Integrator Circuit Stabilization with Adjustable Output Resistance
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Solution Overview
Problem
Analog mixed-signal integrated circuits with continuous-time filters based on elementary integrator cells face issues due to PVT-induced phase shifts, affecting frequency response, especially at high frequencies and wide tunable ranges, as transconductance gain and output resistance are influenced by process spread, supply voltage, and temperature fluctuations.
Innovation Solution
The output resistance of the input transconductance amplifier is made independently adjustable through a dedicated control signal, with a matched transconductance amplifier and a second control loop maintaining constant output voltage, ensuring phase stability across PVT variations by adjusting resistances and maintaining a constant transconductance gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If transconductance amplifier Gi is used with finite output impedance, then the integrator can be implemented with practical components, but the phase of transfer function deviates from 90° due to PVT variations
Solution Approach 1:
A feedback loop comprising error amplifier Aio, DAC, and reference resistor Rr is implemented to automatically adjust the transconductance gain of Gi and Gmi, nullifying the difference between reference current and Gmi*Vr, thereby compensating for PVT-induced phase shifts and maintaining 90° phase accuracy
Solution Approach 2:
The transconductance gain Gi is made tunable over a very large range of values through the feedback control mechanism, allowing the integrator to maintain optimal performance across varying process, voltage, and temperature conditions while implementing the circuit with practical components
2Adaptability or versatility
If transconductance gain Gi is made tunable over wide range, then the integrator can adapt to different frequency requirements, but the phase becomes PVT-dependent and affects filter frequency response
Solution Approach 1:
The feedback control system continuously monitors and adjusts the transconductance gains of Gi and Gmi to maintain consistent phase characteristics across the entire tuning range, ensuring that phase remains independent of PVT variations even when operating frequency is varied
Solution Approach 2:
The feedback mechanism maintains equipotential conditions by nullifying the current difference between the reference current and Gmi*Vr, creating a stable operating point that ensures phase consistency across different frequency settings and environmental conditions
Data Source
AI summary
An integrated circuit integrator includes a first transconductance amplifier having a gain adjustable based upon a first control signal, and receives, as an input, a signal to be filtered, and generates, as an output, a corresponding amplified signal. The first transconductance amplifier includes an R-C output circuit to filter components from the amplified signal, and an output resistance being adjustable based upon a second control signal. A second transconductance amplifier is matched with the first transconductance amplifier, and has a gain adjustable based upon the first control signal, and a matched output resistance adjustable based upon the second control signal. A circuit is configured to force a reference current through the matched output resistance. An error correction circuit is coupled to the second transconductance amplifier and is configured to generate the second control signal so as to keep constant a voltage on an output of the second transconductance amplifier.