Temperature-Compensated Low-Pass Filter for Stable Cutoff Frequency
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Solution Overview
Problem
Conventional low-pass filters with active components face challenges in maintaining a low cutoff frequency due to temperature-induced increases in bias current, which can ruin the fidelity of voltage-controlled oscillators used in applications like ring oscillators.
Innovation Solution
A temperature-compensated low-pass filter design incorporating a subthreshold bias circuit with a differential amplifier and current mirror configuration, which adjusts the gate voltage of transistors to maintain a stable resistance and cutoff frequency despite temperature variations, using a complementary-to-absolute-temperature reference voltage to control the bias current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If active components are used to implement the low-pass filter, then the filter can be manufactured on semiconductor die, but the bias current increases non-linearly with temperature causing the cutoff frequency to rise
Solution Approach 1:
The patent changes the biasing parameters by using a complementary-to-absolute-temperature (CTAT) voltage reference instead of a conventional bandgap reference. This CTAT voltage is used to bias the transistors in the low-pass filter, causing the bias current to decrease linearly with temperature. This parameter change compensates for the inherent temperature-dependent increases in transistor leakage and capacitance, thereby maintaining a stable cutoff frequency across temperature variations while keeping the filter manufacturable on semiconductor die.
2Measurement precision
If the cutoff frequency is reduced to suppress flicker noise, then the reference voltage fidelity improves, but the bias current temperature sensitivity increases
Solution Approach 1:
The patent employs a complementary-to-absolute-temperature (CTAT) voltage reference to bias the transistors, which causes the bias current to decrease linearly with temperature. This parameter change directly addresses the stability issue by compensating for temperature-induced increases in transistor leakage and capacitance that would otherwise cause the cutoff frequency to rise and degrade reference voltage fidelity.
Solution Approach 2:
The patent implements a feedback mechanism where the CTAT voltage reference continuously adjusts the bias current based on temperature variations. This feedback loop ensures that as temperature increases and transistor characteristics change, the bias current is automatically reduced to maintain a stable cutoff frequency, thereby preserving reference voltage fidelity across the operating temperature range.
Data Source
AI summary
A temperature-compensated low-pass filter includes a differential amplifier that controls a first transistor to pass a subthreshold current through the transistor to charge a capacitor with low-pass-filtered output voltage. A second transistor has a first terminal coupled to an input terminal of the low-pass filter and has a second terminal coupled to a current source conducting a bias current. The differential amplifier also controls the second transistor to conduct the bias current responsive to a difference between a complementary-to-absolute-temperature reference voltage and a voltage of the second terminal of the second transistor.