Amplifier Bandwidth Limiting via Load Impedance Ratio Control
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Solution Overview
Problem
Conventional methods for limiting amplifier bandwidth either increase capacitance, making the circuit difficult to drive, or increase resistance, which raises noise density, both of which negatively impact performance.
Innovation Solution
A circuit with a load impedance comprising resistive and capacitive elements, where the ratio of load impedance to the sum of feedback and input impedance is kept constant, reducing closed-loop bandwidth without altering input impedance or increasing noise density, achieved by reducing effective amplifier transconductance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If capacitance is increased to shunt resistance in the signal path to limit bandwidth, then bandwidth is reduced, but the circuit becomes more difficult to drive and may cause instability
Solution Approach 1:
The patent introduces a series resistance element as an intermediary component between the input signal and the capacitive shunt element. This series resistance acts as a mediator that limits the interaction between the input circuit and the large shunt capacitance, thereby reducing bandwidth while maintaining ease of driving the circuit. The series resistance prevents the full impact of the large capacitance from affecting the input impedance, solving the contradiction between bandwidth reduction and driveability.
2Speed
If resistance is increased in series with the signal to limit bandwidth, then bandwidth is reduced, but noise density increases
Solution Approach 1:
The patent applies local quality by placing the series resistance element only at specific locations in the circuit where it is needed for bandwidth limitation, rather than increasing resistance throughout the entire signal path. By strategically positioning the series resistance in series with the shunt capacitance, the bandwidth is limited locally at that node without significantly affecting the noise performance of other circuit stages. The resistance is chosen to be small enough to minimize noise contribution while still achieving the desired bandwidth reduction when combined with the shunt capacitance.
3Power
If DC gain is increased by making load resistance larger, then DC gain improves, but feedback loop stability deteriorates due to parasitic capacitances
Solution Approach 1:
The patent introduces dynamics by making the effective load resistance frequency-dependent through the combination of series resistance and shunt capacitance. At DC and low frequencies, the shunt capacitance has high impedance, allowing the full load resistance to provide high DC gain. At higher frequencies, the shunt capacitance impedance decreases, effectively reducing the load resistance and thereby reducing the gain at those frequencies. This dynamic behavior allows the circuit to maintain high DC gain while automatically providing gain roll-off at higher frequencies, which improves feedback loop stability without requiring separate compensation circuits.
Data Source
AI summary
An apparatus for limiting the bandwidth of an amplifier provides for the design of an input impedance, a feedback impedance, and a load impedance such that the load impedance is proportional to the sum of the input impedance and feedback impedance. A sampling circuit has a load impedance including a resistor and capacitor in series to reduce the effective amplifier transconductance, which decreases bandwidth without increasing noise density or making this circuit more difficult to drive than a conventional circuit.