Capacitive-Feedback LNA Circuit for Wider Gain Bandwidth
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Solution Overview
Problem
Low-noise amplifiers with capacitive feedback provide a matched resistive input impedance with wide bandwidth but are limited by a relatively low gain bandwidth, which restricts the usable frequency range.
Innovation Solution
A low-noise amplifier circuit with a common source or common emitter configuration, utilizing a shunt-feedback capacitor and an output capacitor with a capacitance value less than the transconductance divided by the operating frequency, and a termination circuit with a current input terminal, such as a common-base amplifier or feedback-connected operational amplifier, to achieve frequency-independent transconductance and wideband operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a capacitive feedback LNA is used to achieve matched resistive input impedance with wide bandwidth, then the input impedance matching is improved, but the gain bandwidth is limited which restricts the usable frequency range
Solution Approach 1:
The patent changes the feedback impedance from capacitive to inductive, transforming the frequency-dependent capacitive feedback into frequency-independent inductive feedback. This parameter change enables the LNA to maintain both wideband input impedance matching and extended gain bandwidth, resolving the contradiction between matching bandwidth and gain bandwidth limitations
Solution Approach 2:
The patent substitutes the traditional capacitive feedback mechanism with an inductive feedback mechanism. By replacing the capacitive element with an inductive element in the feedback path, the system achieves frequency-independent transconductance while maintaining resistive input impedance matching across a wide bandwidth, thereby extending the usable frequency range
2Adaptability or versatility
If the LNA is designed for wideband operation to match multiple input frequencies, then the adaptability is improved, but the device complexity increases due to more complex matching networks
Solution Approach 1:
The inductive feedback structure serves multiple functions simultaneously: it provides frequency-independent transconductance, enables resistive input impedance matching across wide bandwidth, and eliminates the need for complex external matching inductors. This multi-functionality allows the LNA to match multiple frequencies without increasing device complexity
Solution Approach 2:
The patent extracts and eliminates the need for external frequency-dependent matching inductors by integrating the matching function directly into the inductive feedback path. This extraction simplifies the overall device structure while maintaining wideband matching capability across multiple frequencies
3Manufacturing precision
If resistive shunt degeneration is used to set the input resistance, then the input impedance control is improved, but the noise figure is degraded due to the presence of the feedback resistor
Solution Approach 1:
The patent substitutes resistive shunt degeneration with inductive series feedback. By replacing the resistive feedback element with an inductive element, the system maintains precise control over input resistance while eliminating the noise figure degradation caused by the feedback resistor, as the inductive element does not introduce the same thermal noise
Data Source
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AI summary
A common source or common emitter LNA circuit (30) for amplifying signals at an operating frequency f in a receiver circuit (10) is disclosed. The LNA circuit (30) comprises an input transistor (50) arranged to, in operation, be biased to have a transconductance gm at the operating frequency f, and having a first terminal (52), which is a gate or base terminal, operatively connected to an input terminal (32) of the LNA circuit (30). The LNA circuit (30) further comprises a shunt-feedback capacitor (60) operatively connected between the first terminal (52) of the input transistor (50) and a second terminal (54), which is a drain or collector terminal, of the input transistor (50). Furthermore, the LNA circuit (30) comprises an output capacitor (65) operatively connected between the second terminal (54) of the input transistor (50) and an output terminal (34) of the LNA circuit. The output capacitor (65) has a capacitance value CL < gm/f.