Current Feedback Amplifier Input Stage for Near-Rail CMVR
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Solution Overview
Problem
Conventional current feedback amplifiers suffer from poor input common mode voltage range (CMVR), limiting their operation to high supply voltages and preventing 'rail to rail' input stages, which restricts their application to low-voltage scenarios.
Innovation Solution
A current feedback amplifier design with a configurable input stage and closed-loop buffer that allows inputs to operate within 800 mV of the supply rails, utilizing current mirrors and an output stage for rail-to-rail performance, enabling operation near the supply rails while maintaining high slew rate and AC flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional CFA topology is used, then excellent performance in terms of slew-rate and bandwidth is achieved, but poor input common mode voltage range (CMVR) limits operation to high supply voltages
Solution Approach 1:
The patent changes the operating parameters of the input stage by using a cross-coupled transistor pair (Q1, Q2) with specific biasing arrangements that allow the input common mode voltage to swing close to the supply rails. The bias voltages VB1 and VB2 are carefully selected to enable rail-to-rail operation while maintaining the high slew-rate and bandwidth characteristics of the CFA topology.
Solution Approach 2:
The patent introduces intermediate biasing circuits and transistor configurations that act as mediators between the input stage and the supply rails. The cross-coupled transistors Q1 and Q2, along with associated biasing network, serve as intermediaries that extend the input voltage range without compromising the high-speed performance of the amplifier.
2Adaptability or versatility
If input stage is designed for rail to rail operation, then input common mode voltage range is improved, but conventional topologies cannot operate properly at input voltages near supply rails
Solution Approach 1:
The patent employs specific parameter selections for the cross-coupled transistors and biasing voltages to ensure reliable operation throughout the entire input voltage range. The bias voltages VB1 and VB2 are designed to maintain proper transistor operation even when input voltages are within 800mV of the supply rails, preventing saturation or cutoff conditions that would compromise reliability.
Solution Approach 2:
The patent uses dynamic biasing arrangements where the bias voltages VB1 and VB2 are designed to adapt to different input common mode voltage levels. This dynamic approach ensures that the transistors remain in their active regions across the full input voltage range, maintaining reliable operation from rail to rail.
3Ease of manufacture
If degeneration resistors are added to input stage, then bias current control is improved, but common mode voltage range is further reduced
Solution Approach 1:
The patent carefully selects and optimizes the values of degeneration resistors R1 and R2 to achieve the desired bias current control while minimizing their impact on the input common mode voltage range. By precise parameter selection, the patent balances the trade-off between ease of manufacture (bias control) and adaptability (voltage range).
Solution Approach 2:
The patent applies degeneration resistors selectively in specific locations within the input stage where they provide maximum bias control benefit with minimum impact on the overall input voltage range. The local placement and value selection of R1 and R2 optimizes the trade-off between manufacturability and voltage range.
Data Source
AI summary
An embodiment of the present invention is directed to a current feedback amplifier. The amplifier is coupleable with a first supply rail and a second supply rail. The current feedback amplifier includes an input stage configurable to provide a first input and a second input for the current feedback amplifier, wherein the first and second inputs are operable to receive input voltages within 800 mV of the first supply rail or the second supply rail. The amplifier further includes a first current mirror coupled with the input stage, a second current mirror coupled with the input stage, and an output stage coupled with the first and second current mirrors. The output stage is operable to provide an output for the current feedback amplifier.