Amplifier Feedback Switch Layout for Lower PSRR Noise
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Solution Overview
Problem
Existing amplifier systems suffer from high Power Supply Rejection Ratio (PSRR) due to parasitic capacitances in MOSFET switches, leading to noise at the output, as supply voltage ripple is amplified and affects the sensitive input nodes.
Innovation Solution
The implementation of a circuit configuration where a switch is controlled to connect its terminal to a clean voltage supply when in a non-conducting state, reducing the impact of parasitic capacitances by isolating the sensitive node from supply voltage ripple, and using an R2R network to adjust feedback paths and input resistance for gain control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a MOSFET switch is used to control feedback paths in an amplifier, then the gain can be adjusted and the circuit can be simplified, but parasitic capacitances in the switch cause supply voltage ripple to appear at the sensitive input node, resulting in high PSRR and noise at the output
Solution Approach 1:
A dummy switch is introduced as an intermediary element that replicates the parasitic capacitance of the feedback switch. This dummy switch is placed in a non-sensitive location where its capacitance does not couple supply ripple to the amplifier input. The dummy capacitance compensates for the harmful effect of the feedback switch's parasitic capacitance, effectively canceling its negative impact on PSRR while preserving the gain control functionality.
2Productivity
If the switch is turned off to isolate the input node from the output node, then feedback is reduced and gain is increased, but the parasitic capacitance at the input node remains connected to the supply voltage, allowing ripple to affect the sensitive input
Solution Approach 1:
The dummy switch acts as a mediator that absorbs the harmful parasitic capacitance effect. By placing an equivalent capacitance in a dummy switch located away from the sensitive input node, the supply voltage ripple is prevented from coupling into the amplifier input through the feedback switch's parasitic capacitance, thereby reducing noise while maintaining the desired gain.
3Object-affected harmful factors
If the parasitic capacitance is reduced by using smaller transistors, then the PSRR improves, but the switch resistance increases and the switching speed decreases
Solution Approach 1:
The solution segments the capacitance function by separating the harmful parasitic capacitance effect from the useful switching function. Instead of reducing transistor size to minimize parasitic capacitance (which would slow switching), the patent uses a dummy switch to replicate and isolate the capacitance effect, allowing the main feedback switch to operate with larger transistors for faster switching speeds while the dummy element handles the PSRR compensation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces the PSRR at the output, minimizing noise and improving the amplifier's performance by effectively canceling parasitic capacitance effects and maintaining constant input resistance.
Implementation Method 1
Switch 20 is disadvantageous due to the parasitic capacitances present between the nodes of the NMOS and PMOS transistors 24, 26
Data Source
AI summary
A circuit including a first sensitive node, a first component connected between the first sensitive node and a first terminal of a first switch, said first switch controlled by a first control signal variable between a supply voltage level and a second voltage level, and a second switch including a first terminal connected to the first terminal of said first switch, and a second terminal connected to a clean voltage supply, said second switch controlled to connect the first node of said first switch to said clean voltage supply when said first switch is in a non-conducting state.