RAIL-TO-RAIL CONSTANT TRANSCONDUCTANCE DIFFERENTIAL INPUT STAGE
First Claim
1. A differential input apparatus, comprising:
- a tail-connected primary differential pair to receive first and second input signals and to pass load current to first and second loads responsive to the first and second input signals, respectively; and
a tail-connected secondary differential pair to receive downward level-shifted versions of the first and second input signals, at least one of the primary or secondary differential pair to conduct current as a common-mode component of the first and second input signals approaches a positive voltage rail such as to present a constant transconductance in series with each of the first and second loads.
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Accused Products
Abstract
A primary differential input pair of transistors and a secondary differential input pair of transistors are capable of operating in parallel to provide load current. A level-shifting pre-stage to the secondary differential pair downwardly level-shifts rail-to-rail input signals. Doing so prevents the secondary differential pair from entering cut-off. A tail current shunt device provides tail current to the secondary differential pair as the primary differential pair approaches cut-off when a common-mode component of the input signals approaches the positive voltage rail. Consequently, the sum of currents through first differential input transistors associated with the primary and secondary differential input pairs remains constant to the first load. Likewise, the sum of currents through the second differential input transistors associated with the primary and secondary differential input pairs remains constant to the second load. Both arms of the composite differential input stage present constant transconductances to their respective loads as a result.
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Citations
20 Claims
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1. A differential input apparatus, comprising:
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a tail-connected primary differential pair to receive first and second input signals and to pass load current to first and second loads responsive to the first and second input signals, respectively; and a tail-connected secondary differential pair to receive downward level-shifted versions of the first and second input signals, at least one of the primary or secondary differential pair to conduct current as a common-mode component of the first and second input signals approaches a positive voltage rail such as to present a constant transconductance in series with each of the first and second loads. - View Dependent Claims (2, 3, 4, 5)
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6. A differential input apparatus, comprising:
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a tail-connected primary differential pair of transistors, first and second transistors of the primary differential pair current channel coupled in series with first and second loads, respectively, to receive first and second input signals at first and second gates, respectively, of the primary differential pair of transistors and to pass load current to the first and second loads responsive to the first and second input signals; and a tail-connected secondary differential pair of transistors, first and second transistors of the secondary differential pair current channel-coupled in series with the first and second loads, respectively, to receive downward level-shifted versions of the first and second input signals at first and second gates, respectively, of the secondary differential pair of transistors, the secondary differential pair to conduct current responsive to the downward level-shifted versions of the differential input signals as the primary differential pair of transistors approaches a cut-off condition such as to present a constant transconductance to the first and second loads as a common-mode component of the first and second input signals approaches a positive voltage rail. - View Dependent Claims (7, 8, 9, 10, 11, 12)
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13. A method of providing a constant-transconductance electronic differential input stage, comprising:
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receiving a first input signal approaching a positive voltage rail at a first gate associated with a first transistor of a tail-connected primary differential pair of transistors; receiving a second input signal approaching the positive voltage rail at a second gate associated with a second transistor of the tail-connected primary differential pair of transistors; decreasing current flow from a tail current source transistor through the primary differential pair, resulting in an increased voltage potential at a tail node associated with the primary differential pair; and providing a tail current to first and second transistors associated with a secondary differential pair of transistors in order to maintain a constant first transconductance in series with a first load associated with the first transistors of the primary and secondary differential pairs and to maintain a constant second transconductance in series with a second load associated with the second transistors of the primary and secondary differential pairs as a common-mode component of the first and second input signals approaches the positive voltage rail. - View Dependent Claims (14, 15, 16, 17, 18, 19, 20)
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Specification