Comparator

Details Comparator Comparator

2002-09-16

2004-06-08

Nguyen, Long (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Specific signal discriminating without subsequent control

By amplitude

C327S066000

Reexamination Certificate

active

06747486

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a comparator for generating an output signal.
FIG. 1
shows a comparator, which is composed of two circuit branches that are connected to one another. The first circuit branch (designated by reference symbol SZ
1
in
FIG. 1
) contains a first pnp-type transistor T
1
and a first power source IQ
1
. A first voltage V
1
, which is one of voltages to be compared with one another, is supplied to the emitter of the transistor T
1
. The base and the collector of the transistor T
1
are connected to ground via the power source IQ
1
.
The second circuit branch, which is designated by reference symbol SZ
2
in
FIG. 1
, contains a first pnp-type transistor T
2
and a first power source IQ
2
. A second voltage V
2
, which is the other of the voltages to be compared with, is applied to the emitter of the transistor T
2
. The collector of the transistor T
2
is connected to ground via the power source IQ
2
, and to an output terminal O via which a signal representing the result of the comparison of the voltages VI and V
2
is output. The base of the first transistor T
1
and the base of the second transistor T
2
are connected to one another.
A current I
1
, which is determined by the power source IQ
1
, flows through the first circuit branch SZ
1
. The potential which depends on the size of the voltage V
1
is set at the base terminal of the transistor T
1
. A current I
2
, the size of which depends, on the difference between the voltages V
1
and V
2
, to be more precise, on the voltage V
2
and the base potential of the transistor T
2
(that is dependent on the voltage V
1
), flows through the second circuit branch SZ
2
. A signal which can be tapped at the output terminal O depends on the the size of the current I
2
.
Such comparators are known, and further details of the same will not be described hereinafter.
The transistors T
1
and T
2
and the power sources IQ
1
and IQ
2
usually have the same dimensions. As a result, V
1
=V
2
in the state of equilibrium (at the switch-over time) of the comparator. Otherwise, if, due to different dimensioning of the transistors and/or of the power sources, different base-emitter voltages are set at the transistors T
1
and T
2
in the state of equilibrium of the comparator, the applicable equation would be V
1
=V
2
±Voff in the state of equilibrium of the comparator. Voff is the known offset voltage.
The occurrence of an offset voltage is generally undesirable. At times, in particular if the comparator is to have a hysteresis, or if one of the voltages to be compared is to be a reference voltage which is not available in the configuration containing the comparator, it may be advantageous if an offset voltage is present, but such an offset voltage should be able to be set precisely and permanently to a specific value, and should preferably be variable within a relatively large range.
It would be possible to generate an offset voltage in the comparator shown in
FIG. 1
by dimensioning the transistors T
1
and T
2
and/or the power sources IQ
1
and IQ
2
differently. The offset voltage Voff which is set as a result is
Voff
=
kT
q
⁢
ln
⁡
(
I1
I2
)
However, a disadvantage with this is that the offset voltage Voff is highly temperature-dependent. In addition, only small offset voltages (up to several mV) can be implemented. Otherwise, the ratio of I
1
to I
2
becomes too extreme.
An offset voltage generated in this way can be set neither precisely nor permanently, nor can it vary within a large range.
Another possible way of generating an offset voltage is illustrated in FIG.
2
.
The configuration shown in
FIG. 2
substantially corresponds to the comparator shown in FIG.
1
. Elements which are designated with the same references refer to identical or corresponding elements.
However, the comparator of
FIG. 2
additionally has a resistor R, which is disposed between the emitter of the second transistor T
2
and the input terminal for the second voltage V
2
.
The offset voltage Voff, which is generated by the comparator is:
Voff=R·I
2
(for I
1
=I
2
), or
Voff
=
R
·
I2
+
kT
q
⁢
ln
⁡
(
I1
I2
)
⁢
 
⁢
(
for
⁢
 
⁢
I1
≠
I2
)
.
A disadvantage with this is that the resistor R limits the current with which a capacitive load connected to the output terminal O can be recharged. The consequence is that the signal which is output via the output terminal O reacts relatively slowly to changes in V
1
and/or V
2
. In other words, the comparator has a relatively slow switching behavior.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a comparator configuration that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, and to provide a comparator whose offset voltage can be set precisely and permanently to a value which can vary within a large range. The offset voltage of the comparator does not lead to the other properties of the comparator to be degraded. In particular, it does not lead to a slower reaction to changes in the input voltages.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a comparator configuration containing a comparator having a first control device, a second control device, a resistor, a first power source and a second power source. The first control device has a first terminal, a second terminal, and a third terminal. The first terminal is used to input the first voltage. The second terminal is connected to the third terminal, and (via the resistor) to the terminal of the second control device. The third terminal is connected to the first power source. A current, whose size depends on the difference between the voltages applied to the first terminal and to the second terminal, flows through the third terminal.
The second control device has a first terminal, a second terminal, a third terminal and a fourth terminal. The first terminal is used to input the second voltage. The second terminal is connected to the fourth terminal, and to the second terminal of the first control device via the resistor.
The third terminal is connected to the second power source. A current, whose size depends on the difference between the voltages applied to the first terminal and to the second terminal, flows via the third terminal. The output signal of the comparator is tapped at the third terminal. The fourth terminal is connected to the second terminal. A current, whose size depends on the difference between the voltages applied to the first terminal and to the second terminal, flows via the fourth terminal.
In such a comparator, a defined current flows through the resistor in the state of equilibrium (at the switch-over time) of the comparator. The voltage drop (which is caused as a result) at the resistor equals to the offset voltage of the comparator or to one of the voltage components from which the offset voltage is composed.
In accordance with another feature of the invention, the difference between the voltages applied to the first terminal and to the second terminal of the first control device and the difference between the voltages applied to the first terminal and to the second terminal of the second control device are preferably equally large. The voltage drop across the resistor is precisely the offset voltage of the comparator.
The offset voltage can be set precisely and permanently to any desired values by appropriately dimensioning the resistor and/or the power sources. Since the resistor which generates the offset voltages is not in the path, via which the output terminal of the comparator is supplied with current, generating the offset voltage does not have any influence on the speed with which the output voltage can change.
In accordance with a further feature of the invention, the first power source outputs a current, and the second power source outputs another current. The control devices and the power sources are constructed and dime

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