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US 7,532,072 B1Granted
Method to control the output common mode in a differential OPAMP with rail-to-rail input stage
AssigneeLinear Technology Corporation
InventorsMaziar Tavakoli Dastjerdi, Kristiaan B. Lokere
Life of this patent
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Abstract
A differential amplifier with a rail-to-rail input stage includes a feedback circuit that maintains the output common mode voltage of the amplifier within a narrow range of an externally supplied value. To reduce harmonic distortions and to further stabilize the output common mode voltage of the amplifier, current is controllably drawn from a biasing circuit coupled to an intermediate stage of the amplifier. Optionally, to reduce harmonic distortions and to further stabilize the output common mode voltage of the amplifier, current is controllably supplied to the intermediate stage of the amplifier.
Claims23
A differential amplifier comprising:
- an input stage, said input stage comprising:
- a first differential amplification stage biased by a first current source and adapted to generate a first pair of intermediate differential signals in response to a pair of differential input signals;
- and
- a second differential amplification stage biased by a second current source and adapted to generate a second pair of intermediate differential signals in response to the pair of differential input signals;
- a second stage adapted to generate a third pair of intermediate differential signals between first and second nodes responsive to the first and second pairs of intermediate differential signals;
- an output stage adapted to supply a pair of differential output signal responsive to the third pair of intermediate differential signals;
- and
- a feedback circuit adapted to supply a feedback signal to define a common mode voltage of the pair of differential output signals in accordance with an external value, wherein
- said second stage comprises:
- first and second transistors responsive to the first pair of intermediate differential signals;
- and
- third and fourth transistors responsive to the second pair of intermediate differential signals;
- said first and third transistors being coupled to the first node and adapted to pass a substantially same first current;
- said second and fourth transistors being coupled to the second node and adapted to pass a substantially same second current,
- each of said first and second transistors forms a cascode amplifier with a different one of the transistors disposed in the first differential amplification stage, and
- each of said third and fourth transistors forms a cascode amplifier with a different one of the transistors disposed in the second differential amplification stage,
- the differential amplifier further comprising a first resistive load disposed between a first supply voltage and the first transistor, and a second resistive load disposed between the first supply voltage and the second transistor.
14 more elements
A method of controlling an output common mode voltage of a differential amplifiers the method comprising:
- generating first and second currents;
- generating a first voltage in response to first portions of the first and second currents;
- generating a second voltage in response to second portions of the first and second currents;
- generating a third voltage defining a first one of a pair of differential output signals in response to the first voltage;
- generating a fourth voltage defining a second one of a pair of differential output signals in response to the second voltage;
- generating a feedback signal adapted to maintain a common mode voltage of the pair of differential output signals in accordance with an externally defined value;
- and
- varying the first and second currents in response to a control signal such that a sum of the first and second currents remains substantially unchanged.
5 more elements
A differential amplifier comprising:
- an input stage, said input stage comprising:
- a first differential amplification stage biased by a first current source and adapted to generate a first pair of intermediate differential signals in response to a pair of differential input signals;
- and
- a second differential amplification stage biased by a second current source and adapted to generate a second pair of intermediate differential signals in response to the pair of differential input signals;
- a second stage adapted to generate a third pair of intermediate differential signals between first and second nodes responsive to the first and second pairs of intermediate differential signals;
- an output stage adapted to supply a pair of differential output signal responsive to the third pair of intermediate differential signals;
- and
- a feedback circuit adapted to supply a feedback signal to define a common mode voltage of the pair of differential output signals in accordance with an external value, wherein
- said output stage comprises:
- a first amplifier having a negative gain and generating a first one of the pair of differential output signals at its output terminal in response to the voltage of the first node;
- and
- a second amplifier having a negative gain and generating a second one of the pair of differential output signals at its output terminal in response to the voltage of the second node.
9 more elements
A differential amplifier comprising:
- an input stage, said input stage comprising:
- a first differential amplification stage biased by a first current source and adapted to generate a first pair of intermediate differential signals in response to a pair of differential input signals;
- and
- a second differential amplification stage biased by a second current source and adapted to generate a second pair of intermediate differential signals in response to the pair of differential input signals;
- a second stage adapted to generate a third pair of intermediate differential signals between first and second nodes responsive to the first and second pairs of intermediate differential signals;
- an output stage adapted to supply a pair of differential output signal responsive to the third pair of intermediate differential signals;
- and
- a feedback circuit adapted to supply a feedback signal to define a common mode voltage of the pair of differential output signals in accordance with an external value, wherein
- said first and second current sources are variable current sources, the differential amplifier further comprising:
- a sensing block adapted to generate a control signal operative to vary the currents flowing through the first and second current sources in response to variations in the common mode of the differential input signals such that a sum of the currents flowing through the first and second current sources remains substantially constant.
7 more elements
Dependencies are inferred from the claim text.