a pnp transistor.
- 3. A function circuit for converting an input signal by a prescribed function, including at least one pair of a first function circuit and a second function circuit each comprising:a first transistor; a second transistor; voltage dividing means connected to the first transistor, for dividing the input signal with a prescribed division ratio; a reference voltage source for applying a prescribed reference voltage to a base terminal of the second transistor; and a current mirror circuit that is connected to the first transistor and the second transistor so that the same constant current flows between a collector terminal and an emitter terminal of the first transistor and between those of the second transistor, wherein the first transistor of the first function circuit is a pnp transistor, the second transistor of the first function circuit is an npn transistor, the first transistor of the second function circuit is an npn transistor and the second transistor of the second function circuit is a pnp transistor.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 2001-324765 |
Oct 2001 |
JP |
|
US Referenced Citations (4)
Foreign Referenced Citations (1)
| Number |
Date |
Country |
| 8-202463 |
Aug 1996 |
JP |
