Differential Amplifier Output Common-Mode Bias Without Stability Loss
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Solution Overview
Problem
Differential amplifiers face challenges with inconvenient output voltage levels, reduced dynamic range, and performance limitations due to high input stage gain, which affect stability and bandwidth, especially when used with feedback or single supply sources.
Innovation Solution
A differential amplifier design with high input impedances at both inputs, independent adjustment of output voltage levels, and a closed-loop input stage that maintains stability, utilizing a reference voltage to bias the output without impacting performance, achieved through a transimpedance circuit with current summation points and current mirrors, allowing for flexible gain and bandwidth settings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the input stage gain is increased to improve signal amplification, then the amplification capability is improved, but the circuit stability deteriorates and oscillations may occur
Solution Approach 1:
The amplifier is divided into separate functional stages: an input stage for signal reception and a reference stage for gain control. The reference stage uses a buffered reference voltage to set the output common mode level independently, allowing the input stage to operate at optimal gain without causing oscillations. This segmentation isolates the gain-setting function from the amplification function, resolving the stability issue.
2Ease of operation
If additional level shifting circuitry is added to adjust output voltage level, then the output voltage level can be positioned correctly, but the device complexity increases
Solution Approach 1:
The reference stage serves multiple functions simultaneously: it sets the output common mode voltage level, provides gain control, and maintains circuit stability. By making the reference stage multi-functional, the patent eliminates the need for separate level shifting circuitry, thereby reducing overall device complexity while achieving proper output voltage positioning.
3Ease of operation
If AC coupling with large capacitors is used to handle inconvenient output voltage levels, then the output voltage level issue is addressed, but the device complexity and size increase
Solution Approach 1:
The patent extracts the output common mode voltage control function from the main amplification path and places it in a dedicated reference stage. This allows the amplifier to directly output voltages suitable for single-supply circuits without requiring AC coupling capacitors. The reference voltage is buffered and fed back to current summation points, enabling direct DC coupling and eliminating the need for large capacitors.
Data Source
AI summary
A fully differential amplifier with a high common mode rejection ratio with an independent output voltage setting is disclosed. The amplifier may be arranged with a single ended output or a differential output. The gain may be set by adjusting a resistor without affecting bandwidth of the circuit. The circuit exhibits high speed and may be implemented with various electronic component types. The DC voltage of the output, single ended or differential, may be adjusted by adjusting a reference voltage, wherein the output voltage adjustment does not substantially affect the performance of the differential amplifier.