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 is designed with symmetric impedance at both drive level terminals, using feedback techniques to apply noise from the power supply to the gate of the output transistor, utilizing an operational amplifier and additional transistors to maintain a constant output current by equalizing the noise voltage, thereby rejecting supply noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional current sources are used with high output impedance, then current stability is improved, but supply noise rejection deteriorates due to asymmetric impedance
Solution Approach 1:
The patent applies asymmetry in reverse - it deliberately creates symmetric impedance at both terminals of the current source. By making the impedance looking into the output terminal equal to the impedance looking into the supply terminal, the circuit achieves both high current stability and excellent supply noise rejection. This symmetric configuration allows the feedback mechanism to effectively cancel supply noise at both terminals simultaneously.
Solution Approach 2:
The patent employs feedback by sensing the supply voltage noise and injecting an equal and opposite noise signal into the gate of the output transistor through the feedback resistor. This feedback mechanism dynamically compensates for supply noise variations, maintaining constant current output despite asymmetric terminal configurations. The feedback loop ensures that supply noise is rejected while preserving high output impedance for current stability.
2Power
If voltage supply rails are used to source current, then current sourcing capability is improved, but noise performance deteriorates due to inherent supply noise
Solution Approach 1:
The patent introduces a feedback resistor as an intermediary element that couples the supply terminal to the gate of the output transistor. This intermediary pathway allows supply noise to be sensed and fed back in a controlled manner, enabling the circuit to reject supply noise while maintaining full current sourcing capability from the voltage supply rails. The feedback resistor acts as a mediator that transforms harmful supply noise into a useful control signal.
3Device complexity
If asymmetric impedance configuration is used in current source, then device complexity is reduced, but noise isolation deteriorates between output and supply terminals
Solution Approach 1:
The patent makes the feedback resistor serve multiple functions simultaneously: it provides the feedback pathway for noise rejection, sets the input impedance at the supply terminal, and determines the noise cancellation characteristics. This multi-functional design achieves symmetric impedance and excellent noise isolation without requiring separate dedicated components for each function, thereby maintaining relatively simple device complexity while dramatically improving noise isolation.
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 achieves a high impedance at the output terminal, effectively rejecting supply noise and maintaining a constant current output, suitable for low-noise applications by ensuring the current source has a high impedance relative to the supply voltage noise.
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.