Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
2000-05-31
2001-07-10
Wells, Kenneth B. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S077000, C327S091000, C327S093000, C327S095000
Reexamination Certificate
active
06259296
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a voltage comparator, and more particularly to an analog voltage comparator having an input voltage offset cancel function.
BACKGROUND OF THE INVENTION
Since voltage comparators generally offset the input voltage, it is necessary to correct or eliminate this offset voltage in order to prevent an erroneous operation of the circuit.
FIG. 1
illustrates an example of a conventional comparator utilizing a single operational amplifier (to be referred to hereinafter as “Op Amp” or “OA”, in short) having the aforementioned offset voltage cancel function. In the example shown, the offset voltage is corrected (i.e., eliminated) by providing a capacitor Cl between the negative input Op Amp (OA)
1
and the input select switch S
1
, and further by providing a feedback circuit using a switch S
2
placed between the negative input and output (Vout) of OA
1
.
FIG. 2
illustrates a second conventional voltage comparator circuit, wherein the comparator of
FIG. 1
is now structured as a voltage comparator having two OAs operating in opposite phases to secure a rapid operation. The voltage comparator shown in
FIG. 2
uses two Op Amps OA
1
and OA
2
operating in opposite phases, and a third one, i.e., output OA
3
, connected to the two inputs OAs such that the respective outputs of the two OAs become the inputs of OA
3
. A switch S
1
is connected to the negative input of OA
1
, while the positive input of OA
2
is coupled to capacitor C
1
. Switch S
1
switches between input voltage Vi and a reference voltage Vref, alternatively supplying these voltages to the negative input and positive input of the OAs in question. Additionally, a feedback circuit consisting of switch S
2
placed between the negative input and the positive output (Vout) of OA
1
is also provided. Moreover, a voltage Vm is supplied to the positive input of OA
1
and to the negative input of OA
2
.
The example shown in
FIG. 2
has a shortcoming of being unable to cancel the offset voltage of OA
2
, due to the circuit configuration, since feedback is provided only to one side of OA
1
. In order to avoid this limitation, it is necessary to provide OAs such that the offset between two OAs
1
and
2
becomes uniform. However, it is difficult and unrealistic to obtain a totally uniform pair of OAs using standard manufacturing semiconductor processes. Accordingly, a limitation exists in suppressing the offset voltage of the Op Amps when they depend on the manufacturing process used.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide an analog voltage comparator capable of suppressing the input voltage offset.
It is another object to provide a comparator that achieves the primary object of the invention by way of two input operational amplifiers (OAs) operating in opposite phases.
It is still an object of the present invention to have the aforementioned circuit suppress the input voltage offset independently of the semiconductor manufacturing process used.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a voltage comparator that includes: two input operational amplifiers operating in opposing phases; and an output operational amplifier wherein outputs of the two input operational amplifiers are connected to the inputs of the output operational amplifier, the circuit being characterized by having an input voltage offset suppression circuit placed between opposite phase inputs of the two input operational amplifiers and inputs of the output operational amplifier.
The input voltage offset cancel circuit of the present invention is characterized by including a capacitor placed between opposite phase inputs of the two input operational amplifiers, and a switch between the positive input of the output operational amplifier and the capacitor.
In another aspect of the invention, there is provided a voltage comparator that includes: a first comparator and a second comparator operating in opposite phases; a third comparator coupled to the two input comparators; a first switch coupled to the negative input of the first comparator and to the positive input of the second comparator by way of a first capacitor, alternating between an input voltage and a reference voltage and supplying these voltages to a negative input and to a positive input the two input comparators; a second capacitor placed between the positive input of the first comparator and the negative input of the second comparator; a second switch positioned between the negative input and the output of the first comparator; and, finally, a third switch placed between the negative input and the output of the second comparator.
REFERENCES:
patent: 4829263 (1989-05-01), Gulczyski
patent: 5355034 (1994-10-01), Nakamura et al.
patent: 6064239 (2000-05-01), Matsuoka
Dinh Paul
International Business Machines - Corporation
Schnurmann H. Daniel
Wells Kenneth B.
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