Common-Gate RF Amplifier Feedback for Broadband 50Ω Matching
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Solution Overview
Problem
Existing radio-frequency amplifiers face challenges in achieving reduced input resistance and high gain with a low noise figure over a large frequency interval, particularly in low power consumption designs, and are often limited by the need for external reactive components and narrow band operation.
Innovation Solution
A common gate amplification stage with a feedback transistor, where the gate-drain voltage is based on the difference between the amplified output and input voltage, allowing for a reduced input resistance and broad frequency band operation without large transistors or bias currents, and is completely integrable without external components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a common source configuration with reactive components is used to achieve better noise performance and impedance matching, then noise figure is improved, but the circuit becomes narrow band and difficult to integrate
Solution Approach 1:
The patent introduces a feedback network comprising a feedback capacitor Cf connected between the drain and gate of the transistor, and a feedback resistor Rf connected in parallel with Cf. This feedback mechanism transforms the input impedance characteristics, enabling broadband operation while maintaining good noise performance. The feedback loop allows the circuit to achieve impedance matching across a wide frequency range without requiring external reactive components, thus resolving the contradiction between noise performance and frequency band adaptability.
2Use of energy by moving object
If the bias current is limited to about one mA for low power consumption, then power consumption is reduced, but the input resistance cannot be matched to 50Ω
Solution Approach 1:
The feedback network with capacitor Cf and resistor Rf transforms the high input resistance (typically several hundreds of ohms) that results from low bias current into a matched 50Ω input impedance. The feedback mechanism effectively multiplies the transconductance effect, allowing impedance matching to be achieved even with limited bias current of about one mA, thus resolving the contradiction between power consumption and impedance matching capability.
3Ease of operation
If external reactive components are used for impedance matching, then impedance matching is achieved, but the circuit becomes encumbering and expensive
Solution Approach 1:
The patent merges the impedance matching function with the existing transistor and feedback network, eliminating the need for external inductors and capacitors. The feedback capacitor Cf can be implemented as an on-chip component, and the feedback resistor Rf can be integrated using standard CMOS resistors. This integration approach reduces component count, lowers cost, and simplifies the circuit while maintaining excellent impedance matching performance across the frequency band.
4Ease of operation
If the transistor aspect ratio W/L is increased to satisfy the input resistance condition, then input resistance matching is improved, but the capacitance increases making the reactive part non-negligible
Solution Approach 1:
The feedback network compensates for the capacitive effect of large W/L transistors. By introducing the feedback capacitor Cf and resistor Rf, the circuit transforms the input impedance in a way that counteracts the detrimental effect of increased transistor capacitance. This allows the use of larger W/L ratios for better input resistance matching without degrading the frequency response, as the feedback mechanism effectively neutralizes the capacitive loading effect.
Data Source
AI summary
A radio-frequency amplifier includes a common gate amplification stage configured to be biased in a saturation condition with a first current and configured to receive an input signal as a gate-source voltage and to generate an output voltage as an amplified replica of the input signal. A feedback transistor is configured to be biased in a saturation condition with a second current and coupled to the common gate amplification stage so as to have a gate-drain voltage corresponding to a difference between the output voltage and the input signal.