Fully Differential Op-Amp Feedback with Level-Shifted Common-Mode Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In fully differential operational amplifier circuits, the input common mode voltage (CM) is often set to match the output CM, limiting the freedom in setting input CM, especially when using resistive feedback paths, which can be unfavorable, particularly at low supply voltages.
Innovation Solution
The use of a combination of coupling resistors and level shifters connecting amplifier outputs to inverting and noninverting inputs, allowing for independent setting of input CM relative to output CM, particularly with capacitive coupling paths, and incorporating transistors and current sources for level shifting, enables flexible control of input-side potential ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If resistive feedback paths are used to set input CM to match output CM, then input common mode voltage is defined, but freedom in setting input CM is limited
Solution Approach 1:
The patent introduces an intermediary circuit block between the output and the feedback network that performs level shifting. This mediator allows the feedback network to operate with a different voltage reference than the input stage, enabling independent setting of input CM and output CM without directly coupling them through resistive feedback alone.
Solution Approach 2:
The feedback path is segmented into multiple stages: the output stage, a level shifting stage, and the feedback network stage. This segmentation allows each stage to be optimized independently, with the level shifter translating between different voltage domains to enable flexible CM setting.
2Adaptability or versatility
If output CM is set to approximately half of supply voltage for large control range, then amplifier control range is maximized, but input CM becomes equally large which is unfavorable under certain circumstances
Solution Approach 1:
A level shifting intermediary is introduced between the output CM setting stage and the input CM stage. This mediator translates the output voltage reference (optimized for maximum control range at half supply voltage) to an appropriate input voltage reference, decoupling the optimal output CM setting from the input CM level.
Solution Approach 2:
The patent changes the voltage reference parameter through the level shifter, allowing the output CM to be set at half supply voltage for maximum dynamic range while the input CM is transformed to a different level through the level shifting operation, optimizing both stages for their respective requirements.
3Reliability
If coupling resistors are used in feedback paths, then DC feedback is provided, but influence on amplification increases and high-frequency performance degrades
Solution Approach 1:
The patent applies different coupling characteristics to different parts of the feedback network. Capacitive coupling is used in paths where high-frequency performance is critical, while resistive coupling is used where DC stability is the primary concern. This local differentiation of coupling quality allows optimization of both DC stability and high-frequency response in different portions of the circuit.
Solution Approach 2:
The feedback network uses composite coupling structures combining resistive and capacitive elements. By strategically placing capacitive couplings in parallel or series with resistive feedback paths, the circuit achieves both DC feedback stability (through the resistive component) and high-frequency performance (through the capacitive component that bypasses or reduces the resistive influence at high frequencies).
Data Source
AI summary
The present invention relates to a circuit configuration having a feedback operational amplifier (AMP), which is implemented as fully differential, for amplifying an input signal differentially input to the circuit configuration and for outputting the amplified input signal as a differential output signal. In order to increase the freedom in setting the input common mode voltage, according to the present invention, a combination made of a coupling resistor (R1b) and a level shifter (I1b, Nsfb) connecting the positive amplifier output (y1) to the inverting amplifier input (x2) and a combination made of a coupling resistor (R1a) and a level shifter (I1a, Nsfa) connecting the negative amplifier output (y2) to the noninverting amplifier input (x1) are provided.