Amplifier offset cancellation using current copier

A differential amplifier circuit for driving a load and incorporating offset cancellation, said differential amplifier comprising in combination:

• a. a differential amplifier having a positive input terminal and a negative input terminal for receiving an input differential voltage, said differential amplifier including an output terminal for producing an output current for driving the load, the output current being derived from the input differential voltage;
• b. a first switch for selectively shorting together the positive input terminal and negative input terminal of said differential amplifier to null any input differential voltage thereacross;
• c. a second switch for selectively coupling the output terminal of said differential amplifier to the negative input terminal of said differential amplifier for closed-loop operation, or selectively decoupling the output terminal of said differential amplifier from the negative input terminal of said differential amplifier for open-loop operation;
• d. a current copier circuit, said current copier circuit having a first terminal for setting and maintaining a voltage, said current copier circuit also including a second terminal coupled to the output terminal of said differential amplifier for supplying an offset current having a magnitude based upon the voltage at the first terminal of said current copier circuit, said current copier circuit also including a storage for storing the voltage at the first terminal thereof;
• e. a third switch for selectively coupling the first terminal of said current copier circuit to the output terminal of said differential amplifier;
• and
• f. a control circuit for selecting between a cancellation mode and an operating mode of said differential amplifier, said control circuit causing, during the cancellation mode, the first switch to be closed, the second switch to be opened, and the third switch to be closed, for allowing said current copier circuit to sense any output current offset at the output terminal of said differential amplifier with a null input differential voltage during open-loop operation, and to supply an equal and opposite offset current to the output terminal of said differential amplifier, said control circuit causing, during the operating mode, the first switch to be opened, the second switch to be closed, and the third switch to be opened, for allowing said differential amplifier to respond to the input differential voltage while continuing to supply the offset current to the output terminal of said differential amplifier to cancel any output current offset inherent in said differential amplifier.