Current-Feedback Capacitance Multiplier for Stable RC Filters
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Solution Overview
Problem
Conventional capacitance multiplier circuits require additional power and circuitry, which can destabilize the frequency response and increase power consumption in system-on-chip (SoC) integration, especially when used in transimpedance amplifiers (TIAs).
Innovation Solution
A capacitance multiplier circuit that splits the output current into two branches, one to sense and the other to multiply the current by a factor K, with the multiplied current fed back to the filter input, effectively increasing the capacitance without adding extra circuitry or power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If a Miller capacitance multiplier is used to reduce capacitor surface area, then the capacitance multiplication effect is achieved, but additional power and circuitry are required which affects frequency response stability
Solution Approach 1:
The output current is segmented into two separate branches: one branch senses the output current through the filter, while the other branch multiplies the sensed current by factor K. This segmentation allows the circuit to achieve capacitance multiplication without requiring additional active capacitor multiplier circuitry that would destabilize the frequency response.
Solution Approach 2:
A current mirror circuit is introduced as an intermediary mechanism to multiply the sensed current. The current mirror takes the sensed current as input and produces a multiplied current output (K times the sensed current), which is then fed back to the filter input. This intermediary approach achieves capacitance multiplication without directly adding unstable active capacitor multiplier circuitry to the signal path.
2Area of stationary object
If a conventional current sensing capacitance multiplier is used to reduce capacitor size, then capacitance multiplication is achieved, but substantial additional power is required to reduce the series lossy resistance
Solution Approach 1:
The current sensing function and current multiplication function are merged into a single integrated circuit structure. The same circuit elements that sense the output current also perform the current multiplication through the current mirror mechanism, eliminating the need for separate high-power sensing circuits and multiplication circuits that would consume substantial additional power.
Solution Approach 2:
The circuit uses the existing output current from the filter to provide both the sensing function and the multiplication function. By feeding back a portion of the output current through the current mirror, the circuit achieves capacitance multiplication using the signal itself rather than requiring external power-intensive active capacitor multiplier circuitry.
Data Source
AI summary
A capacitance multiplier circuit is configured to sense a current through a capacitor in an RC filter of the circuit and to multiply the current so as to achieve a capacitance multiplier effect without adding additional circuitry or requiring additional power. The circuit includes an RC filter, a first signal path connected to a filter output, and a second signal path connected to an input to the filter. A current output through the filter (iout) is split between the two paths, sensed in the first path and multiplied in the second path. The multiplied current is fed back from the second path to the filter input to raise the effective capacitance of capacitor C. The capacitance multiplier circuit, in raising the effective capacitance of the capacitor in the filter, does not affect the frequency response, linearity performance and/or stability of the overall circuit.