Supply-Noise-Rejecting Current Source With Symmetric Impedance
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Solution Overview
Problem
Conventional current sources have asymmetric impedance, leading to poor isolation of output current from voltage noise in supply rails, making them unsuitable for applications requiring very low-noise operation due to significant feedthrough of power supply noise.
Innovation Solution
A supply-noise-rejecting current source with symmetric impedance at both drive level terminals is achieved through a feedback technique that applies noise from the power supply to the gate of the output transistor, using an operational amplifier and a feedback resistor to maintain a constant output current by equalizing the noise voltage, thereby rejecting supply noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional current sources are used, then current output is provided, but supply noise feeds through to the output due to asymmetric impedance
Solution Approach 1:
The patent employs feedback by sensing the supply voltage noise and injecting it through a feedback resistor to the gate of the output transistor. The operational amplifier detects voltage changes at its inputs and adjusts the gate voltage to counteract the noise, creating a closed-loop system that actively compensates for supply variations.
Solution Approach 2:
The feedback resistor acts as an intermediary element that couples the supply noise to the gate terminal. The operational amplifier serves as a mediator that processes the noise signal and generates the compensating gate voltage, isolating the output from direct noise coupling while maintaining control.
2Ease of manufacture
If asymmetric impedance is used in conventional current sources, then manufacturing is simplified, but noise isolation performance deteriorates
Solution Approach 1:
The feedback mechanism continuously monitors supply voltage variations and adjusts the gate voltage in real-time, creating dynamic noise rejection without requiring complex asymmetric impedance matching networks or specialized transistor configurations.
Solution Approach 2:
The patent changes the gate voltage parameter dynamically in response to supply noise. By modulating the gate voltage to match and counteract supply variations, the system adapts its operating parameters to maintain noise rejection performance across different supply conditions.
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
The solution effectively rejects supply noise, ensuring high impedance at the output terminal and reducing noise performance issues, making it suitable for applications requiring low-noise operation.
Implementation Method 1
the op-amp drives its output to equalize the voltage at the first op-amp input to the second op-amp input
Implementation Method 2
Noise on a voltage supply induces a current from the source to the drain of the second transistor causing a change in voltage at the first input of the op-amp via the feedback resistor
Data Source
AI summary
Various technologies pertaining to a high-impedance current source are described herein. The current source outputs a substantially constant current by way of a first transistor that draws current from a supply. The current source is configured to feed back noise from the supply to a feedback resistor at an input of an operational amplifier (op-amp) by way of a second transistor. The feedback resistor and the op-amp are configured such that responsive to receiving the supply noise feedback, the op-amp drives a gate voltage of the first transistor to cause the first transistor to reject the supply noise and cause the output of the current source to remain substantially constant.