Rail-to-Rail Input Circuit With Current Steering for Constant Gm
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Solution Overview
Problem
Existing rail-to-rail input circuits for amplifiers and comparators require complex and power-hungry control circuitry to maintain constant transconductance across the entire rail-to-rail common-mode range, making them expensive and inefficient.
Innovation Solution
A rail-to-rail input circuit using two differential pairs with different threshold voltages, where a simple control circuit steers bias current between the pairs based on common-mode voltage, ensuring constant transconductance across the range, and sharing a common tail current source to reduce power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If two or more complementary differential pairs are used to handle rail-to-rail input common mode range, then the input common mode range is extended to cover the entire rail-to-rail voltage swing, but the device complexity increases due to the need for multiple differential pairs and control circuitry
Solution Approach 1:
The patent uses differential pairs with different threshold voltage parameters (e.g., NMOS with negative threshold voltage and PMOS with positive threshold voltage) to enable each pair to operate effectively in different portions of the rail-to-rail common mode range. This parameter variation allows the circuit to achieve full rail-to-rail coverage without requiring complex switching control circuitry, as the threshold voltage differences naturally cause the differential pairs to activate in appropriate voltage ranges.
2Reliability
If complicated control circuitry is used to maintain constant transconductance across rail-to-rail range, then the transconductance remains constant, but the power consumption increases and the circuit becomes more expensive to design and manufacture
Solution Approach 1:
The patent employs a self-service mechanism where the differential pairs with different threshold voltages automatically adjust their conduction states based on the input common mode voltage level. The circuit inherently maintains constant transconductance through the natural characteristics of the depletion-mode and enhancement-mode devices, eliminating the need for external control circuitry that would consume additional power. The system serves itself by utilizing the inherent properties of the transistors to achieve the desired performance.
3Adaptability or versatility
If multiple differential pairs are used to cover rail-to-rail range, then the input common mode range is extended, but the manufacturing cost increases due to more components and complex assembly
Solution Approach 1:
The patent merges the functionality of multiple differential pairs into a unified circuit architecture where depletion-mode and enhancement-mode differential pairs operate in parallel. By combining these pairs with different threshold voltage characteristics, the circuit achieves rail-to-rail common mode range coverage while maintaining a compact structure that is easier to manufacture. The merging of functions reduces the need for separate control circuits and minimizes the overall component count.
Data Source
AI summary
A power-efficient, rail-to-rail input circuit includes two differential pairs, one having devices of a first threshold voltage and one having devices of a second, different threshold voltage. In various embodiments, the two differential pairs receive a differential input in parallel and are supplied a tail current, which a control circuit steers between the pairs in accordance with a common-mode level of the input.