Analog comparator
AssigneeAmerican Telephone and Telegraph Company
InventorsJerrell P. Hein, Thayamkulangara R. Viswanathan
Life of this patent
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Abstract
A novel analog comparator having cascaded gain stages powered by two buses, the voltages on which are dependent on a reference input voltage. A network, responsive to the reference input voltage, sets the voltages on the buses and isolates the buses from external power and ground to achieve high power supply and ground noise immunity. An alternative design of the network is provided which removes errors in the accuracy of the comparator resulting from differing drain-to-source voltages across the various transistors. The accuracy of the comparator is then dependent on the accuracy of matching predetermined ratios of the sizes of the transistors.
Claims17
An analog comparator formed on an integrated circuit for comparing first voltage input to a second voltage input characterized by:
- at least one cascadable gain stage, the stage having an input, an output and first and second power terminals, the corresponding power terminals coupling to first and second buses, the input of the cascadable gain stage coupling to the first voltage input of the comparator;
- a first transistor of a first polarity type having two output terminals and an input terminal, corresponding output terminals coupling to the first bus and directly to a first voltage source, and the input terminal coupling to the second voltage input;
- a second transistor of a second polarity type having two output terminals and an input terminal, corresponding output terminals coupling to the second bus and directly to a second voltage source, the input terminal coupling to the second voltage input;
- a first current source for supplying current to the first bus;
- and,
- a second current source for supplying current to the second bus, the amount of current being substantially equal to the current supplied by the first current source;
- wherein the voltage of the first voltage source is different from the voltage of the second voltage source.
An analog comparator in an integrated circuit for comparing a first voltage input to a second voltage input characterized by:
- at least one cascadable gain stage, the stage having an input, an output and first and second power terminals, the corresponding power terminals being coupling to first and second buses, the input of the first of the cascaded gain stages coupling to the first voltage input;
- a first transistor of a first polarity type having two output terminals and an input terminal, corresponding output terminals coupling to the first bus and to a node, the input terminal coupling to the second voltage input;
- a second transistor of a second polarity type having two output terminals and an input terminal, corresponding output terminals coupling between the second bus and the node, the input terminal coupling to the second voltage input;
- a current source coupling to the first bus;
- a subtractor having two inputs and an output, corresponding inputs coupling to the node and the second voltage input;
- an integrator responsive to the output of the subtractor;
- and,
- a variable current source, responsive to the integrator, for sinking current from the second bus;
- wherein the current supplied by the variable current source varies in response to the integrator until the voltage on the node is substantially the same as the voltage on the second voltage input.
A method for comparing first and second voltages, comprising the steps of:
- biasing first and second buses to predetermined voltages substantially dependent only on the first voltage;
- and,
- amplifying the second voltage with at least one cascadable gain stage powered by the first and second buses;
- wherein the amplified second voltage indicates whether the second voltage is greater than or less than the first voltage.
Dependencies are inferred from the claim text.