Resistively Folded Differential Amplifier for Zero-Volt Common Mode
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Solution Overview
Problem
Existing differential amplifiers struggle to accommodate a low input common mode voltage without impacting the performance of the bias current source and maintaining high input impedance, leading to distorted output signals and reduced bandwidth.
Innovation Solution
A single-stage differential amplifier design using p-channel MOS transistors with a resistively folded input stage and NMOS transistors to provide a high input impedance and accommodate zero volt common mode, allowing for linear termination resistors and direct impact of input signals on the output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a low input common mode voltage is used, then the amplifier can operate with zero volt common mode input, but the performance of the bias current source is impacted and the input impedance decreases
Solution Approach 1:
The amplifier is divided into two independent sub-stages: an input sub-stage with PMOS transistors that handles the low common mode input, and a transistor sub-stage with NMOS transistors that provides the bias current source. This segmentation allows each sub-stage to be optimized independently, so the input sub-stage can accommodate zero volt common mode input while the transistor sub-stage maintains stable bias current performance.
Solution Approach 2:
The resistive load serves as an intermediary between the input sub-stage and the transistor sub-stage. It resistively folds the input sub-stage while maintaining electrical independence between the two sub-stages, allowing the input sub-stage to operate at low common mode voltages without directly impacting the bias current source performance in the transistor sub-stage.
2Adaptability or versatility
If a low input common mode voltage is used, then the amplifier can accommodate zero volt common mode input, but the input impedance is reduced
Solution Approach 1:
The input sub-stage uses PMOS transistors specifically configured to provide high input impedance at the gate terminals, while the rest of the circuit uses NMOS transistors optimized for low common mode operation. This local quality assignment allows the input stage to maintain high impedance characteristics even when operating with zero volt common mode input.
3Adaptability or versatility
If multiple stages are used to accommodate low common mode input, then the common mode input range is improved, but the amplifier becomes multi-stage increasing complexity
Solution Approach 1:
The patent merges the functions of accommodating low common mode input and providing bias current into a single integrated stage. The resistive load folds the input sub-stage directly into the transistor sub-stage, creating a single-stage amplifier that eliminates the need for separate compensation stages or additional circuitry that would otherwise be required to handle low common mode voltages.
Data Source
AI summary
A resistively folded single stage differential amplifier capable of accommodating a low input common mode without impacting the performance of a bias current source, while also providing a high input impedance to allow for the use of the linear termination resistors. The differential amplifier provides an amplified output signal with a common mode referenced to an upper bound of an input power supply. The differential amplifier includes an input sub-stage and a transistor sub-stage resistively folding the input sub-stage.