Comparator circuit

Details Comparator circuit Comparator circuit

2003-08-22

2004-10-12

Nguyen, Long (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S065000, C327S563000, C330S260000

Reexamination Certificate

active

06803795

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application incorporates by reference the subject matter of Application No. 2002-243270 filed in Japan on Aug. 23, 2002, on which a priority claim is based under 35 U.S.C. §119(a).
FIELD OF THE INVENTION
The present invention relates to a comparator circuit, and in particular to a comparator circuit having both current output and voltage output and suitable for a monolithic integrated circuit.
BACKGROUND OF THE INVENTION
A voltage output type comparator is disclosed, for example, in Japanese Laid-Open Patent Application Tokukai 2000-188517A (published on Jul. 4, 2000), or in Japanese Laid-Open Patent Application Tokukai 2001-189633A (published on Jul. 10, 2001).
FIG. 2
shows a conventional comparator circuit
11
having both current output and voltage output. The comparator circuit
11
includes a differential amplifier
12
, transistors Q
13
and Q
14
, and constant current sources CS
12
and CS
13
. The differential amplifier
12
is made up of transistors Q
11
and Q
12
, resistors R
15
and R
16
, and a constant current source CS
11
. The comparator circuit
11
further includes an emitter follower circuit
13
as a positive feedback circuit for applying feedback to each base of the transistors Q
11
and Q
12
of the differential amplifier
12
and for outputting an output voltage of the comparator circuit
11
, and means for supplying an input voltage of the comparator circuit
11
to the respective junctions between each base of the transistors Q
11
and Q
12
of the differential amplifier
12
and resistors R
13
and R
14
via the resistors R
11
and R
12
.
Also, for obtaining current output, the comparator circuit
11
further includes a current switch circuit
20
made up of the transistors Q
15
and Q
16
, and a constant current source CS
14
. The output voltage of the comparator
11
is supplied to each base of the transistors Q
15
and Q
16
so that the collector currents of the transistors Q
15
and Q
16
are switched with the switching of the output voltage of the comparator circuit
11
, thus realizing current output of the comparator circuit
11
.
The following will explain operation of the comparator circuit
11
and the current switch circuit
20
. Note that, in the operation explained below, hFE of the transistor is so high that the base current does not need to be taken into account. Further, a voltage between base/emitter of the transistor Q
13
and a voltage between base/emitter of the transistor Q
14
are approximately equal.
In
FIG. 2
, v
1
and v
2
express the input voltages of the comparator circuit
11
, respectively, while v
3
express a base voltage of the transistors Q
1
, v
4
expresses a base voltage of the transistor Q
12
, v
5
expresses an emitter voltage (comparator output voltage) of the transistor Q
14
, and v
6
expresses an emitter voltage (comparator output voltage) of the transistor Q
13
. Firstly, it is assumed that a current I
11
from the constant current source CS
11
flows into the transistor Q
11
with the input condition=v
1
>v
2
. Here, the respective output voltages v
5
and v
6
are denoted by the following equations.
v
5
=
Vr−VBE
(
Q
14
)
v
6
=
Vr−R
15
×
I
11
−
VBE
(
Q
13
)
where Vr expresses a reference voltage, VBE (Q
13
) expresses the voltage between base/emitter of the transistor Q
13
. Accordingly, the relation between the comparator output voltages v
5
and v
6
are expressed as v
5
>v
6
.
Further, v
3
is supplied by dividing v
1
and v
5
by the resistors R
11
and R
13
, and v
4
is supplied by dividing v
2
and v
6
by the resistors R
12
and R
14
. Thus, v
3
and v
4
are denoted by the following equations.
v
3
=(
R
11
×
v
5
+
R
13
×
v
1
)/(
R
11
+
R
13
)
v
4
=(
R
12
×
v
6
+
R
14
×
v
2
)/(
R
12
+
R
14
)
According to the relations v
1
>v
2
and v
5
>v
6
, and when the resistors are designed to satisfy R
11
=R
12
and R
13
=R
14
, v
3
becomes greater than v
4
. Therefore, the constant current I
11
of the constant current source CS
11
flows into the transistor Q
11
, and the output voltages of the comparator circuit
11
stably maintain the relation of v
5
>v
6
.
Further, according to the relation of v
5
>v
6
, the constant current I
14
flows into the transistor Q
15
in a current switch circuit
20
as the second-stage circuit, and the output current of the comparator becomes io
1
=I
14
, io
2
=0. Thus, the output currents also stably maintain the relation of io
1
>io
2
.
Next, in the condition above, when v
1
is decreased with respect to v
2
(v
1
<v
2
), the difference between v
3
and v
4
is reduced, and when the difference between v
3
and v
4
satisfies a certain condition with the application of positive feedback by the resistors R
13
and
14
, the flow of the constant current I
11
is switched from the transistor Q
11
to the transistor Q
12
. This switching is carried out on condition that the open loop gain=1 when the positive feedback loop is removed. According to this condition, the relation between v
3
and v
4
can be denoted as follows.
v
3
−
v
4
=
VT×
ln((
I
11
/2
+A
)/(
I
11
/2
−A
))
where
A=I
11
×(
I
11
/4
−VT/R
),
R=R
11
×
R
16
/(
R
11
+
R
13
)+
R
12
×
R
15
/(
R
12
+
R
14
),
VT=K×T/q.
(K expresses Boltzmann's constant, T expresses absolute temperature, q expresses elementary charge)
When the difference between v
3
and v
4
satisfies the foregoing equation, the operation of the comparator circuit
11
is instantly reversed by the positive feedback circuit, and the constant current I
11
flows into the transistor Q
12
. Here, the output voltages of the comparator are expressed as follows.
v
5
=
Vr−I
11
×
R
16
−
VBE
(
Q
14
)
v
6
=
Vr−VBE
(
Q
13
)
Accordingly, the relation between the comparator output voltages v
5
and v
6
are denoted as v
5
<v
6
.
Further, as with the case above, the relation between v
3
and v
4
are expressed by the following equations.
v
3
=(
R
11
×
v
5
+
R
13
×
v
1
)/(
R
11
+
R
13
)
v
4
=(
R
12
×
v
6
+
R
14
×
v
2
)/(
R
12
+
R
14
)
According to the relations v
1
<v
2
and v
5
<v
6
, and when the resistors are designed to satisfy R
11
=R
12
and R
13
=R
14
, v
4
becomes greater than v
3
, and therefore, the constant current I
11
flows into the transistor Q
12
, and the output voltages of the comparator circuit
11
stably maintain the relation of v
5
<v
6
. Here, since the constant current I
14
flows into the transistor Q
16
in the current switch circuit
20
as the second-stage circuit, the output currents of the comparator become io
1
=0, io
2
=i
4
. Accordingly, the relation of io
1
<io
2
is maintained.
Next, in the condition above, when v
1
is decreased with respect to v
2
(v
1
<v
2
), the operation of the comparator
11
is reversed when the difference between v
3
and v
4
satisfies a certain condition. As with the case above, the open loop gain=1 also in this condition, and the following equation is satisfied.
v
3
−
v
4
=
VT
×ln((
I
11
/2
+A
)/(
I
11
/2
−A
))
When the difference between v
3
and v
4
satisfies the foregoing equation with the application of positive feedback, the operation of the comparator circuit
11
is instantly reversed, and the constant current I
11
flows in the transistor Q
11
, i.e., the current flow is again in the original state.
In order to realize both voltage output and current output in the comparator circuit
11
, it is necessary to additionally provide the current switch circuit
20
. In, this configuration, the circuit current increases, and provision of the current switch circuit increases the chip size.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a comparator circuit with a simpler structure, and a smaller circu

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