Rail-to-rail input stage
AssigneeTexas Instruments Incorporated
InventorMarco Corsi
Life of this patent
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Abstract
A rail-to-rail input stage (20) for an operational amplifier having a constant transconductance (Gm) over a common mode range. The input stage has a cross-coupled quad circuit (Q9, Q10, Q15, Q16) having an essentially infinite transconductance, and pair of transistors (Q5, 6) running at the same current as input transistors (Q1, Q2) when active, whereby the pair of transistors (Q5, Q6) establish a constant transconductance of the input stage (20).
Claims8
A rail-to-rail input stage forming a portion of an operational amplifier having a first rail and a second rail, comprising:
- a first pair of transistors adapted to receive a first differential input signal and coupled between the first rail and the second rail;
- a cross-coupled quad adapted to receive a second differential input signal and coupled to the first rail;
- a second pair of transistors coupled between said cross-coupled quad and the second rail such that a transconductance of the input stage is primarily established by said second pair of transistors;
- and
- a current mirror adapted to establish a first current conducting through said first pair of transistors being substantially the same as a second current conducting through said second pair of transistors.
A method of operating a rail-to-rail input stage for an operational amplifier, comprising the step of:
- providing a cross-coupled quad in the input stage having essentially infinite transconductance and being coupled to a pair of transistors that establish a transconductance of the input stage, wherein said cross-coupled quad is operated such that the transconductance of the input stage is constant over a common mode range;
- and
- providing a current mirror in the input stage to establish a first current through said cross-coupled quad that is substantially identical to a second current conducting through said pair of transistors.
Dependencies are inferred from the claim text.