Active folded cascode
AssigneeElantec, Inc.
InventorsDouglas S. Smith, Edward C. Bee
Life of this patent
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Abstract
An active folded cascode includes an amplifier transistor and a source follower transistor configured as a folded cascode with the drain of the amplifier transistor and the source of the follower transistor connected to form a gain node. A feedback transistor has its gate and drain connected to the source and gate of the follower transistor while bias current provided to the drain of the feedback transistor by a current source maintains the gain node at a fixed voltage with respect to a reference voltage. Coupling of the voltage at the gain node to the gate of the source follower transistor by the feedback transistor reduces the effective source impedance of the source follower transistor, providing improved gain and bandwidth properties for the active folded cascode circuit.
Claims10
An active folded cascode circuit having improved impedance, gain, and bandwidth properties, the active folded cascode circuit comprising:
- first and second reference voltages;
- an amplifier transistor having first, second, and third electrodes, the first electrode forming a gain node, the second electrode forming an input of the active folded cascode circuit, and the third electrode being connected to the second reference voltage;
- a follower transistor having first, second, and third electrodes, the third electrode being connected to the gain node and the first electrode forming an output of the active folded cascode circuit;
- a feedback transistor having first, second, and third electrodes, the first and second electrodes being connected to the second and third electrodes, respectively, of the follower transistor, and the third electrode being connected to the first reference voltage;
- a first current source connected to the first electrode of the amplifier transistor, for providing bias current to the amplifier transistor and output current to the follower transistor, according to a signal applied to the input of the active folded cascode circuit;
- and
- a second current source connected to the first electrode of the feedback transistor to provide bias current to the feedback transistor;
- the biased feedback transistor maintaining the third electrode of the follower circuit at a fixed voltage relative to the first reference voltage and coupling the voltage at the third electrode of the follower transistor to the second electrode of the follower transistor to reduce the impedance of the gain node of the active folded cascode circuit.
Dependencies are inferred from the claim text.