Two-Stage Fully Differential Op-Amp With 9-Transistor Bias Feedback
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Solution Overview
Problem
Traditional fully-differential two-stage operational amplifiers have complex structures with high hardware consumption and production costs due to the large number of transistors required for common-mode feedback and bias circuits, which is not conducive to area savings in integrated circuits.
Innovation Solution
A fully-differential two-stage operational amplifier circuit design that includes a telescopic first-stage amplification circuit, a common-source second-stage amplification circuit, a simplified common-mode feedback circuit with only 4 transistors and 1 resistor, and a bias circuit with 4 NMOS transistors and 1 resistor, reducing the overall transistor count and simplifying the structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional common-mode feedback circuit with 7 transistors (MP7-MP11 and MN7-MN8) and a bias circuit with 14 transistors (MP12-MP19 and MN9-MN14) are used, then the operational amplifier can provide stable common-mode feedback and proper bias voltages, but the total transistor count reaches 21, resulting in complex structure, high hardware consumption and large area occupation
Solution Approach 1:
The patent merges the common-mode feedback circuit and bias circuit into a highly integrated structure. The common-mode feedback circuit uses only 4 transistors (MP1-MP4) and the bias circuit uses only 5 transistors (MP5-MP9), totaling 9 transistors compared to the traditional 21 transistors. This merging achieves both common-mode feedback stability and bias voltage generation with minimal hardware, directly resolving the contradiction between reliability and device complexity.
Solution Approach 2:
The transistors in the simplified circuits serve multiple functions. For example, MP1 and MP2 in the common-mode feedback circuit not only provide feedback but also function as active loads and bias elements. The bias circuit transistors generate multiple bias voltages (VB1-VB5) simultaneously. This multi-functionality reduces the total transistor count while maintaining circuit performance.
2Reliability
If a traditional common-mode feedback circuit and bias circuit are implemented, then the operational amplifier can maintain proper operating points and common-mode levels, but the hardware consumption and production cost increase due to 21 transistors
Solution Approach 1:
The patent combines multiple circuit functions into fewer transistors, reducing the total count from 21 to 9 transistors. This merging maintains operating point stability through the simplified common-mode feedback loop while significantly reducing hardware consumption and production cost.
Solution Approach 2:
The simplified bias circuit automatically generates all necessary bias voltages (VB1-VB5) for the operational amplifier stages without requiring additional dedicated bias circuits. The common-mode feedback circuit self-regulates the output common-mode voltage, eliminating the need for separate control circuits and reducing overall hardware requirements.
3Reliability
If traditional common-mode feedback and bias circuits are used, then the operational amplifier can provide proper voltage signals, but the area of the integrated circuit increases due to the complex structure
Solution Approach 1:
The patent merges the common-mode feedback and bias generation functions into a compact integrated structure using only 9 transistors total. This integration maintains all necessary voltage signal generation capabilities while occupying minimal silicon area, directly addressing the area reduction requirement.
Solution Approach 2:
The bias circuit is nested within the common-mode feedback circuit structure, where the bias transistors (MP5-MP9) are interconnected with the feedback transistors (MP1-MP4) to share common nodes and signals. This nesting arrangement maximizes space utilization and minimizes the overall circuit footprint while maintaining full functionality.
Data Source
AI summary
A fully-differential two-stage operational amplifier circuit is provided, and it includes a first-stage amplification circuit, a second-stage amplification circuit, a common-mode signal acquisition circuit, a common-mode feedback circuit and a bias circuit. The first-stage amplification circuit has a telescopic structure and receives differential input signals INP and INN. The second-stage amplification circuit has a common-source structure and outputs differential output signals OUTP and OUTN. The common-mode signal acquisition circuit receives differential output signals, and outputs an operational amplifier output common-mode signal VCMO. The common-mode feedback circuit outputs common-mode feedback signals VB1 and VB2 to the first-stage amplifier circuit and the second-stage amplifier circuit respectively; The bias circuit outputs a bias voltage VB3 to the first-stage amplifier circuit, and outputs bias voltages VB4 and VB5 to the first-stage amplifier circuit respectively.